EXPLORATORY WORK ON THE.PRECISION WAVELENGTH
OF
MEASUREMENT
THE HYDROGEN LYMAN.SPECTRA
by
FREDERICK WILLIAM DALBY
A THESIS SUBMITTED I N PARTIAL FULFILMENT OF
THE
REQUIREMENTS FOR THE DEGREE OF
MASTER OF ARTS
I n t h e Department
of
P h y s i c s ..
We a c c e p t , t h i s
standard
t h e s i s as conforming t o the
required, from candidates
f o r the
d e g r e e o f MASTER OF ARTS
Members o f t h e D e p a r t m e n t o f
Physics
THE
UNIVERSITY OF B R I T I S H COLUMBIA
April,
1952
ABSTRACT
The
L u b z i n s k i . s p e c t r o g r a p h has
f o c u s s e d and
ultraviolet
i t s p e r f o r m a n c e a s a h i g h d i s p e r s i o n vacuum
instrument
Light
sources
has
been s t u d i e d .
have b e e n d e s i g n e d
t i o n o f t h e h y d r o g e n Lyman s e r i e s .
s e r i e s has
hydrogen-deuterium
seven hours.
of
A
spectrogram
of
the
i n the
has
i d e n t i f i c a t i o n of the
sources
f o r the
e x c i t a t i o n of the
been
hydrogen
first-
s p e c t r a o f c o p p e r have, b e e n c o n s t r u c t e d a n d
studied.
because of
low
experiment-
T h i s s p e c t r u m c o n t a i n s a l a r g e number:of
s t a n d a r d w a v e l e n g t h s , i n t h e vacuum u l t r a v i o l e t ;
g r a t i n g i n t e n s i t y we
were u n a b l e
however,
to
observe
lines.
An
e x i s t e n t d i s c r e p a n c y between c a l c u l a t e d
measured, v a l u e s f o r t h e
atoms h a s
netic
this
line•
Light
these
excita-
ranging
g r a t i n g order a t a d i s p e r s i o n of 1 A ° / W «
Lyman a
ally
a line
isotope structure obtained
obtained permitting positive
spark
The
f o r the
been o b t a i n e d u s i n g exposure times
from twenty minutes t o
fifth
been a c c u r a t e l y
i o n i z a t i o n p o t e n t i a l of
b e e n r e s o l v e d by a g e n e r a l i z e d Lamb
shift.
and
Helium-like
electromag-
iv
ACKNOWLEDGMENTS
I am p l e a s e d
t o acknowlege t h e h e l p and a d v i c e
g i v e n b y D r . A . M. C r o o k e r who s u g g e s t e d
To
foil
the problem.
D r . J . B. W a r r e n , f o r t h e l o a n o f p l a t i n u m ,
and a g i f t
o f h e a v y w a t e r , t o Mr. J . L e e s ,
generous h e l p w i t h glass, blowing
Reesor f o r h i s a i d i n d r a f t i n g ,
for hiss
p r o b l e m s , a n d t o Mr. T.
I extend
my warmest
thanks.
I am i n d e b t e d
t o D r . G. H e r z b e r g f o r v a l u a b l e
d i s c u s s i o n o f many o f t h e p r o b l e m s e n t a i l e d
It
was
i s a pleasure
t o acknowledge t h a t t h i s
completed d u r i n g t h e tenure
Council
Bursary.
i n this
of a National
work.
work
Research
iii
TABLE OF CONTENTS
Pap;e
1
Acknowledgements
Abstract
i i
Table of Contents
List
i i i
of Illustrations
iv
Introduction
1
I.
3
Theory
o f H y d r o g e n - l i k e Spectra..
(a) E a r l y Work
3
(b) W e l t o n ' s T h e o r y
of the Electromagnetic S h i f t
7
I I . P r e v i o u s E x p e r i m e n t a l Work o n Lyman S e r i e s
Spectra
12
(a) On t h e H y d r o g e n Lyman S e r i e s
12
(b) On O t h e r H y d r o g e n i c
15
I I I . tamb S h i f t
Lyman S e r i e s
16
i n H e l i u m - l i k e Atoms
19
I V . D e s i g n o f E x p e r i m e n t a l Programme
(a) E s t i m a t e o f O p t i c a l P e r f o r m a n c e
of the
Lubzinski Spectrograph
19
(b) S t a n d a r d W a v e l e n g t h s I n t h e Vacuum
(c)
V.
Ultraviolet
25
Widths o f S p e c t r a l L i n e s .
31
Experimental
-
33
(a) The L u b z i n s k i S p e c t r o g r a p h
33
(b) F o c u s s i n g t h e S p e c t r o g r a p h
37
(c)
41
Lyman S e r i e s
(d) F i r s t
Spark
Spectra
Spectrum o f Copper
45
V I . C o n c l u s i o n s a n d Recommendations
48
Bibliography
50
iv
LIST OF ILLUSTRATIONS
A.
Figures
I.
Energy L e v e l Diagram of the Hydrogen
Atom.
II.
to f o l l o w page 5
O p t i c s of the L u b z i n s k i S p e c t r o g r a p h . t o
f o l l o w page 19
I I I . Vacuum Wavelength Standards from
the R i t z Combination P r i n c i p l e
I V . Spectrograph P r e s s u r e A g a i n s t
t o f o l l o w page 19
Time
from C e s s a t i o n of Pumping
V.
page 34
C i r c u i t Diagrams of Power Supply and
High Frequency O s c i l l a t o r
B.
to f o l l o w page 4 l
Tables
—,,,
I.
Experimental Hydrogen Lyman Wavelengths
A c c o r d i n g to Boyce and Rieke
II.
I o n i z a t i o n E n e r g i e s f o r the
page 14
Ground S t a t e s
of H e l i u m - l i k e Atoms
I I I . L i n e a r D i s p e r s i o n and Wavelength
IV. Vacuum Wavelength Standards i n the
C.
17,
page
21
Against
D i f f r a c t i o n Angle
V.
page
First
Spark Spectrum of Copper
page 30
Doppler H a l f Widths- f o r Hydrogen and Copper
page 32
Plates
I.
Reproduction of T y p i c a l Spectragrams.
to f o l l o w page 42
INTRODUCTION
The. t h e o r y o f D i r a e e ( l 8 ) p r e d i c t s t h a t
a_Sj_ e n e r g y
l
have t h e
l e v e l and
same.energy
Retherford
P± l e v e l
the
the
o f t h e h y d r o g e n atom
(he d e g e n e r a t e ) .
However, Lamb
and
(26) have shown e x p e r i m e n t a l l y t h a t t h e r e i s a n
-1
energy
difference
o f a b o u t 0.03
Bethe. ( 1 4 ) , has. d e d u c e d f r o m
cm
between these
t h e new. quantum
an e q u a t i o n w h i c h p r e d i c t s an upward s h i f t
relative
this
t o the P l e v e l s .
shift
cm
f o r the A
agreement w i t h t h e . e x p e r i m e n t a l . v a l u e .
/ *" 5j.
, a shift
line
a t 1216
A°.
wavelength of t h i s
o r d e r t o be
o f 0.004 A
I t was
L
a
line
able to t e s t
I t was
available
0
(1) To
i n good:,
F o r the ground
w i t h an a c c u r a c y
the
state,
corresponds
Bethe
the
o f 0.001
A°
in
prediction.
L u b z l n s k i vacuum
only
spetrograph
such e x c e s s i v e l y l o n g exposure times
experimental
study
level,
our p u r p o s e t o measure
the
theory
f o r t h e h y d r o g e n Lyman
precieionviWorkoonttheLL^mansseries,
Our
Jx
levels
soon d i s c o v e r e d t h a t the b l a z e of the
grating.for
necessitated
Bethe
o f 0.264 cm"""*- i s p r e d i c t e d , w h i c h
to a wavelength s h i f t
a
electrodynamics,
in a l lS
A c c o r d i n g to the
amounts t o 0.03
levels.
that
w o u l d be I m p r a c t i c a l .
programme t h e n became:
focus the
s p e c t r o g r a p h a c c u r a t e l y and
to
I t s performance w i t h the e x i s t i n g g r a t i n g i n o r d e r
to f a c i l i t a t e
u t i l i z a t i o n when t h e improved g r a t i n g
(2) To
develop
a
suitable
source f o r the
arrived.
excitation
of
the
h y d r o g e n Lyman s e r i e s
(3)
involved
To
attempt
A
solve
the
fundamental problem
i n a l l p r e c i s i o n w a v e l e n g t h measurements i n
vacuum u l t r a - r v i o l e t
of
to
spectra^
spectral region
s u i t a b l e wavelength
d e t a i l e d account of
i . e . the
the
development
standards,
the
results
of
t h i s programme
is
presented.
An
i n t e r e s t i n g d i s c r e p a n c y between measured
and
calculated
was
encountered.
be
ionization potentials
I t i s shown t h a t
readily resolved
by
considering
of h e l i u m - l i k e
t h i s discrepancy
a modified
atomscan
Bethe
equation..
We
the
commence w i t h a n
theory of
elementary
magnetic
semi-quantitative treatment
the
the
h y d r o g e n - l i k e atoms i n c l u d i n g
shift,
a l work on
o u t l i n e , of
f o l l o w e d by
Lyman s e r i e s
of
history
Walton's
the
electro-
a review of p r e v i o u s
spectra*
of
experiment-
If".' THEORY OF
(a)
3 -
HYDROGEN-LIKE. SPECTRA,
E a r l y - Work.
The
problem of the
o f t h e h y d r o g e n atom has
i n t e r p r e t a t i o n of the
p l a y e d an a l m o s t . u n i q u e
t h e d e v e l o p m e n t o f t h e new
quantum.mechanics.
spectra
role
From
t i m e , o f B a l m e r , c o n t i n u a l major, a d v a n c e s were made
either
i n v e n t i n g more r e f i n e d t h e o r i e s t o a c c o u n t
experimental
elaborate
o f new
data,
on t h e . one
experimental
thought
other.
t h a t t h i s p r o c e s s had
range of p r e c i s e e x p e r i m e n t a l
f o r by
the t h e o r y of D i r a c .
work by
Lamb
and R e t h e r f o r d
Until
recently
exhausted
itself..
expressed
h
i n 1885
*
by
the
great
accounted
experimental
(26)
wartime
exploiting
and
subsequent
(14), have
shown
showed, t h a t t h e w a v e l e n g t h s .
simple
hydrogen
empirical formula:
.
where b i s . a n e m p i r i c a l l y
This, formula
A
been
However, r e c e n t
( >\ ) o f a l l t h e t h e n known l i n e s a t t r i b u t e d , t o
c o u l d be
new
i n the Dirac: t h e o r y .
(13)
Balmer
for
i t . had
dataseemed well
t h e o r e t i c a l work, e s p e c i a l l y by. B e t h e
deficiencies
by
to observe p r e d i c t i o n s
developments i n microwave t e c h n i q u e ,
real
the
hand, o r d e v i s i n g more-
techniques
t h e o r i e s on t h e
in
d e r i v e d c o n s t a n t and
n = 3,
p r e d i c t e d , the. w a v e l e n g t h s o f t h e o b s e r v e d
with errors.of
less
than
one
p a r t i n one
thousand.
4,
...
lines?
Michelson and Morley
in
( J I ) s o o n showed t h a t t h e f i r s t
t h e s e r i e s . ( H ) was
a
therefore
4 line
i n r e a l i t y a c l o s e d o u b l e t ..and t h a t
the Balmer f o r m u l a
c o u l d n o t be s t r i c t l y
correct.
I t was n e v e r t h e l e s s a n i m p o r t a n t , s t i m u l u s f o r f u r t h e r
work.
The
first
s u c c e s s f u l t h e o r e t i c a l treatment
of
t h e h y d r o g e n atom problem.was; r e p o r t e d by B o h r (15)
1913.
