OPTICAL
Die Expansionsgeschwindigkeit
EXCITATION
Fs muß
dabei
FUNCTIONS
OF
Zuverlässigere A u s s a g e n über die Entstehung des
grö-
ß e r sein als die Schallgeschwindigkeit des Füllgases,
Vorläufers
u m einen M a s s e n t r a n s p o r t zu ermöglichen.
raum-zeitlichen
A u s Gin.
(7)
1
o0H
>V
und
d 0 JL
vn2
=
D V,
(8)
11
V\3 '
{
ergibt sich, daß F
H 2
)
/*V
Argon
hat.
für
Mit
Helium
bzw.
plausiblen
im
(D = 1 0 c m , Va ^
0 , 3 1 fxsec/cm
für D
Werten
Messungen
während
einer
seiner
Entla-
in A r g o n e n t l a d u n g e n
ein enger Z u s a m m e n h a n g .
Ein
13
und
gefungleicher
direkter
Vergleich ist nicht m ö g l i c h , da ihre M e s s u n g e n
niederen Fülldrucken, kleineren
bei
Gefäßdurchmessern
und kleineren S p a n n u n g e n erfolgten.
für
Vs
und
1 0 0 0 m / s e c ) ergibt sich d0 ^
0,3
c m , d. h. ein e b e n f a l l s plausibler W e r t .
13
„Anodenfuß"
Geometrie
untersuchten Druckbereich konstant ist und den W e r t
0 , 3 5 jusec/cm
durch
dem v o n J A H N , JASKOWSKI und O B E R T H
denen
=
nur
Entwicklung
der Ursache der hier beschriebenen V o r l ä u f e r
Vy-r
g e l a u f e n . D a m i t gilt
gov
können
d u n g erhalten werden. Vermutlich besteht zwischen
In der Zeit r ist der V o r l ä u f e r eine Strecke
D =
1937
NEON
R. G. J A H N U. W . V . JASKOWSKI, Guggenheim Aerospace
Propulsion Lab. Princeton University Reports 634 k und
634 j zum N A S A Grant NSG-306-63.
Diese Arbeit entstand im Rahmen des Förderungsvorhabens F G 1 0 0 3 des Bundesministeriums für wissenschaftliche Forschung. A l l e Experimente und ein Teil
der Auswertung wurden in Kiel ausgeführt. W i r danken Herrn Prof. W . LOCHTE-HOLTGREVEN für die uns
dabei gewährte großzügige Unterstützung. Herrn U .
GROTH verdanken wir viele technische Details und
Hilfe bei den Experimenten.
Optical Excitation Functions of Neon in the Vacuum Ultraviolet
H . HERTZ
Institut für Angewandte Physik der Universität Hamburg
(Z. Naturforsch. 24 a, 1937—1940 [1969] ; received 6 September 1969)
Optical excitation functions of Ne I, Ne II, and Ne III spectral lines in the wavelength range
of 400 — 800 Ä have been measured with electron energies from 0 to 500 eV. T h e excited ions are
formed by electron impact in a single collision process. The ( 2 p ) 4 3s levels of Ne II become populated via autoionizing states of the neon atom. The excitation functions of the neon resonance
lines at 736 and 744 A are strongly pressure dependent due to radiation imprisonment.
S o f a r litte is k n o w n a b o u t the electron
1. Apparatus and Experimental
impact
Procedure
excitation of the v a c u u m ultraviolet lines in the rare
gases, especially a b o u t simultaneous ionization and
excitation. General considerations
on the measure-
ment of optical excitation functions can b e f o u n d
in the papers
2.
A
synopsis
of
the literature
on
excitation f u n c t i o n s is also given in 1 .
In this w o r k the f o l l o w i n g processes leading
to
the e m i s s i o n of V U V spectral lines are investigated:
N e + e~
N e * + e~,
N e + e~ —> N e + * + 2 e - ,
N e + e~
N e + + * + 3 e".