By c o m b i n i n g
Rutherford's
idea
(34)
of a n u c l e a r
atom w i t h t h e quantum c o n d i t i o n s o f P l a n c k a n d
(19),
in
Einstein
B o h r was a b l e t o show t h a t , o n l y . d i s c r e t e , e n e r g y
l e v e l s were p o s s i b l e a n d g i v e n by f h e formula:.7
2
E(n) = -
...
hcR
(1-2)
n
The
number
to n.
of " o r b i t s "
p o s s i b l e f o r ea,ch e n e r g y
T h u s f o r n —. 1 o n l y a c i r c u l a r
w h e r e a s f o r n = 2 one c i r c u l a r
were p e r m i t t e d ,
that
etcetera.
the r e l a t i v i s t i c
o r b i t was
was
permitted
and,one e l l i p t i c a l
S o m m e r f e l d (37)
equal
orbit
I n 1916
showed
v a r i a t i o n o f mass w i t h v e l o c i t y l e d
to. s m a l l e n e r g y
d i f f e r e n c e s between o r b i t s
quantum number,
n , . a n d t h e . t h e o r y e x p l a i n e d a l l t h e known
experimental r e s u l t s
o f t h e same
- Including the doublet
s p l i t t i n g of
Michelson-Morley•.
However, t h e B o h r t h e o r y was n o t
adequate.
completely
No s a t i s f a c t o r y m e t h o d f o r t r e a t i n g atoms
more t h a n one . . e l e c t r o n c o u l d be d e v i s e d ; . f u r t h e r
i e s were e n c o u n t e r e d
i n deducing
with
difficult-
the r u l e s f o r l i n e
Intensities
5 -
- the s o - c a l l e d s e l e c t i o n . r u l e s .
These
difficult-
i e s were surmounted i n 1926 by the new wave mechanical
> theory of Schrttdinger
m a t r i x mechanics
(35) ( o r the e q u i v a l e n t
(22)) based upon s p e c u l a t i o n s
The p o s s i b l e energy
levels,
of d e B r o g l i e .
E , and the s t a t e f u n c t i o n s , Y ,
which r e p l a c e the Bohr o r b i t ,
eigenvalue
Heisenberg
c o u l d be deduced from the
equation
HY - EY
. . . (1-3)
where H Is. a s u i t a b l e . H a m i l t o n i a n . o p e r a t o r .
Rules were
g i v e n f o r deducing the a p p r o p r i a t e form of H from
analogues...
classical
A f t e r the new mechanics had been m o d i f i e d
by the i n t r o d u c t i o n o f r e l a t i v i t y
mass v a r i a t i o n
effects,
and the s p i n o r b i t i n t e r a c t i o n of Uhlenbeck and Goudsmit
(39) r e s u l t s were o b t a i n e d f o r the hydrogen energy
equivalent
t o those o f Sommerfeld.
function,
levels
The e l e c t r o n wave
Y , was a f u n c t i o n of the p r i n c i p a l quantum
number, n , the o r b i t a l quantum.number, JL , whose p o s s i b l e
v a l u e s are n , ( n - 1 ) ,
a possible
m=>e,
(n-2) . . . 0
Bohr orbit., a magnetic
U-2), ... 0,-1,
(-e-1),
each c o r r e s p o n d i n g t o
quantum number
. . . -JL,
and f i n a l l y a
s p i n quantum number S whose p o s s i b l e v a l u e s a r e +1/2 and
-l/2.
The d i s c r e t e
energy
s t a t e s which a r e deduced from
the Schrtidinger e q u a t i o n a r e (7)
S(n.,Q
=
_ RZ£
he
n'
where a = 2 n e / c h
2
constant,
+
rP
4n
j + 1/2
( o r 1/137) i s the f i n e
• • •
(1-4)
structure
Z and R a r e : t h e a p p r o p r i a t e charge and Rydberg
Bethe
! B,
1
1
t
D irac
FIGURE I,
ENERGY
LEVEL DIAGRAM F O R
ATOM.
To f o l l o w page 5
THE
HYDROGEN
constant, respectively,
6 -
f o r t h e atom c o n s i d e r e d , a n d J i s
t h e t o t a l a n g u l a r momentum whose p o s s i b l e v a l u e s a r e X +
and A its
l / 2 except
value
just
i s 1/2.
f o r S states,
The f i r s t
term
( s t a t e s w i t h i = 0 ) , when
of equation
term
g i v e s t h e combined
spin-relativity
p
which i s , f o r hydrogen, an order o f a
first.
An important
prediction
and
that s t a t e s of d i f f e r e n t ^
t h e 2*$x a n d 2 * ^
Figure
Lyman s e r i e s .
called
levels
t o the ground
The p r i n c i p a l
theory
in
2 ?JL
1
states,
, but i d e n t i c a l
level
that
degenerate.
scheme f o r
The l i n e s
arising
state are c a l l e d the
line
shown i n t h e f i g u r e i s
Q
s h o u l d be a d o u b l e t whose
called
n
( L ) a n d a c c o r d i n g t o t h e wave
intensities are Indicated.
and
smaller than the .
i n h y d r o g e n - l i k e atoms be
I g i v e s t h e energy
t h e Lyman a l i n e
mechanical
correction
Thus t h e t h e o r y p r e d i c t s
hydrogen on the Schrtidlnger t h e o r y .
transitions
The
consequence o f t h i s . e q u a t i o n . i s the
j have t h e same e n e r g y .
from
(1-4) i s
the f a m i l i a r n o n - r e l a t i v i s t i c ; B o h r expression.
second
1/2
The l i n e s
theoretical
ending
t h e Balmer s e r i e s ,
on t h e 2 S i 1
a r e n o t shown
the diagram.
The
relativistic
quantum, t h e o r y o f D i r a c ..(18)
w h i c h a u t o m a t i c a l l y endowed.the e l e c t r o n w i t h
spin
a n g u l a r momentum, i s c o n s i d e r e d a more s a t i s f a c t o r y
than the o l d SchrBdinger
theory.
it
t h a t f o r t h e h y d r o g e n atom p r o b l e m
i s sufficient
t o note
However, f o r o u r
theory
purposes
D i r a c s theory g i v e s the same r e s u l t ^ as the SchrBdinger
1
theory.
For hydrogen-like atoms,.the
energy l e v e l s are
a g a i n g i v e n by e q u a t i o n ( 1 - 4 ) , and i n p a r t i c u l a r the
2*"Si.and. 2 . P i
l e v e l s are
degenerate.
From 1887 t o 1940 soma.thirty--two
o p t i c a l studies
(27)
careful
were made of the h y p e r f i n e
of the Balmer a l i n e . .
structure
The . s l i g h t d i s c r e p a n c y , which seemed
to be i n d i c a t e d , between the Dirac. theory and most of
t h i s work c o u l d be r e s o l v e d by r a i s i n g the 2 ' " S i
level
about 0.03 c m " r e l a t i v e to the 2
1947
1
x
level.
In
Lamb..: and R e t h e r f o r d (26) by o b s e r v i n g t r a n s i t i o n s between
fine
s t r u c t u r e l e v e l s using.new microwave
techniques
found.that
indeed the S l e v e l was s h i f t e d upwards.
Bethe
t h e n deduced from the quantum e l e c t r o d y n a m i c s
(14)
the expected magnitude of such a s h i f t .
account, of t h i s Bethe s h i f t ,
A simplified
d u e . t o Welton (40),
will
now be p r e s e n t e d .
(b)
Welton*s Theory of the E l e c t r o m a g n e t i c
Shift
The quantum theory of r a d i a t i o n p r e d i c t s an
infinite
e l e c t r o n mass and hence an i n f i n i t e
a s s o c i a t e d w i t h t h i s mass.
As such q u a n t i t i e s
not observable t h i s . r e s u l t must be I n c o r r e c t •
#
energy
are.clearly
Bethe
The. p r e c i s e r e s u l t of the D i r a c theory leads _to e q u a t i o n
(1-4) when expanded In.powers of a t o a .
Successive
terms are n e g l i g i b l y s m a l l .
2
suggested
finite,
that the i n f i n i t e
by a s u b t r a c t i o n procedure.
be p e r f o r m e d
result
and
mass, be r e n o r m a l i z e d ,
"correctly"
the theory
I f this renormalization
leads to the f e l i c i t o u s
of zero electromagnetic.energy
a small, displacement
o f energy
or. made
f o r a free
levels
electron
f o r a bound
electron.
Welton's
the p h y s i c a l o r i g i n
for
in
s e m i - c l a s s i c a l theory
of this
a bound e l e c t r o n .
vacua,
displacement, of energy
E
K
levels
Consider a quantized e l e c t r i c
as a F o u r i e r expansion
E -- l_
(40) i l l u s t r a t e s
i n terms o f plane
e
field,
waves^
... ( 1 - 5 )
k
—»
,
where
i s the. a m p l i t u d e
and c i r c u l a r
of. the. wave w i t h
*c i
vector
k
energy
i s not zero, f o r a quantized, r a d i a t i o n . f i e l d , f o r
there are the so-called
those
f o r a.simple
energy
frequency
propagation
The l o w e s t
zero p o i n t e n e r g i e s
harmonic
oscillator.
similar to
The z e r o p o i n t
a s s o c i a t e d . w i t h e a c h F o u r i e r component
so t h a t t h e a m p l i t u d e
£ (r+l]>)-i-^t)~E r
T
<
where M-M£j
c a n be d e t e r m i n e d
= J*£
1£K\
C
.
T^v
is T ^ ^
by t h e e q u a t i o n
H-IVCSJ
= ~jp^s i s t h e number.of r a d i a t i o n
whose wave v e c t o r s l i e i n ^ k .
# We
an
3
... ( i _ 6 )
oscillators
Hence
d-7)
employ t h e c o n v e n t i o n t h a t t h e . summation be c o n s i d e r e d
i n t e g r a l f o r n o n - i n t e g e r k.
-
9 -
These f l u c t u a t i n g e l e c t r i c - f i e l d s
p o s i t i o n of an
electron w i l l
the nucleus,
e l e c t r o n over a
n o t be
so
tend to spread
s m a l l volume.
the
Such
an
s t r o n g l y a t t r a c t e d c l when c l o s e t o
thus r a i s i n g . i n energy
a n g u l a r momentum.relative to those
the
s t a t e s of
s t a t e s of
zero
higher
a n g u l a r momentum i n w h i c h t h e e l e c t r o n has. a s m a l l
bility
of b e i n g found
Now
from
let.us
where t h e
fluctuations.
contributes
The
the
nucleus.
t r y t o . e s t i m a t e the
such f l u c t u a t i o n s . .
electron,
and
near
L e t r + d r be
second
term
kinetic
change i n
a r i s e s from
energy
ignored.
of the
V
s m a l l we
(? ^)=
The
time
term
+
average
value of the
second
contribution.
there
Since
dr
The
term
...
i s zero
; i_ )
(
8
and
c o n t r i b u t i o n of
the
is
*v-- tC^)U. v v
l
To
electron
assume
t h e r e f o r e makes no
last
zero p o i n t
F o r a bound s t a t e
vc?j C ^ * ) V ( ? J +it^-*)V?; + -
+
the
oscillations
e x i s t s an e f f e c t upon the . p o t e n t i a l energy.
is
energy
the p o s i t i o n of
t o t h e - e l e c t r o m a g n e t i c mass o f t h e
c a n t h e r e f o r e be
proba-
calculate
... _
(1
9)
c^Mi, r e c a l l
—i
^
or
and
<Ly
- €. E
- - e
f o r the r e s u l t a n t
E
fc
Vf- »-/-w*n
....
(1-10)
-
V
I
* <- V ^ c /
/
e
. The
limits
integral
effect
^
k-^ a n d
from
of the
k
10
2
[A
l
which are
-
4 fe
fe
inserted
...
to prevent
the
d i v e r g i n g . w i l l , b e . discussed, later..
zero p o i n t f l u c t u a t i o n s
(1-11)
The
is.then
- (1
When t h i s , i s a v e r a g e d
energy
shift
over a
quantum s t a t e
¥ the
l
only the S
structure
constant
The
••• (1
W
states are
and
nuclear
(a i s the
the
same a s
c a l c u l a t e d and
v a l u e : f o r kjj c o u l d be
13)
fine
charge.)
Bethe e q u a t i o n o b t a i n e d from
t h a t k]_ c o u l d be
reasonable
shifted,
Z the
quantum . . e l e c t r o d y n a m i c s , was
except
resulting
is
i« (^)/* PM$)
so.that
12)
non-r-relativistic
equation
a
guessed.
(1-13)
physically
Bethe f o u n d
for
hydrogen
k
The
x
=
17.8R
upper, l i m i t
'
Is^ was
taken
...
e q u a l t o mc/h
expectation that a r e l a t i v i s t i c
convergence f o r l i g h t
energy.of
was
the
(1-14)
because of
c a l c u l a t i o n would l e a d
quantum e n e r g i e s e x c e e d i n g . t h e
electron.
The
the
logarithm appearing
to
rest
above
a c c o r d i n g l y , f o r hydrogen
log
~
or l o g
1 7
^Q
h
c
R
or
7.63
...
(1-15)
11
Relativistic
-
calculations(27)
have been
subse-
quently c a r r i e d out t o second o r d e r p e r t u r b a t i o n y i e l d i n g
results
equivalent
t o those obtained here, w i t h i n the
accuracy r e q u i r e d f o r o p t i c a l ,
spectroscopy.
Whe n. hydro geni c - wave- f u n o t i o n s . a r e
i n t o e q u a t i o n (1-13) the r e s u l t a n t
shift
is
sub at 1 t u t ed
explicitly
dE = ^ h c l o g ( ^ ) ^
f o r the S s t a t e s .
stant
...
Other s t a t e s a r e u n a f f e c t e d .
(1-16)
The c o n -
k^ f o r helium t u r n s out to be f o u r times t h a t f o r
hydrogen (1-14) and v a r i e s
slowly f o r s u c c e s s i v e
hydrogenlc
atoms•
The p o s i t i o n s of the S l e v e l s
p r e d i c t e d by e q u a t i o n
Figure.I.
for.hydrogen
(1-16) are shown c r o s s h a t c h e d . i n
As i s i n d i c a t e d / . i n . t h e f i g u r e ,
2 " " f x energy
levels
x
^
a
n
d
a r e s h i f t e d upward by 0.265 cm""'' and
0.03 c m " r e s p e c t i v e l y .
1
the l
This shifts
the H
Balmer s e r i e s by about 0.01 A ° , and the L
each
a
a
l i n e of the
l i n e o f the
Lyman s e r i e s by about 0.004 A / t o l o n g e r wavelengths*
0
those p r e d i c t e d by the D i r a c
theory.
than
XTI,
PREVIOUS EXPERIMENTAL WORK ON LYMAN SERIES SPECTRA
(a) On t h e H y d r o g e n Lyman S e r l e s
It
predict
t h a t Lg f o r h y d r o g e n be a d o u b l e t
0.367 cm"
by
has been .seen,that, t h e o r e t i c a l
considerations
of separation
whose c e n t r e o f g r a v i t y l i e s a t 1216...664 A °
1
the unmodified
o r a t 1216.664 A
Dirac theory
according t o the Bethe.theory
of electromagnetic
The
Lyman.a l i n e
doublet
observed.
structure of this
Further
sufficiently
shift.
placed, such confidence
the
Dirac equation
Indeed, u n t i l
is
a
a
recently
been
observors
commonly c a l c u l a t e d Lyman
lines
Because, s u c h . g r e a t
i n the t h e o r e t i c a l l y c a l c u l a t e d
considerable
experimental
encountered i n precision,measurement
Lyman l i n e s ,
has never
i n the. w a v e l e n g t h s c a l c u l a t e d f r o m
that very
was p l a c e d
wavelengths,rand
shift.
v e r i f i c a t i o n of .the.
were u s e d a s w a v e l e n g t h s t a n d a r d s .
confidence
0
i t s wavelength has.never been measured
a c c u r a t e l y t o permit
electromagnetic
+ 0.004 A
0
t h e p u b l i s h e d work o n t h i s
difficulty
of the hydrogen
subject
I s meagre.
I n t h e s p e c t r a l r e g i o n o f t h e hydrogen.Lyman
line,
indeed
f r o m a b o u t 1900 A ° t o b e l o w 10 A , a i r ,
0
q u a r t z , and g e l a t i n a r e opaque.
the
a
quartz
o p t i c s b y t h e more, t r a n s p a r e n t
s p e c i a l l y prepared
spectrograph
and
thus
Schumann (4) b y r e p l a c i n g
was a b l e
photographic
to register
fluorite,
using
plate., and. e v a c u a t i n g h i s
s p e c t r a t o a b o u t 1250 A °
c r e a t e d vacuum u l t r a v i o l e t
spectroscopy.
As
f l u o r i t e becomes a l m o s t opaque . i n t h e h y d r o g e n Lyman a
13
region,
as
Schumann p r o b a b l y
the.refractive
was
d i d . not
observe
index of f l u o r i t e
i n this
spectral
unknown n o . w a v e l e n g t h measurements were
Lyman . ( 2 8 ) , who.was t h e f i r s t
fluorite
by
t h i s . line,.
1914
A
t o r e p l a c e , the.
grating,
Lp a t 1026
and
0
A°.
His
w a v e l e n g t h measurements were b a s e d . u p o n . t h e g e o m e t r y
his
instrument
a n d were a c c u r a t e t o p e r h a p s 1
1922
In
i n the f i r s t
imeter, g r a t i n g
s p e c t r o g r a p h w i t h two
=
a
1215.68 ± 0.03
Takamine a n d
minute
a
observed
the
A
first
0
(38),
Suga
Lyman s e r i e s o u t
and
Rao
intensity
anomalies
(inelastic)
and
Budami
t o the t w e n t i e t h
orders respectively, a t t r i b u t e d
to c o l l i s i o n s
o f the
w i t h f o r e i g n .gas m o l e c u l e s
(32),
and
discovered
second., k i n d
.( a n d
atoms).
the. method...of o v e r l a p p i n g o r d e r s , w a v e l e n g t h s were
measured t o about 0.02
B a l l a r d and
of
cent-
exposures..
with the
fifteenth
By
0
he e s t a b l i s h e d
L
who
A .
f o u r o r d e r s of a f i f t y
c o i n c i d e n c e o f the. f o u r t h o r d e r L
o r d e r Hp
of
J.. J . Hopf leld...(.24.) o b s e r v e d . t h e . h y d r o g e n
Lyman s p e c t r u m
By
region
possible.
prism, by a c o n c a v e R o w l a n d d i f f r a c t i o n
d i s c o v e r e d LQ a t 1216
Further
t h e hydrogen and
first
order
also
observed
A .
0
White
(12)
deuterium
obtained
Lyman . s e r i e s l i n e s , i n t h e
o f a t h r e e metre, g r a t i n g .
i n the
second
spectrograms
order.
The
Values
L
l i n e s , were
a
of
A
A , the
- 14
-
w a v e l e n g t h i n t e r v a l f o r t h e h y d r o g e n i s o t o p e s , were
mined u s i n g
i r o n arc:: l i n e
deter-
standards, i n second o r d e r
spectra a n d . c a l c u l a t e d hydrogen wavelengths i n the
first.
T h e r e was
theory.
n a t u r a l l y r a t h e r g o o d agreement, w i t h t h e
P r o b a b l y the, most ..accurate e x i s t i n g , m e a s u r e m e n t s
o n - t h e h y d r o g e n Lyman w a v e l e n g t h s - a r e t h o s e o f Boyce
Rieke
(17).
U s i n g a t w o metre
grating,
;
and
wavelengths of
t h e h y d r o g e n Lyman l i n e s were determined..by c o m p a r i s o n i n
higher grating orders against
T h e i r r e s u l t s are,,summarized
first.order
i n Table I .
iron
The
differences
between
the. m e a s u r e d
wavelengths and the D i r a c
results
a r e compared
w i t h the s h i f t
theory.
Some s h i f t
indicated..
can e a s i l y
rather
lines.
theoretical
p r e d i c t e d b y t h e Bethe,.
t o w a r d s t h e B e t h e r e s u l t . s e e m s t o be
However, a s t h i s . m e t h o d o f o v e r l a p p i n g
lead to considerable error
orders
such evidence i s
inconclusive.
. - TABLE. I
E x p e r i m e n t a l . H y d r o g e n Lyman W a v e l e n g t h s
t o Boyce a n d R i e k e
Experimental
A°
Dirac
Theory
A°
1215.666
1215.664
1025.725
1025.717
972.538
Experimental
- Calculated
IO"
3
A
0
Accordlnp;
Bethe
Shift
10~ ~A°
3
+3.9
+ 8..
+3.5
972.532
+ 6
+3.1
949.740
949.739
+ 1
+3.0
939.792
939.799
- 7
+2.-.9
+10
+16.4
-
(b)
On
Other
Hydrogen!c
Hydrogen-like
observed
0
f o r He
VIII(6).
lines
measurements
Calculated
of
Lyman
because
spectral
-
Lyman S e r i e s
III,, L l I I I ,
However,
in this
15
series
spectra, have
Be
IV,
B V,
of
the
scarcity
region
(306
A
0
to
w a v e l e n g t h were u s u a l l y
wavelengths
f o r these
C
lines
VI,
19
K
of
been
VII,
and
standard
A°),
precision
impossible.
were u s e d
as
standards.
For
careful
orders
measurement
up
calculated
agrees
L i III, Edlen
to
the
from
well with
calculated
by
of
the
(6)
Dirac
the
Mack
Is
(6).
p o i n t e d out
wavelength of
t w e l f t h show
the
has
it'to
theory
by
the
Lyman a
red
about
electromagnetic
that
of
20
shift
lines
the
cm" .
value
This
1
of
in
19
cm"
1
- 16 III.
LAMB SHIFT I N HELlUM-LIKE ATOMS
A c c u r a t e work o n t h e s p e c t r a o f atoms
w i t h He I , m o s t l y , b y
t h e Swedish s c h o o l , permit
iso-electronic.
experimental
d e t e r m i n a t i o n o f t h e p o s i t i o n o f the ground energy
o f t h e s e atoms...
Atomic Energy
The
These e x p e r i m e n t a l v a l u e s t a k e n
Tables
(6) a r e l i s t e d
calculated.results
a r e t h o s e o f E r i k s s o n (20) who
p o l a r i z a t i o n corrections..
results
a r e throughout
g i v e n - a noteworthy
point
of view."
It
two e l e m e n t s ^
circumstance from
Although
expected
shift
c a n be r e a d i l y made.
f u n c t i o n s .so t h a t
a r e made
4 EC?-)
The
the t h e o r e t i c a l
c a l c u l a t i o n s , have
We a p p r o x i m a t e
equation
the
by h y d r o g e n i c
wave-
( 1 - 1 3 ) becomes when n u m e r i c a l
explicit
=
Lamb s h i f t s
i n Helium-like
f o r s u c h atoms a n e s t i m a t e , o f t h e
ground s t a t e h e l i u m - l i k e wavefunctions
f ctors
l i m i t s of
f o rthe S levels
quantum e l e c t r o d y m a m i c
not y e t been performed
a
the discrepancy
i s i n t e r e s t i n g t o ask i f such a d i s c r e p a n c y
may be r e s o l v e d b y a Lamb s h i f t
atoms.
- "the
g r e a t e r than t h e exper-
considerably greater than the experimental
error
used
a n d mas-s-
(11) h a s n o t e d
Tyren
i m e n t a l ones, and f o r t h e l a s t
is
from
i n Column 3 T a b l e I I .
a modified Hylleras formula with r e l a t i v i t y
theoretical
states
(O-AJT)
•
c a l c u l a t e d from
Column 5 o f T a b l e
II.
# 0 V I I and F V I I I .
and A l X I I .
'
. . .