It is r e m a r k a b l e that even the excited d o u b l y ionized
n e o n is f o r m e d in a single step collision
with a cross section of the order of
1
process,
l-10~18cm2.
Reprint requests to H. HERTZ, Institut für Angewandte
Physik der Universität Hamburg, D-2000 Hamburg 36,
Jungiusstraße 11.
B. L. M O I S E I W I T S C H and S . J. SMITH. Rev. Mod. Phys. 40.
238 [ 1 9 6 8 ] .
A n electron beam ( 1 0 — 2 0 0 /uA) of variable energy
(0 — 5 0 0 eV) passes through a collision chamber. The
beam is confined by a magnetic field ( 1 0 0 — 4 0 0 gauss).
The electrode system is heated to 2 5 0 ° C during the
operation, in order to reduce surface contaminations.
The apparatus has been described in detail by SROKA 3 .
The gas pressure (3 • 1 0 - 5 — 5 • 1 0 - 3 Torr) within the
collision chamber is measured with a capacitance manometer. Therefore the pressure indication is independent
of the special gas.
The radiation emitted from the excited gas is observed at right angles to the beam with a vacuum
monochromator ( 5 0 cm, Seya-Namioka mounting) and
a magnetically focussed electron multiplier. A n X - Y
recorder registers the light intensity in dependence on
the electron energy. T h e smothed traces of these ex2
3
D. W . O. HEDDLE, Methods of Exp. Phys. 7, Part A , 43
[1968].
W . SROKA, Z. Naturforsch. 23 a, 2004 [1968].
Unauthenticated
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1938
H.
citation
functions
are corrected
with respect
to
HERTZ
fluc-
tuations of the gas pressure and beam current.
T h e energy scale is calibrated b y m e a n s of the well-
The
error of
the measured
respectively.
threshold
potentials
is
lower than 0 . 6 e V .
T h e absolute excitation cross sections of the helium
resonance lines at 5 8 4 Ä ,
from
4
and
intensity scale.
tion 2 . 1 . )
537 A
are used as in
5
and 5 2 2 A
obtained
in order to calibrate the
3
(For the calibration at 4 6 1 A see Sec-
For this purpose gas mixtures with
were used in
3.
helium
Because of the gas independent
pres-
sure indication this is no more necessary here. But the
absolute cross sections in neon can only be estimated,
because the wavelength
of
the
apparatus
dependence
is not
well
of
known,
the
sensitivity
particularly
at
eV
40
60
16.9
17.0
16.7
16.9
250
48.6
48.5
170
50.8
175 ( 5 4 )
300 <89
49.5
52.2
88.0
A
known threshold potentials of the helium and neon resonance lines at 5 8 4 A and 7 3 6 / 7 4 4 Ä ,
threshold
potential
(eV)
exp.
calc.
ETn
upper level
configuration
743.7
735.9
2p5(2P°/2)3s
2p5(2P?/2)3s
3 •10-18 cm2
2 • 10-17 c m 2
arbitrary
units
A 460.7
2s2p62Si/2
10
462.4
5
B 4 4 5 - 4 4 8 2p4 (3P) 3s 2P
3
4
2
D 406/407
2p (!D)3s D
—
C 488-491 2s2p53P°
1
Table 1. T h e wavelengths have been taken f r o m 6 - 8 configura7 . Q m is the estimated m a x i m u m excitation cross
tions f r o m
section at the energy E m . T h e calculated threshold potential
is the sum of h v and the ionization energies.
wavelengths b e l o w 5 0 0 A . T h e r e f o r e the cross sections
for this wavelength region in T a b l e 1 are given in ar-
2.1.
bitrary units only.
Radiation
The
from
measured
Singly
and
relative
Doubly
Ionized
excitation
functions
s h o w n in F i g . 2 . T h e e s t i m a t e d m a x i m u m
2. E x p e r i m e n t a l Results and
Neon
are
excitation
c r o s s sections are listed in c o l u m n 3 of T a b l e 1 in
Discussion
a r b i t r a r y units. T h e t r a n s i t i o n s A , B , D a r e m a r k e d
F i g u r e 1 s h o w s a n e o n s p e c t r u m e x c i t e d b y electrons
with
an
energy
of
150 eV.