(3-1)
this, formula: a r e g i v e n i n
The r a t h e r g o o d . a g r e e m e n t o f d a t a
We h a v e a d d e d new r e s u l t s
o f Mg X I
i n the l a s t
17
-
two columns of Table I I ,
In view of the
large
e x p e r i m e n t a l . e r r o r s and the c r u d i t y of our t h e o r e t i c a l
treatment,
suggests s t r o n g l y that a. Lamb s h i f t
After
is
operative.
c o m p l e t i o n of the above c o n s i d e r a t i o n s
was d i s c o v e r e d t h a t
it
s u b s t a n t i a l l y the sameargument had
been p r e v i o u s l y employed by E r i k s s o n
TABLE
(21)
II
I o n i z a t i o n E n e r g i e s f o r the 1 ' S
Ground S t a t e s of
H e l i u m - l i k e , Atoms.
0
Atom .
Calculated
cms
, Experimental],Calculated
Lamb,Shift
-Experimental
'
cms""
cmd" cms .
3
He I
198319
198305115
+
14
+
2
Li
II
610092
610079+25
+
13
+
19
Be
I I I 1241308
1241225.1100
+
83
+
60
B IV
2092151
20919601200
+
191
+ 150
C V
3162759
31624501300
+ 309
+ 300
N VI
4453336
44528001500
+ 536
+ 570
0 VII
5964057
59630001600
+1057
+ 960
V I I I 7695209
76934001800
+1809
+15^0
F
Ne IX
not
yet observed
Na X
not
yet observed
Mg XI
14213753
1420920012500
+4553
+4850
A l XII 16829563
1682500013000
+4563
+6700
I t may
be
which s u f f i c i e n t l y
obtained,
observed
those
the
is
18
concluded
-
that
t h e few.atoms f o r
a c c u r a t e , e x p e r i m e n t a l d a t a have, b e e n
i . e . L l I I I and
some H e l i u m - l i k e atoms,
S ground state, s h i f t s
agree, q u i t e w e l l
c a l c u l a t e d f r o m .the Bethe.. e q u a t i o n . ( 1 - 1 . 6 ) *
with
j
e x p e r i m e n t a l d a t a on the. h y d r o g e n atom g r o u n d
too u n c e r t a i n to permit
theoretical
the
However,
state
verification.
IV.
DESIGN. OF EXPERIMENTAL PROGRAMME
To d e t e r m i n e t h e p o s i t i o n , of: t h e 1 5_<_
x
energy state
o f t h e h y d r o g e n atom r e l a t i v e t o t h e 2
state,
necessitates
series
lines.
The
the
x
m e a s u r e m e n t . o f t h e w a v e l e n g t h s o f Lyman
elegant, microwave
Retherford, which f i r s t
level,
ground
established
i s not a p p l i c a b l e
t e c h n i q u e s of. Lamb a n d
the s h i f t . i n the 2
t o the ground s t a t e .
Bethe p r e d i c t i o n t o w i t h i n
even t h i r t y
To v e r i f y
per..cent
requires
a n a b s o l u t e w a v e l e n g t h measurement o f a c c u r a c y o f
one p a r t
i n a m i l l i o n ( t o 0.001
A°)>
. A l t h o u g h the. a t t a i n -
ment o f s u c h a c c u r a c y p r e s e n t s no d i f f i c u l t y
i n the
visible
spectrum
and near u l t r a - v i o l e t
regions of the
where p o w e r f u l i n t e r f e r o m e t r i c ^ methods a r e a v a i l a b l e ,
in
t h e vacuum u l t r a - v i o l e t
encountered..
The two
f o r m i d a b l e problems are
most m a j o r p r o b l e m s , -the n e e d
a vacuum s p e c t r o g r a p h o f a d e q u a t e
development
will
(a)
now
of a s u i t a b l e
be c o n s i d e r e d
r e s o l v i n g power a n d
source of s t a n d a r d
i n some
of
the
wavelengths,
detail.
E s t i m a t e of O p t i c a l Performance
of the
Lubzlnski
Spectrograph
The
spectroscopy laboratory
B r i t i s h Columbia
two
metre
at the U n i v e r s i t y
i s fortunate, i n possessing the
spectrograph.
of
Lubzinski
The m e c h a n i c a l c o n s t r u c t i o n
and
rx
•,
"# Note, however, t h a t t h e L a m b . s h i f t i n t h e 2
level,
w h i c h c o r r e s p o n d s t o a w a v e l e n g t h s h i f t o f 0.01 A ° i n a
component o f t h e B a l m e r a l i n e , - two a n d a h a l f t i m e s a s
l a r g e as the s h i f t i n the L
, was n e v e r c l e a r l y
established optically.
5p&
a
*
FIGURE II OPTICS OF THE LUBZINSKI
FIGURE HI
VACUUM WAVELENGTH
RITZ
SPECTROMETER
STANDARDS FROM
COMBINATION PRINCIPLE
To f o l l o w page 19
- 20
performance
described
of t h i s
(5).
We
i n s t r u m e n t ' h a v e been
shall
o p t i c a l performance
in
now
discuss
previously
i t s e x p e c t e d optimum
when u s e d w i t h o u r S l e g b a h n
o r d e r t o answer t h e f o l l o w i n g
(1) Can
-
t h e 0.368 cm"
1
grating
questions:
doublet structure
of L
be
a
resolved?
(2) Can t h e w a v e l e n g t h o f t h e c e n t r e o f g r a v i t y
at
1216
A
be m e a s u r e d
0
the
e x p e c t e d Lamb
Our
discussion w i l l
sufficiently
shift?
show t h a t
the f i r s t
q u e s t i o n must
be
affirmative.
o p t i c a l mounting, i s i n d i c a t e d . i n F i g u r e I I .
from the s l i t
S i n c i d e n t u p o n t h e two
metre
Slegbahn
g r a t i n g G a t a n a n g l e o f a b o u t .20° a f t e r
diffraction is
focussed upon the p l a t e h o l d e r P to P .
The a n g l e o f
1
d i f f r a c t i o n 0 ranges from 85° at P to 40° a t P .
This
1
g r a t i n g mounting
I s . q u i t e u n i q u e . b e i n g ..the. o n l y
mounting u t i l i z i n g
negative orders.
S t e h n a n d E d l e n (30) have, shown, t h a t
sion increases without l i m i t
(8
20°).
severe l i m i t a t i o n s
a p p r o a c h i n g P may
performance
be
of t h i s
theory of p h y s i c a l
existent
Mack,
a l t h o u g h t h e ..disper-
as © i n c r e a s e s to 90°, a t the
same t i m e t h e r e s o l v i n g power a p p r o a c h e s
rather
Q
of the L u b z i n s k i Spectrograph
The
Light
L
accurately to detect
a n s w e r e d . i n t h e n e g a t i v e and. t h e s e c o n d I n t h e
(1)"Optics
of
zero.
on t h e use o f t h e
Hence
region
e x p e c t e d . ..For a r o u g h e s t i m a t e o f t h e
s p e c t r o g r a p h mounting
optics
gives
first
order
--21 -
h \ z. d(*t^ & - <**~s i )
where
in
i s the wavelength d i f f r a c t e d
grating order
grating line
with
d =
So
P
1
spacing.
(1/576) 1 G
e
F o r the L u b z i n s k i
spectrograph
grating
T
A
0
= 17360
A
0
= 20°
that
.
KA
v a r i e s f r o m a b o u t 4800 A ° a t P t o 11500 A
Thus t h e h y d r o g e n Lyman a l i n e at. 1216 A
observed only
(2)
through an angle
n; i i s t h e i n c i d e n t a n g l e a n d d t h e
the Siegbahn
i
... ( 4 - 1 )
i n grating- orders
0
0
at
c a n be
exceeding the f o u r t h .
L i n e a r ^ D i s p e r s i o n o f the: S p e c t r o g r a p h
The
graph, i s g i v e n
h
linear
d i s p e r s i o n of the Lubzinski
by
linear
^
•••
TT'~
where r i s t h e R o w l a n d c i r c l e r a d i u s
along
spectro-
the p l a t e holder.
and. S
Table. I I I . g i v e s
the.distance
the r e s u l t a n t
d i s p e r s i o n f o r various u s e f u l angles
of
diffraction.
TABLE .III
L i n e a r D i s p e r s i o n and Wavelength A g a i n s t
Diffraction
(A°)
(A°/mm)
40°
5220
6.7
50°
7360
5.6
60°
$100
4.3
70°
10400
2.9
80°
11200
1.5
Angle
-
As
the
p o s i t i o n of a
sharp
to
one
micron,
be
(n
=
0.001
5 x 1216
A .
a
A )
-
spectral line
observed i n the
i t s w a v e l e n g t h may
0
I f orders
0
employed the
be
i f L
22
as h i g h . a s the
c a n be
fifth
be
determined
order,
measured
to
seventh could
be
w a v e l e n g t h o f t h i s h y d r o g e n Lyman l i n e
could
d e t e r m i n e d t o 0.0007 A ° .
(3)
Resolving
The
g r a t i n g of
Power
t h e o r e t i c a l r e s o l v i n g power,R, o f a
N l i n e s , employed
i n the
n* *
1
order
1
i s given
R = n N
For the
...
by
(4-3)
Slegbahn g r a t i n g
R = n
The
diffraction
(576
(81 mm)
lines/mm) =
46700
minimum w a v e l e n g t h s e p a r a t i o n t h a t may
be
n
resolved
Is
then
Thus t h i s minimum w a v e l e n g t h s e p a r a t i o n f o r t h e
Lyman a l i n e . w o u l d
and
0.002 A °
be
0.003 A °
f o r s i x t h order..
for fifth
The
i n such h i g h
violet
appreciably
theoretical
spectra
practical resolving
power a c t u a l l y , a c h i e v e d
s p e c t r a w o u l d be
order
hydrogen
order
vacuum
smaller
than
ultrathese
values.
Further,
the
above c o n s i d e r a t i o n s a p p l y
the
r e s o l v i n g power o f t h e
for
the
the
spectrographs
grating.
Lubzinski spectrograph
plate.
The
i s the
The
real
only
limitation
r e s o l v i n g power
most f i n e
to
grained
of
plates
-
sensitive
50
lines
23
to f a rultraviolet
per millimetre.
As
radiation can resolve
only
the dispersion of the
spectro-
graph
i s about. 1 A ° / m m
plate
r e s o l u t i o n corresponds.to
of
0.02
vation
Lyman
(4)
A°.
Lyman
lines,
this
a wavelength r e s o l u t i o n
separation
f o r the
o f 0.006 A
obser-
i n the.hydrogen
0
line.
Optimum
Grating
As
surface
and S l i t
the concave
the ordinary
are. e n c o u n t e r e d .
important
of
f o r the hydrogen
Such r e s o l u t i o n i s inadequate
of the doublet
a
-
Width
grating
optical
These
i s ruled
aberrations
aberrations
or diffraction
spherical
f o r s u c h a. s u r f a c e
Seoomee'e s p e c i a l l y
f o r g r a t i n g mountings with
incidence
on a
rather
and l i m i t
large
the useful
angles
grating
width.
Mack,
grating
width
Stehn,
and E d l e r
f o r maximum
(30) have
r e s o l v i n g power
* A R.
shown t h a t
w
0
p.j.
the
i s given
by
3
.... ( 4 - 5 )
where
R
i s the grating radius.
graph
this
raction
of
40°
optimum, w i d t h v a r i e s
angle
80°.
o f 4 0 ° t o 4.9
Siegbahn
Mack,
cms. f o r a d i f f r a c t i o n
diffraction
grating width
Stehm and E d l e r
i s not, l i k e
extra
spectro-
from..8.3 c m s . f o r a
A3 a l l o u r work u t i l i z e d
the f u l l
length
For the Lubzinski
width,
o f 8.1
further
angle
angles
cms. was
state
positively
diff-
near
employed.
"Extra
harmful".
- 24 The s l i t
to
permit
width
attainment
s h o u l d he s u f f i c i e n t l y
of the f u l l
without excessive loss
(30)
i n light
g r a t i n g r e s o l v i n g power ,
intensity.
I t c a n be
shown/that t h e w i d t h S g i v e n by t h e f o l l o w i n g
satisfies
this
it
slit
width f o r
1200 A
o u r s p e c t r o g r a p h i s about f i v e
i s found
tolerable
usually
... (4-6)
•
Hence t h e " i d e a l "
orderof
that
slit
equation
criterion.
a RA
S =
narrow
0
microns.
imperfections, i n the l i n e
width
i n the f i f t h
Practically
image, make t h e
l a r g e r than, t h i s . t h e o r e t i c a l v a l u e a n d
the best width
is. d e t e r m i n e d
We may c o n c l u d e
from
optimum o p t i c a l p e r f o r m a n c e
this
experimentally.
d i s c u s s i o n of the
of the Lubzinski spectrograph
that
(1) t h e d o u b l e t
structure
o f t h e h y d r o g e n Lyman a l i n e
c a n n o t be r e s o l v e d w i t h t h e e x i s t i n g
(2) a n d t h e m o s t : a c c u r a t e
wavelength
instrument.
measurements
t h e o r e t i c a l l y p o s s i b l e with the spectrograph permit
d e t e r m i n a t i o n o f t h e hydrogen ground s t a t e
only t h i r t y
per cent.
Lamb s h i f t t o
•(b) S t a n d a r d W a v e l e n g t h s
25
-
I n t h e Vacuum
The most d i f f i c u l t
problem
Ultraviolet
involved
a b s o l u t e w a v e l e n g t h measurements i n t h i s
is
the choice
encountered
standards.
i n vacuum.spectroscopy
ment o f l i g h t
studied,
of s u i t a b l e
spectral
Most o t h e r
region
problems
- s u c h as. t h e d e v e l o p -
s o u r c e s f o r t h e e x c i t a t i o n o f s p e c t r a t o be
or the d e s i g n of i n s t r u m e n t s o f
power a n d d i s p e r s i o n - c a n be r e a d i l y
experienced
i n precision
spectroscopist
adequate
r e s o l v e d by
of s u f f i c i e n t
resolving
an
ingenuity.
the p r o b l e m of s t a n d a r d w a v e l e n g t h s below
2000 A °
Yet
remains
largely unsolved.
F o u r methods o f m e e t i n g t h i s p r o b l e m o f w a v e l e n g t h
s t a n d a r d s may
be
considered:
(1) By e m p l o y i n g c a l c u l a t e d w a v e l e n g t h s o f h y d r o g e n i c
o r h e l i u m - l i k e atoms f o r w h i c h t h e t h e o r y
i s rather
well
established.
(2) By t h e u s e
(3) By
reference
interferometer.
s u p e r i m p o s i n g t h e unknown sp;:ectra u p o n known
lines
(4) F i n a l l y ,
the
of a .reflection..echelon
i n lower g r a t i n g
by t h e u s e
orders.
of wavelengths d e t e r m i n e d f r o m
" R i t z combination p r i n c i p l e " .
The f i r s t
method w h i c h i s o f t e n t h e o n l y
available
method o f e s t a b l i s h i n g
s t a n d a r d wavelengths has been u s e d
extensively.
The a c c u r a c y o f t h e
(17, 11)
determined wavelengths, i s l i m i t e d
experimentally
o n l y by t h e r a n g e o f
validity
26
of the theory employed.
method i s inadequate
Clearly,
however,
this
f o r our Lyman s e r i e s problem.
F o r an i d e a l g r a t i n g i n p e r f e c t
focus
comparison
#
between s p e c t r a i n d i f f e r e n t
exceedingly
accurate.
grating orders
should be
However, as.no g r a t i n g i s i d e a l and
as s m a l l i m p e r f e c t i o n s , i n focus are c e r t a i n l y , present
this
method of o v e r l a p p i n g orders i s subject t o unknown e r r o r s ,
(cf.
Boyce
of Wu (41)
(16))
F o r t h i s r e a s o n the proposed experiment
and the measurements,previously
Boyce and Rieke
sufficiently
(17)
are s u s p e c t .
presented,
T h i s method i s
standards
a
to
the
discarded.
The only work r e p o r t e d on the use of a
echelon f o r
not
r e l i a b l e to permit d e t e r m i n a t i o n of L
accuracy r e q u i r e d and was t h e r e f o r e
by
reflection
i n t e r f e r o m e t r i c d e t e r m i n a t i o n of wavelength
i n the vacuum u l t r a v i o l e t
i s t h a t of MacAdam (29),
who p u b l i s h e d a p r e l i m i n a r y account of some
but no r e s u l t s .
experiments
A s r t h e r e f l e c t i o n echelon i s the
only
instrument of v e r y . h i g h r e s o l u t i o n which can be employed
i n the vacuum u l t r a v i o l e t
there would be
considerable
interest
i n i t s f u r t h e r development f o r use i n t h i s
region..
However, such work would be t e c h n i c a l l y
difficult.
Tolansky
(10)
has d i s c u s s e d some of
problems which are encountered.
spectral
very
the
The maximum e r r o r s p e r m i t t e d
f o r the o p t i c a l surface are o n e - e i g h t i e t h
of a
wavelength.
# I f the g r a t i n g be I l l u m i n a t e d e x a c t l y the same way f o r
both r e f e r e n c e and unknown spectra-;
This is usually
achieved by simultaneous exposure of both s p e c t r a .
- 27 If
t h e i n s t r u m e n t be b u i l t up u s i n g s a y g r e e n
light, these
wavelength
same e r r o r s amount t o o n e - f i f t e e n t h o f a
for
M000 A°.
worse a s t h e w a v e l e n g t h
shows t h a t
produced
mercury
Thus t h e f r i n g e s become
decreases.
t o achieve an analysis
Tolansky
steadily
further
of the fringe;
system
r e q u i r e s t a k i n g a p l a t e w i t h a f o r e i g n gas i n t h e
s p e c t r o g r a p h whose p r e s s u r e be m a i n t a i n e d
b e t t e r t h a n one t e n t h o f a m i l l i m e t r e
p r o b l e m s a r e much more d i f f i c u l t
encountered
i n more o r t h o d o x
constant.to
of mercury.
Such
of s o l u t i o n than
those
spectroscopy.
r e f l e c t i o n e c h e l o n s ;?of s u f f i c i e n t l y
A l s o . as.
high quality for
vacuum s p e c t r o s c o p y a r e n o t y e t a v a i l a b l e
t h i s method o f
e s t a b l i s h i n g s t a n d a r d w a v e l e n g t h s was d i s c a r d e d .
The
here.
f i n a l ..possibility
.If t h e e n e r g y
a r e so p l a c e d t h a t
i n the v i s i b l e
i s t h e one w h i c h was a d o p t e d
l e v e l s A, C, a n d B ( s e e F i g u r e I I I )
t r a n s i t i o n s A—*C a n d C—>B a r e o b s e r v e d
where t h e i r a s s o c i a t e d w a v e l e n g t h
c a n be
v e r y a c c u r a t e l y measured t h e n t h e c o n s i d e r a b l y lower
wavelength
a s s o c i a t e d w i t h A—>B c a n be d e d u c e d f r o m t h e
" R l t z combination
principle".
The e n e r g y
difference
b e t w e e n A a n d B i s s i m p l y t h e sum o f t h e o t h e r two d i f f e r ences.
F o r example, i f
A— >C,
a n d C—>-B e a c h
l i e a t about
2500 A ° (40000 c m ) a n d c a n be m e a s u r e d t o 0.001 A
- 1
( O i 0 1 6 c m " ) t h e n . t h e A-*B w i l l
1
and
o c c u r a t 1250 A
c a n be c a l c u l a t e d w i t h a n a c c u r a c y
(O.O32 c m " ) .
1
o f about
G
0
(80,000 c m " )
1
0.0005 A °
- 28
-
A t p r e s e n t , t h e s p e c t r a o f v e r y few atoms have
been
of
studied
sufficiently
such standards.
Those
standards i n the r e g i o n
be c o n s i d e r e d .
The
few atoms, f r o m w h i c h
M200 A
energy l e v e l
were f o u n d v e r y h e l p f u l
He
e x t e n s i v e l y f o p e r m i t wide use
may
0
wavelength
be d e d u c e d ,
will
d i a g r a m s ....of G r o t r i a n
now
(2)
i n the s e a r c h f o r such atoms.
II
The w a v e l e n g t h a s s o c i a t e d w i t h t h e
J - f o r once
4*"Dx—-v 2
P
n e a r 1215
A ,
values.
Balmer
0
This
ionized
transition
Helium, w h i c h . s h o u l d o c c u r
can. be a c c u r a t e l y c a l c u l a t e d f r o m known t e r m
line
series.
i s the analogue
As h e l i u m c c a n be
tube i n gaseous
form,
o f Hp
i n the
introduced
hydrogen
into
and a s o n l y a v e r y s h o r t
a discharge
extrapolation
Would be n e c e s s a r y f o r t h e c a l c u l a t i o n o f t h e w a v e l e n g t h o f
L ,
a
this
1215
However, t h i s
A
s t a n d a r d would
0
line
i s very d i f f i c u l t
p r o b a b l y n e v e r been o b s e r v e d .
extensive
be most c o n v e n i e n t .
search f o r this,
to excite
Herzberg
^1215
A
0
(25)
line
c a n a l s o be
series
h a s made a n
third
spectrum of
c a l c u l a t e d f r o m known t e r m v a l u e s .
member, w h i c h l i e s
at.
observed w i t h r a t h e r
at
i n the. f i r s t
X1402 A ,
0
has been..only
l o n g exposure times;
A1268 A ' W o u l d be v e r y much l e s s
0
has
without success.
The w a v e l e n g t h s . o f t h e s e c o n d . a n d
of the p r i n c i p a l
and
The
memberss
mercury
The
second
weakly
third
intense than the
member
-
second and has not yet been
29
-
observed^
CO
The r o t a t i o n a l s t r u c t u r e of the e l e c t r o n i c
of t h i s diatomic molecule might be used
standards.
The t r a n s i t i o n n B ' E
+
as.wavelength
— ,
which has been
observed i n e m i s s i o n and a b s o r p t i o n can be
from known term v a l u e s
expensive
(3).
band.-systemsaof
calculated
However, as other v e r y
t h i s molecule would o v e r - l a p
these known l i n e s c a u s i n g severe e x p e r i m e n t a l
and as at present the accuracy
low we have not yet
b|nds
difficulty
of these standards i s
s t u d i e d t h i s method
rather
experimentally.
Cu I I
Shenstone
(36) as a r e s u l t of h i s
exhaustive
study of the a p e c t r a of once i o n i z e d Copper, has been
able to. c a l c u l a t e
and f i f t y
the wavelengths of more than one hundred
l i n e s between 2000 A° and 685 A ° .
T h e i r accuracy
i s b e l i e v e d to v a r y from about 0.003 A° at 1700 A
0
l e s s , than 0.001 A° at 800 A ° .
the
wavelengths of those
their intensities
Table IV p r e s e n t s
to
l i n e s i n the r e g i o n o f hydrogen L ,
on an a r b i t r a r y s c a l e ,
f l
and t h e i r p r o b a b l e
errors.
A l t h o u g h these copper l i n e s are somewhat
t o e x c i t e they have c e r t a i n l y been observed
difficult
(25, 3 6 ) .
# Herzberg (23) has not been a b l e to e x c i t e t h i s " l i n e i n
emission.
He i s now a t t e m p t i n g o b s e r v a t i o n i n a b s o r p t i o n .
- R O TABLE IV
Vacuum Wavelength Standards
Wavelengths
(A°)
i n the F i r s t Spark Specta
Copper..
.
Intensities
_ _ _ _ _
Probable E r r o r
(10-3 AQ)
.1299.26?
10
2
1298.394
15
2
1297.549
2
2
1281.458
8
4
1275.570
30
2
1274.463
3
2
1266*308
10
1
1265.504
15
1
I25O.045
10
2
5
2
1241.961
2
1
1219.332
1
2
1214.553
1
2
1185.899
2
2
1109.742
1
2
1106.446
3
2
1088.393
20
2
1248.790
•
Careful-vacuum i n t e r f e r o n s try. .of the v i s i b l e
would reduce the above p r o b a b l e . e r r o r s .
t h a t these Cu II
wavelength
line..