The
resonance
in the s i m p l i f i e d e n e r g y
about
575 Ä
appear
towards
longer
wavelengths.
there is a m u l t i p l e t of
Nelll,
the
lines at s h o r t e r w a v e l e n g t h s are m u l t i p l e t s o f N e I I .
The N e I spectrum
The
spectrum
pressure up
to
is s t r o n g l y p r e s s u r e
below
l-10
500 A
_ 2
Torr.
levels of the t r a n s i t i o n s 6 column.
is
7
dependent.
independent
I n T a b l e 1 the
of
the
upper
a r e listed i n the s e c o n d
T h e l o w e r level is the g r o u n d
state of
the
term s y s t e m in q u e s t i o n .
diagram
T h e u p p e r levels a r e p o p u l a t e d
series of n e o n c o n v e r g i n g to the i o n i z a t i o n l i m i t s at
A t about 4 9 0 A
level
of
N e II
in
Fig. 3.
by
simultaneous
i o n i z a t i o n a n d e x c i t a t i o n f r o m the N e I g r o u n d state.
Some
of
the
ionization
Ne I
limits
series
which
corresponding
converge
to
to
the N e II
the
levels
are i n d i c a t e d in F i g . 3 .
T h e e x c i t a t i o n f u n c t i o n A f o r t h e e j e c t i o n of a 2 s
electron h a s a light u p w a r d c u r v a t u r e w i t h i n the
10 eV
above
threshold.
at 4 6 0 . 7 / 4 6 2 . 4 A
order
2 : 1
spectrum.
The
h a s been
The
doublet
resolved
measured
2 Si/ s
in the
intensity
first
— 2 P 3 / 2 , v2
second
ratio
in a c c o r d a n c e to the statistical w e i g h t s of
is
the
x2
736 &
A 461/62 Ä
744 >S
0 445 -46 &
D406/7
A
C 468-91 A
I/»
400
4
5
45 0
500
55 0
600
J . D . JOBE and R . M . S T . J O H N , Phys. Rev. 164. 1 1 7
650
700
750
Fig. 1. Neon emission spectrum excited by
electrons of £ = 1 5 0 e V . p = 1 . 8 - 1 0 - 3 Torr.
T h e Ne I-lines have to be corrected with
respect to the pressure d e p e n d e n c e .
[1967].
6
J . C . B O Y C E , Phys. Rev. 4 6 . 3 7 8
A. H . GABRIEL and D. W . O. HEDDLE, P r o c . R o y . Soc. Lon-
7
CH. E. MOORE. A t o m i c Energy Levels,
Stand., Washington 1949.
don A 258, 124
[I960],
[1934].
V o l . I, Nat. Bur.
Unauthenticated
Download Date | 6/15/17 4:28 AM
OPTICAL
EXCITATION
FUNCTIONS
upper
OF
level
1939
NEON
can b e estimated
from9
to b e
of
the
T h e upper levels of the multiplets B a n d D
are
order of 1 • 1 0 - 1 7 c m 2 .
populated b y the d o u b l e excitation of two 2 p electrons, one of them is ejected and the other one is
excited to 3 s . T h e excitation f u n c t i o n s a p p e a r to rise
linearly
above
threshold.
The
measured
threshold
potential of the multiplet B is 1 . 3 e V greater
than
the sum of h v and the ionization e n e r g y .