It
parent
seems
the best hope of
lines
clear
satisfactory-/
s t a n d a r d s . i n the r e g i o n of the hydrogen Lyman a
Therefore,
i m e n t a l study
these
l i n e s offer
of
lines.
our programme Includes a c a r e f u l
of l i g h t
exper-
sources f o r the p r o d u c t i o n of
- 31
-
(c) Widths of S p e c t r a l L i n e s
No
factors
radiation
i s p e r f e c t l y monochromatic*
most, commonly p r o d u c i n g f i n i t e
lines.are
- intrinsic
S t a r k and
Zeeman b r o a d e n i n g ,
finally
the
Doppler width.
discussed here.
If
the
s e l f - r e v e r s a l width,
For the c o n d i t i o n s
( c f . Tolansky
intrinsic
atom a t r e s t w o u l d e m i t
radiation
by
and w i l l
be
and
experiments
the only
(10))
w i d t h be
neglected, an
light.
An
the observor would
i s displaced to higher frequencies
ti v where
This
Now
of our
s t r i c t l y monochromatic
atom m o v i n g w i t h a v e l o c i t y v t o w a r d s
emit r a d i a t i o n which
spectral
r a d i a t i o n width,, p r e s s u r e b r o a d e n i n g ,
D o p p l e r w i d t h i s much t h e l a r g e s t
effect
breadth of
The
i s s i m p l y a consequence
of the f a m i l i a r . Doppler
t h e atoms o f a gas move w i t h a M a x w e l l i a n
distribution.
which
Effect.
velocity
Hence t h e r a d i a t i o n r e s u l t i n g f r o m
atoms,
i f a t r e s t would emit m o n o c h r o m a t i c . r a d i a t i o n of
f r e q u e n c y v , w i l l .byyvirtue, of t h i s M a x w e l l i a n
d i s t r i b u t i o n have f i n i t e
resultant
equation
line
(4-7)
width.
The
velocity
h a l f w i d t h of
i s simply d e r i v e d from k i n e t i c
the
theory
and
as
(4-8)
where T i s t h e t e m p e r a t u r e
a n d R i s t h e gas
constant.
o f t h e gas o f a t o m i c w e i g h t
m
I f V be m e a s u r e d i n wave numbers
- 32.we
nave
Jiv
The
(p-fi)
=
Doppler h a l f
spectral
lines
if"
T
[
....(4-9)
w i d t h s d e r i v e d from, e q u a t i o n (4-9) f o r
of hydrogen
r e g i o n a t temperatures
temperature.(30°
t-HJ.
and copper i n the
of l i q u i d
nitrogen
1200
of copper
(1083° C) a r e p r e s e n t e d i n T a b l e V.
TABLE V
D o p p l e r H a l f Widths, f o r Hydrogen, a n d
Half. Width
(°C)
Copper
i n cm""
1
Cu
H
-210
0.06
0.47
30
0.13
1.0
0.28
2.2
1083
Even a t the temperature
of l i q u i d
nitrogen
the.. D o p p l e r h a l f w i d t h o f t h e Lyman a h y d r o g e n
(0.47 cm" )
1
structure
would
line
p r o b a b l y mask t h e e x p e c t e d d o u b l e t
(0.36 cm"" ) •
1
0
(r210° C ) , room
C ) , and the m e l t i n g p o i n t
Temperature.,,
A
V.
All
quartz,
33 -
EXPERIMENTAL
t h e common o p t i c a l m a t e r i a l s ,
etc.),
a n d most, gases,*; ( n o t a b l y a i r ) , a r e opaque
t o r a d i a t i o n i n t h e s p e c t r a l r e g i o n below
Further,
because, of. t h e s t r o n g a b s o r p t i o n
a l l conventional
to
A
The s p e c t r o s c o p y
of t h e i r
emulsion,
insensitive
of t h i s r e g i o n
2000 A , c a l l e d vacuum u l t r a v i o l e t , s p e c t r o s c o p y ,
0
below
requires
e l i m i n a t i o n o f a l l s u c h opaque m a t e r i a l f r o m t h e
spectrograph.light path.
the
ordinary
ible
techniques
However, i n a l l o t h e r
of spectroscopy
Boyce
(16)., t r e a t i s e s b y Bomke
helpful
information
t h e vacuum
(a) The
on t h e t e c h n i q u e s
(4), and
(6) a l l c o n t a i n
very
of spectroscopy i n
ultravioleti
Lubzinski.Spectrograph
A d e t a i l e d account
of t h i s
The r e v i e w o u t l i n e d
(1) a n d Lyman
i n Sawyer's t e x t
respects
i n the more-access-
s p e c t r a l regions, are applicable,.
the r e l e v a n t chapter
spectrograph
hg,ve a l r e a d y
of the mechanical c o n s t r u c t i o n
has been g i v e n by L u b z i n s k i
examined, t h e m a i n f e a t u r e s
system o f t h i s , instrument
p e r f o r m a n c e of. t h e r e m a i n i n g
described.
(5)•
We
of the o p t i c a l
a n d have, e s p e c i a l l y . . . n o t e d t h e
u n i q u e mounting, w h i c h u t i l i z e s
be
A 2000 A ° .
p h o t o g r a p h i c , . m a t e r i a l s aree a l s o
such radiation..
the
by
(glass,
negative
critical
orders.
The
components w i l l
now
Vacuum
A large
o i l . d i f f u s i o n pump b a c k e d
Kenney pump s e r v e s t o e v a c u a t e
f o r t y minutes..
The. p r e s s u r e
by a
mechanical
t h e s p e c t r o g r a p h i n about
i s recorded on a B i r a n i .
guage.
The u l t i m a t e p r e s s u r e , w h i c h i s . l i m i t e d b y the.
natural
leak of the spectrograph, d e s o r p t i o n o f gases, and
t h e w a t e r c o o l i n g employed, w i t h .the. d i f f u s i o n pump, i s
about.one micron.of
factor
mercury.
The. I m p o r t a n c e o f t h e s e c o n d
i s illustrated, i n Figure IV.
FIGURE.IV
^
S p e c t r o g r a p h . P r e s s u r e . A g a i n s t Time. From C e s s a t i o n o f Pumping
o
U
o
>
w
ft;
lo
Iv
It
c a n be s e e n t h a t
the rate
Jo
%
9
of p r e s s u r e r i s e upon c e s s a t i o n
o f pumping d e c r e a s e s w i t h t o t a l e l a p s e d pumping t i m e .
phenomenon was a t t r i b u t e d
t o water vapour a d s o r p t i o n I n
t h e porous, i r o n a n d i r o n o x i d e
spectrograph
housing.
" R i n s i n g " the spectrograph w i t h hydrogen o r helium
reduced
This
greatly
t h e time r e q u i r e d f o r d e g a s s i n g .
I n a l l exposures
a continuous
flow.of
circulating
- 35 gas from the. d i s c h a r g e through the s p e c t r o g r a p h s l i t
maintained..
The r e s u l t a n t
c o n d i t i o n s was about f i v e
was
spectrograph p r e s s u r e under such
microns, r e s u l t i n g i n
negligible
a b s o r p t i o n f o r the s p e c t r a l r e g i o n of the hydrogen Lyman
series
(cf.
Hopfield
(24)).
The G r a t i n g
The g r a t i n g employed (#257655) was r u l e d at
Physical Institute
by Slegbahn.
i n Uppsala on a r u l i n g engine
the
designed
576 l i n e s p e r m i l l i m e t r e were r u l e d over an
81 m i l l i m e t r e w i d t h of the two metre g r a t i n g a l u m i n l z e d
blank.
The l e n g t h of the r u l i n g s was 48 m i l l i m e t r e s .
g r a t i n g was a p p r a i s e d by Mr. David. R i c h a r d s o n (33)
Bausch.and Lomb and we r e p o r t his. f i n d i n g s .
"(1)
This
of
He s t a t e s :
Weak Lyman ghosts, from t o o l bounce.
(2) Rowland ghosts ••approximately 0.2$ i n f i r s t
(3) E r r o r of r u n not
order.
serious.
(4) Target p a t t e r n .quite
strong.
(5) S a t t e l i t e s seen near parent l i n e i n f i r s t and
second o r d e r s .
(6) R e s o l v i n g power good (estimate 15%
of
theoretical
value i n t h i r d o r d e r ) .
(7) Energy d i s t r i b u t i o n v e r y good - there b e i n g f i v e
as much l i g h t
the
i n the f i r s t order on one s i d e as on
other.
(8) Diamond set down very h a r d and bounced.
evidence
Little,
of diamond wear. ,
(9) We b e l i e v e
times:;
the g r a t i n g to be q u i t e
acceptable."
-
Most o f t h e
be
diffracted
(c.f.
i n the
(4) .and
items
normal f o r the
diffracted
observed.
from
this
g r a t i n g tends
general, . d i r e c t i o n of the
(7)
above) - 20°
from
to
central
the
i n t h i s , s p e c t r o g r a p h , (c . f .
image.
grating
can
i n a g r e a t waste of l i g h t
r a t h e r l o n g exposure
the
F i g u r e .II)
t h r o u g h a n g l e s g r e a t e r t h a n 40°
This results
consequently
-
L u b z i n s k i s p e c t r o g r a p h . , However, w i t h
mounting employed
light
light
36
only
be
and
times.
MechanlcalThe
g r a t i n g mounting, p l a t e h o l d e r , and
mechanism w h i c h L u b z i n s k i h a s
all
design proved
the f o c u s s i n g of the g r a t i n g .
e r e d i n any
of the exposures
(5)
described i n d e t a i l
f u n c t i o n quite adequately.
f e a t u r e s of t h e i r
slit
Many o f t h e
excellent
a great convenience
No
in.
d i f f u i c u l t y . was
encount-
w i t h v^bratlionsiiofi the
optical
components.
A
slit
width
o f 30 m i c r o n s ,
give best d e f i n i t i o n with reasonable
w h i c h was
exposure
e m p l o y e d f o r most o f t h e s p e c t r o g r a m s .
theoretical
slit
w i d t h was
(The
found
times,
to
was
optimum
p r e v i o u s l y shown t o be
5
microns).
R e g i s t r a t i o n of the
All
Ilford
Q-2,
Spectra
t h e vacuum s p e c t r a were r e c o r d e d on
a mexllum g r a i n ,
T h e . p l a t e s were d e v e l o p e d
• about
68°
F. and
rather high sensitivity
i n Kodak D-19
then f i x e d
commercial
plate.
f o r f o u r minutes a t
i n Kodak.F-5 s o l u t i o n
and
-
washed.
two
37
-
For these p l a t e s of very t h i n , emulsion
processes
treated,
take
these
c o n t r a s t , and
less
than a minute each.
p l a t e s were f o u n d
low
to give s p e c t r a of
T h e y were p r o c e s s e d
i n the
same manner as- t h e
p a r t i c u l a r l y w i t h the
grating,
the
and
as., i t s d i a m e t e r
blank.
F u r t h e r the
plates.
103ar-0 p l a t e s , p l a t e s
metre r a d i u s
Lubzinski
The
tangent
to the
grating, centre
the r a d i u s of c u r v a t u r e
slit
which
of the g r a t i n g
should..be a c c u r a t e l y p a r a l l e l
The
spectrograph
system
i s then
will
now
be
r a d i u s of c u r v a t u r e R of
±
observations
limited
diffraction
said, to
to
be
The. method o f a c h i e v i n g t h i s f o c u s f o r t h e
determined
199*55 cms
concave
the diffracted...spectrum should
grating irulings.
focussed.
was
slit
l i e on a c i r c l e
1.
Q,-2
Spectrograph
I n a l l mountings of the
the
employed.
of the p l a t e h o l d e r .
(b) F o c u s s i n g t h e
has
region
p l a t e s were
w o u l d b r e a k a f t e r b e i n g f o r c e d t o t h e one
both
good
fog.
e i t h e r Kodak 103a-0 o r Kodak. I X F - 3
curve
last
When so
For r e c o r d i n g s p e c t r a i n the v i s i b l e
Occasionally,
the
by a F o u c a u l t > k n i f e
0.1
cms.,
s h a r e d by
o n l y by
as
two
described.
the.Slegbahn.grating
edge method
the average of e l e v e n
observors.
The
(9) t o
independent
accuracy
the u n c e r t a i n t y i n measurements.of
w i t h a.good s t e e l
rule.
be
was
distance
2.
A Zeiss precise level
38 (#5672) k i n d l y l e n t u s
by:the
of the U n i v e r s i t y of
British
C i v i l . Engineering.Department
C o l u m b i a was,employed
to set the centre
centre
o f t h e g r a t i n g , and t h e m i d p o i n t
holder
from P to P
was, d i s c o v e r e d
placed
1
that
(see F i g u r e
the.plate
I I ) i n the. same plane..
about. 0.34
h o r i z o n t a l .plane t h r o u g h t h e s l i t .
was
of
the
It
the mechanical c o n s t r u c t i o n n e c e s s a r i l y
the grating. centre
Rowland, c i r c l e
of t h e . s l i t ,
therefore
s l o p i n g upward from t h e s l i t
cms.... above t h e
The p l a n e
of the
not h o r i z o n t a l , but r a t h e r
to the plate
.
holder.
3» The g r a t i n g o r i e n t a t i o n was, f i x e d b y
visually
f o c u s s i n g ^ a . d i f f r a c t e d i r o n , a r c .spectrum u p o n t h e p l a t e
..holder.
T h i s ..defines .the .Rowland c i r c l e . - a
perpendicular
to the midpoint
circle
o f t h e g r a t i n g and o f r a d i u s
199.55/2 cms.
4.
To p o s i t i o n . t h e
-slit-grating
distance
slit..upon..this Rowland.circle
was. a d j u s t e d
g r a t i n g - c e n t r a l , image.distance..
was,.determined w i t h
until
a good s t e e l t a p e and. a d e p t h
the s t e e l tape to the focus
image, a s d e t e r m i n e d u s i n g a F o u c a u l t
method p l a c e d
one m i l l i m e t r e
5.
focus
to the
The s l i t - g r a t i n g
The g r a t i n g - c e n t r a l image, d i s t a n c e , was
ment w i t h
equal
the. s l i t
the
distance
guage.
deduced by measureof the c e n t r a l
k n i f e , edge.
u p o n t h e Rowland c i r c l e
This
within
accuracy.
P l a c i n g the p l a t e h o l d e r
ensures that
i t also w i l l
i n the p o s i t i o n of
be o n t h e R o w l a n d
best
circle.
39
T h i s a d j u s t m e n t was
first
-
made, v i s u a l l y u s i n g
s c o p e whose f o c a l p l a n e , c o u l d be
of
the. p l a t e h o l d e r .
graphically using
one
an
The
1(a)
plate holder
t o the
The
focus
p o s i t i o n of b e s t
centre
spectrum.
from the
This
f o c u s s i n g of
changing the
plate.
badly
out
minimize the
A
large gratings.
reproduction.
Wandering, o f t h e
o p t i c s may. w e l l
hence . the
arc,
change
to
on
appear
c a r e f u l , alignment, of
d u p l i c a t i o n of t r i a l
in
line profile
spectral lines
Frequent
easily
the
exposures served
to
difficulty.
conventional
e l e c t r o d e s was
a i r a t a few
to give f i n e diatomic
convenient
i n focussing.
millimeters pressure,
ampere p r o d u c e d v e r y
However, w i t h
type G e i s s l e r tube with
constructed
spectra which are very
currents
b r i g h t s p e c t r a o f NO,
reasonable
required precluded
gratings
in
the
not.commonly e m p l o y e d
a l i g n m e n t .of t h e
a r c p o s i t i o n and
exposure
c o r r e s p o n d e d to t h a t of
s p e c t r a are
of focus.
plates.
inches
c o n c l u s i o n f o l l o w s more
T h i s causes, t h e
about
Rowland r a d i u s b e t w e e n e x p o s u r e s .
the. g r a t i n g i l l u m i n a t i o n and
in
Exposures of
of a t y p i c a l t r i a l
o r i g i n a l p l a t e t h a n from the
Iron arc
that
made, p h o t o -
moved a b o u t 0.01
was
a direction parallel
the
spectrum.
shows a r e p r o d u c t i o n
(H-73)•• The
by
made c o i n c i d e n t w i t h
f i n a l , a d j u s t m e n t , was
i r o n arc
micro-
minute g i v e w e l l developed s p e c t r a upon I I F - 3
Plate
the
a
slit
the use
widths the
N,
2
aluminum
band
When
of
and
run
one
Ng .
long exposure
of this, source i n f o c u s s i n g
+
times
the
- 40
6.
Finally
"parallel
to
t h e . s l i t , was
the
a c c o m p l i s h e d by
sharp l i n e s ,
detected
by
viewing
of the
while
the
rotated; until.. very
grating milinga.
spectrum placed.behind
Parallelism
-
This
accurately
operation
t h r o u g h a microscope an
the
slit
small
slit
w h i c h was
and.grating
was
iron
slowly
arc
rotated.
r u l i n g s , ensures
fine
e r r o r s o f f p a r a l l e l i s m are
astigmatic
a d j u s t m e n t was. f o u n d to. be
easily
b r o a d e n i n g . ... This...visual,
more s e n s i t i v e t h a n p h o t o g r a p h i c
methods.
T h i s , .completes, t h e
region.
As
this--ensures
vacuum u l t r a v i o l e t
spectra
f o c u s s i n g f o r the
rather accurate
r e g i o n and
a l s o as
focussing for
long,
performed.
time
consuming.
example, a f t e r , f i x i n g , the. g r a t i n g o r i e n t a t i o n a n d
slit-grating
discover
the
further
l i n e s was.not
T h i s f o c u s s i n g problem, c a n . b e v e r y
For
d i s t a n c e 'by.-«steps. (3) and. (4) one
insufficient.mechanical.freedom
plate holder
by
step
(5).
steps
process
(4) a n d . ( 5 ) .
several
to e f f e c t
focussing
slight
t h e n a r e p e t i t i o n of
I t i s e a s i l y p o s s i b l e to repeat
times.
the
might
This necessitates a
change, i n g r a t i n g o r i e n t a t i o n and
the
exposure times f o r
i n the. vacuum. r e g i o n were r a t h e r
f o c u s s i n g u s i n g vacuum u l t r a v i o l e t
visible
this
- 41 ( c ) Lyman S e r i e s S p e c t r a
Two methods o f o b t a i n i n g s p e c t r a i n t h e r e g i o n
of Hydrogen L
tube.could
a
were c o n s i d e r e d .
be"coupled
directly
o p t i c a l material, i n the l i g h t
fluoride,
isolate
very
Either
t o the spectrograph
path or f l u o r i t e ,
windows a n d c o n d e n s i n g
the spectrograph
the discharge
t o absorb
\ 1200 A
o p t i c a l . r e g i o n near
the
only other transparent m a t e r i a l i n t h i s
and l i t h i u m
0
c o l o u r s v e r y b a d l y u n d e r the. e l e c t r o n i c
gaseous d i s c h a r g e ,
using direct
discharge
valve.
oil
region,
dis-
bombardment i n
(16)) t h e f i r s t
method
hydrogen
through
( o r h e l i u m ) g a s was i n t r o d u c e d
a smalllMathew's needle
type
into
leak
C o n t i n u o u s e v a c u a t i o n by a Hyvac pump s e r v e d t o
maintain
slit
fluoride,
c o u p l i n g was e m p l o y e d .
The
the
( c f . Boyce
As even t h e
rather strongly i n
the
a
or lithium
l e n s c o u l d be u s e d t o
f r o m the. d i s c h a r g e .
best fluorite.commences
w i t h no
low p r e s s u r e s .
The g a s w h i c h e s c a p e d
through the
i n t o .the s p e c t r o g r a p h was e l i m i n a t e d b y t h e l a r g e
d i f f u s i o n pump.
c o u l d be r e a l i z e d
I n t h i s way a g r e a t
i n the discharge
g r a p h p r e s s u r e was a l w a y s v e r y
range o f p r e s s u r e s
tube, w h i l e
low (about
the. s p e c t r o -
f i v e microns o f
mercury).
The
cms.,
s p e c t r a were e x c i t e d i n a p y r e x
i n l e n g t h and about three
high frequency
energy.
cms.in diameter
tube
forty
by e x t e r n a l
The c i r c u i t . d i a g r a m o f t h e - p o w e r
s u p p l y a n d t h e o s c i l l a t o r w h i c h were e m p l o y e d a r e g i v e n i n
200O
V A.
2000
VOLT
POWER
SUPPLY
ET
RADIO FREQUENCY OSCILLATOR
FIGURE IL
To f o l l o w page 41
(
F i g u r e V.
up
42 -
V o l t a g e s u p t o two t h o u s a n d
t o one ampere were u s e d .
o s c i l l a t o r was f o u n d
v o l t s with, c u r r e n t s
The o u t p u t
frequency
of the
t o he f i f t e e n m e g a c y c l e s , , w i t h a
s t r o n g harmonic a t t h i r t y megacycles.
I n a l l exposures
t h e e l e c t r o d e s were p l a c e d a t t h e e x t r e m i t i e s o f t h e d i s charge
tube.
The d i s c h a r g e was a l w a y s c o n c e n t r a t e d
between
these e l e c t r o d e s .
All
of
the plate
Thus, near
t h e p l a t e s were e x p o s e d I n t h e f i r s t
h o l d e r where
v a r i e s from
L , spectra, i n the regions
A 2000 A ° , a n d
1.
h\
section
4800 A ° t o 6 2 0 0 . A ° .
X 1200 A ° ,
A 1500 A ° ,
A 3200 A ° were a l l o v e r l a p p e d .
E x c i t a t i o n . . . o f Tank H y d r o g e n
By
energy,
spectrum
exciting
a bright
pink
tank
hydrogen w i t h the h i g h
d i s c h a r g e was o b t a i n e d .
c o n s i s t e d o f s t r o n g Balmer l i n e s
frequency
The v i s i b l e
and v e r y
much
weaker h y d r o g e n b a n d s p e c t r a .
Although
v a r i e d very
relative
the c h a r a c t e r i s t i c s
slowly with pressure
some t e n d e n c y
development o f the atomic
s p e c t r a was o b s e r v e d
a t lower
were t h e r e f o r e employed
(about
of the.discharge
f o rbetter
to the molecular
pressures.
Low p r e s s u r e s
100 m i c r o n s ) .
I n c r e a s i n g the v o l t a g e on the o s c i l l a t o r
the b r i g h t n e s s of the source.
characteristics
No o t h e r e f f e c t u p o n t h e
o f t h e d i s c h a r g e was o b s e r v e d
r a n g e 500 t o 2000 v o l t s .
increases
i n the voltage
Plate 1(a)
Iron Arc Spectra used i n Focussing..
(H-73)
E a c h e x p o s e d f o r40 s e c .
P l a t e h o l d e r moved
a b o u t 0.01 i n c h e s b e t w e e n e a c h e x p o s u r e .
*<
I.
III I I
Plate
1(b)
H y d r o g e n E x c i t e d i n H i g h F r e q u e n c y D i s c h a r g e . (H-78)
Most o f t h e l i n e s a r i s e from t h e band spectrum
of t h e hydrogen m o l e c u l e .
Plate
1(c)
Trace o f Hydrogen
i n High Frequency
Plate
1(d)
T r a c e o f Hydrogen a n d Heavy Water E x c i t e d i n
Excess of Helium i n High Frequency Discharge.
The o b s e r v e d i s o t o p e s h i f t p e r m i t s p o s i t i v e
i d e n t i f i c a t i o n o f t h e h y d r o g e n Lyman a l i n e .
To
Excited I n Excess of Helium
D i s c h a r g e . (H-88)
follow
page
42
- 43 Plate
w h i c h was
o f 1600
1(b)
i s a r e p r o d u c t i o n of p l a t e
exposed f o r f o u r hours a t an
volts.
Most o f t h e
r o t a t i o n a l molecular
broad
line
hydrogen L
specjjra. n e a r
a
i n the f i f t h
line positive
The
and
the
order.
very
work o n t h e
voltage
the p l a t e a r e
A.1506 A ° .
The
hydrogen
feeble
attributed
Because o f t h e . v e r y
lines relative
to
to
high
this.supposed
identification.was. impossible.
l o n g e x p o s u r e s w h i c h were r e q u i r e d ,
strong overlapping molecular
efficient
oscillator
marked on t h e r e p r o d u c t i o n was
i n t e n s i t y of the m o l e c u l a r
atomic
l i n e s on
H-78
Lyman, s e r i e s
l i n e s made a c c u r a t e
impossible..
method o f e x c i t i n g t h e
C l e a r l y a. more
Lyman s e r i e s
lines
was
necessary.
A
the
condensed discharge, of the tank hydrogen i n
G e i s s l e r type
d i s c h a r g e t u b e was
v i s u a l ..spectroscope.
pressures
was
complete range of
available.outside extinction,
s e r s f r o m 0 t o 1/2
the atomic
Over the
examined w i t h
F j
l i n e s over
no
improvement
and
with
a
hydrogen
conden-
i n brightness
t h a t of the h i g h frequency
in
discharge
observed.
•
2. E x c i t a t i o n o f Tank H e l i u m When h e l i u m
was
e x c i t e d i n the h i g h
d i s c h a r g e a v e r y m a r k e d improvement was
frequency
observed.
The
# We have b e e n u n a b l e t o f i n d a p u b l i s h e d a n a l y s i s o f t h e s e
. l i n e s . . A c a r e f u l study of t h i s spectrum might w e l l prove
rewarding.
- 44 h y d r o g e n band, s p e c t r a were v i r t u a l l y
line
a t t r i b u t e d to L
times of l e s s
into
a
c o u l d be e a s i l y
than an hour.
fEurther,
impurity
very bright
No h y d r o g e n w a s . i n t r o d u c e d
i n t h e h e l i u m were s u f f i c i e n t
L
a
i s marked.
with Plate.1(b) directly
attributed
H-88
The
line,
should.be
a b o v e w h i c h was
excited
with
spectral
line
could
be
i d e n t i f i c a t i o n o f t h e e m i t t i n g atom became p o s s i b l e
drops o f heavy water
a p l a t e was
to the helium
obtained; of b o t h the hydrogen
and
spectra.... The...observed I s o t o p e s h i f t p e r m i t t e d
identification.of
reproduces p l a t e
t o note t h a t
the L
a
line.
Plate
1(d)
w h i c h was. exposed, f o r f o r t y
doublet, isotope
structure
the, d e u t e r i u m L
Q
i s marked.
i s much s h a r p e r t h a n t h e
discussion
o f t h e D o p p l e r w i d t h , t h e h y d r o g e n . l i n e B h o u l d be
time's a s b r o a d a s t h e d e u t e r i u m
-The
low
light
minutes.
It is. interesting
c o r r e s p o n d i n g hydrogen, l i n e . ' A c c o r d i n g t o o u r
permit
compared
t o LQ w i t h r e a s o n a b l e , e x p o s u r e t i m e a
supply, l i n e
positive
minutes.
This plate
that a d i s t i n c t
By a d d i t i o n o f a few
deuterium
produce
i n . a f o u r hour, e x p o s u r e . .
How
positive
to
1(c) i s a r e p r o d u c t i o n of p l a t e
exposed f o r about f i f t y
attributed.to
hydrogen
or
spectra.
Plate
w h i c h was
the
o b s e r v e d with-..exposure
the d i s c h a r g e - traces, of e i t h e r water vapour
hydrogen
The
absent.
1.41
line.
i n t e n s i t y from, t h e g r a t i n g d i d n o t
o b s e r v a t i o n o f h i g h e r members o f t h e h y d r o g e n
Lyman
<
- 45
series.
No
effort
was
-
made t o c o o l . t h e d i s c h a r g e t u b e
h e n c e decrease., t h e h y d r o g e n : D o p p l e r
teoudlening..
When
measurements o f w a v e l e n g t h s o o f
these
with accurate.standards l i q u i d
a i r c o o l i n g w o u l d much
reduce
the
(d) F i r s t
line
by
Shenstone
spectrum
(36).
Copper
has.been v e r y thoroughly s t u d i e d
A c c o r d i n g t o S h e n s t o n e ..it. I t . . - f u t i l e
endeavor to o b t a i n the
metal
complete
spectrum
atom by means o f a s p a r k . a l o n e .
was. f i n a l l y
chosen
into
tungsten wires.
l o n g and was
I t was
cathode. Was
about
usually half
1 cm.
filled
w i t h copper
was
s u p p l i e d u s u a l l y w i t h a 700
volt
was
controlled.by resistances.
The
ampere a n d
helium.
As
200
hundreds of copper
visible.
At
the
and. 5.
foil.
same t i m e
generator
the
of the
but
current
and
when c o l d a n d
pressure,
the helium
spark
cm.
Some
The
filled
operates
spectrum
cathode
the d i s c h a r g e changes
green,
a r c and
tube
volts., and
the m e l t i n g p o i n t of copper,
becomes, a b r i l l i a n t
D.C.
3 mm.
the temperature
metal
thin
e x c i t a t i o n helium.is necessary.
w i t h c i r c u l a t i n g h e l i u m a t about
and
whose
the c o n d u c t i n g gas,
the complete
mainly
which
made o f
i n diameter
for
one
source
follows:
o b s e r v a t i o n s were made w i t h n e o n a s
on about
The
o r d e r t o get r a p i d e v a p o r a t i o n of the
the discharge, t h e h o l l o w
to
of a o n c e . i o n i z e d
by. t h i s w o r k e r was. a S c h u l e r t u b e
c h a r a c t e r i s t i c s were d e s c r i b e d a s
"In
c a n be made
widths.
S p a r k Spectrum o f
This
lines
and
is
reaches,
completely
l i n e s f a d i n g and
l i n e s a p p e a r i n g . i n the
the p o t e n t i a l
rises
t o 500
or
- 46 600 v o l t s , the c u r r e n t d r o p p i n g t o perhaps 0.4 amperes.
When the tube, i s ..in t h i s c o n d i t i o n the e x t e r n a l r e s i s t a n c e .
. can be v a r i e d over a wide range without any great, e f f e c t
on the c u r r e n t .
A still.further
i n t e n s i f i c a t i o n of the
dopper l i n e s can.be produced by l o w e r i n g the helium
pressure."
As no t u n g s t e n o r molybdenum was a v a i l a b l e
when t h i s work was begun an.attempt.was made.to e x c i t e
t h i s copper spark spectrum
The f i r s t
i n other type s o u r c e s .
source s t u d i e d was.a more c o n v e n t i o n a l
hollow cathode type, d i s c h a r g e ( c f . Tolansky (10)0'»
the aluminum^, water..cooled cathode ^werepplaced
of
copper.
Into
thin f o i l s
Because of the l a r g e mass of aluminum employed,
and because
of the water c o o l i n g and. f i n a l l y ..because the
m e l t i n g p o i n t of aluminum was. much lower than t h a t of
copper, i t was found i m p o s s i b l e t o r e a c h the c o n d i t i o n
which Shenstone
the copper.
d e s c r i b e s as. o c c u r r i n g upon m e l t i n g of
The spectrum of helium
predominated.
The next source was i d e n t i c a l w i t h the S c h u l e r
tube of Shenstone
except t h a t fatherofthan tungsten^,
i r o n $ was used as a.cathode.
When the I r o n cathode was
h a l f f i l l e d w i t h copper f o i l a n d . e x c i t e d i n helium the
b r i g h t green d i s c h a r g e i n d i c a t i v e of copper spark spectra.
# The m e l t i n g p o i n t s . o f tungsten, platinum, i r o n , copper
and aluminum a r e 3387oC, 1773°C, 1527°C, 1083°C, and
660°C r e s p e c t i v e l y .
47
c o u l d be o b t a i n e d . .
However, a t t h e same t i m e ,
s t r o n g i r o n a r c and spark
of t h e copper
gain their
-
s p e c t r a , e x c i t e d made
lines, very d i f f i c u l t .
the very
observation
The i r o n s p e c t r a l
i n t e n s i t y a t t h e e x p e n s e o f t h e copper..
lines
This
s o u r c e was n o t . c o n s i d e r e d a d e q u a t e f o r . t h e e x c i t a t i o n o f
vacuum u l t r a v i o l e t
As
type
copper
spark
spectra.
some p l a t i n u m ^ f o i l
source u s i n g such a cathode
was a v a i l a b l e , a S h e n s t o n e
was c o n s t r u c t e d .
Platinum
has, a m e l t i n g p o i n t a b o u t 700°C h i g h e r t h a n t h a t o f
so we e x p e c t e d , t h a t , t h e
less
s p e c t r a , o f p l a t i n u m w o u l d be much
intense than that of copper.
however, t h a t t h e p l a t i n u m
strong spectrum.
spectrum
I t was. d i s c o v e r e d ,
sputtered very badly
The e x c i t a t i o n o f t h e c o p p e r
was n e g l i g i b l e .
atom.
# See f o o t n o t e o n p r e v i o u s
giving a
spark
No change i n t h e c o n d i t i o n s o f
the.discharge effected better r e l a t i v e
copper
copper
page
e x c i t a t i o n of the
- 48 V I . CONCLUSIONS AND
The
RECOMMENDATIONS
p r o b l e m u n d e r t a k e n , was
l e n g t h of the hydrogen
a c c u r a c y o f 0.001
Lyman a l i n e
t o m e a s u r e t h e wavewith an.absolute
A .
0
Some p r o g r e s s h a s b e e n made t o w a r d s
this
problem...
The. h y d r o g e n
a n d d e u t e r i u m Lyman a
have been o b t a i n e d i n the f i f t h
p e r s i o n s of 1 A°/mm.
This
- i f reliable
G
We
i s sufficient
standards, which.are
is
Principle".
standards..
The
experimentally.
t h e new
light
available.
a c c u r a c y and
c a l c u l a t e d from the " R i t z
copper spark spectrum
However we
d i s p e r s i o n to
t h e o n l y method o f
of s u f f i c i e n t
to use wavelengths
dis-
t o the r e q u i r e d a c c u r a c y
s t a n d a r d s were
have shown, t h a t
lines
grating order with
e n a b l e measurement ,of w a v e l e n g t h
(0.001 A )
s o l u t i o n of
obtaining
reliability,
Combination
should supply such
were u n a b l e t o p r o d u c e
When t h e t u n g s t e n f o i l
these
arrives,
lines
and i f
g r a t i n g h a s . t h e p r o p e r t y of good u t i l i s a t i o n
near
M 2 0 0 A ° , the Shenstone
has been d e s c r i b e d
exposures under
hollow cathode
should give the d e s i r e d
two
The m a j o r
spectra
of
wMfeh
with
hours.
experimental.difficulty
encountered
i n t h i s work a r o s e f r o m t h e l o n g e x p o s u r e s w h i c h were;found necessary.
for
good p l a t e s
Exposures
o f one
of the hydrogen
L
Q
t o two
h o u r s were
line.
Commonly, w i t h
o t h e r vacuum s p e c t r o g r a p h s , e x p o s u r e s o f a s many
required
minutes
were a d e q u a t e
(17, 24,
ordered which w i l l
3 6 ) . . A new
g r a t i n g has
been
have a m o d i f i e d g r o o v e f o r m t o
t r a t e r a d i a t i o n i n the d i r e c t i o n
of the
plate.holder
where I t c a n be u t i l i z e d .
Such a g r a t i n g
exposure
tenfold.
times better
than
concen-
should reduce
When t h e c o p p e r s p a r k s t a n d a r d l i n e s . h a v e
obtained with t h i s
new
o n t h e Lyman l i n e s
e m i t t e d by a l i q u i d
discharge
should prove worthwhile.
required w i l l
will
grating,.accurate
n o t be e a s i l y
remain very
been
measurements,
air. cooled deuterium
However, t h e a c c u r a c y
a c h i e v e d and t h e experiment
challenging.
--50
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