This fact suggests, that the levels ( 2 p ) 4 ( 3 P ) 3 s
2P
of N e II at l o w energies are populated via the h i g h l y
excited
(ra ^
by
Fig. 2. Relative excitation functions of radiation from the excited ions Ne II and Ne III. The cross sections are listed in
Table 1.
an
( 2 p ) 4 (*D) 3s ( 2 D ) n p
states
4,
see
Fig. 3 ) .
autoionizing
This
of
excitation
transition
into
the
is
the
continuum
( 2 p ) 4 ( 3 P ) 3 s £ p. T h e first of these h i g h l y
levels
(n = 4 )
atom
followed
excited
is 1 . 2 e V a b o v e the ionization
( 2 p ) 4 ( 3 P ) 3 s £ p in a c c o r d a n c e with the
limit
measured
threshold potential. T h e autoionizing states a n d their
interaction with the c o n t i n u u m have been observed
in absorption m e a s u r e m e n t s
10.
T h e m e a s u r e d thresh-
old potential of D is also greater than the calculated
one. But this m e a s u r e m e n t is less accurate because
of the low intensity of this multiplet.
T h e excitation f u n c t i o n C f o r d o u b l e
ionization
displays an u p w a r d curvature within the first 2 0 e V
above
threshold.
The
upper
levels
2s
(2p)5
are
excited b y the simultaneous ejection of a 2 s and a
2 p electron. T h e excitation cross section is smaller
b y a factor 1 0 or 2 0 than that f o r the ejection of
one 2 s electron
(A).
A l l these excitation f u n c t i o n s depend linearly on
the b e a m current and the g a s pressure in the r a n g e
from
1-10
processes
- 4
to
6
10~3Torr.
other than single
Therefore
collision
step collisions
can
be
excluded. T h e estimated error of the relative excitation functions is lower than 5%,
only in the case of
D it is about 1 0 % .
2.2. The 3s Resonance
Lines
T h e intensity of the n e o n resonance lines is not
proportional
2s2 2p6 Ne I
to
the g a s
pressure.
As
an
example
of a typical pressure dependence the apparent excita-
Fig. 3. Simplified energy diagram of Ne II, after 7. The wavelengths of the transitions are given in A. Some series of Ne I
are indicated in order to ilustrate the text.
tion cross section (defined as light intensity, divided
lower
F i g . 4 . T h e pressure dependence is due to
levels.
The
relative excitation
functions
are
identical. T h e absolute excitation cross section of the
8
9
R. L. KELLY, University of California, Lawrence Radiation
Laboratory U.C.R.L. 5612.
W . L Ö T Z , Z . Phys. 2 1 6 , 2 4 1
b y the b e a m current a n d the gas pressure)
736 A
line is plotted
against
of the
the g a s pressure
in
absorp-
tion p h e n o m e n a . T h e deviation f r o m the exponential
10
K . CODLING,
155,26
R. P.
MADDEN,
and D. L.
EDERER,
Phys. Rev.
[1967].
[1968],
Unauthenticated
Download Date | 6/15/17 4:28 AM
1940
OPTICAL
l a w is caused b y the change of
EXCITATION
FUNCTIONS
OF
N E O N 1940
the line profile of
the registered radiation with increasing gas pressure
a n d b y the multiple a b s o r p t i o n within the collision
chamber.
The
intensity
fraction
due
to
cascading
f r o m higher levels is also expected to b e pressure
dependent, b e c a u s e the radiation f r o m these levels is
also
influenced
by
resonance
radiation
imprison-
ment.
The
shape
(Fig. 6 )
of
the
excitation
is not pressure
function
dependent
up
at 7 4 4 Ä
to
4-10
- 3
T o r r , although the line is already s t r o n g l y a b s o r b e d
at this pressure. T h u s it appears to b e , at the m o s t ,
weakly polarized, which can b e e x p l a i n e d as f o l l o w s .
T h e excitation cross sections in T a b l e 1 include the
population of the resonance levels d u e to cascading
Fig. 4. Apparent excitation cross section Q for the Ne I 736 A
line in arbitrary units versus gas pressure. The Q scale is
logarithmic. The electron energy is 64 eV.
from
for
higher
this
levels.
The
cascading
excitation
fraction
cross
which
sections
can
be
ob-
Fig. 5. Relative excitation function at 736 Ä at
various pressures. For all functions the same arbitrary value of ( V has been chosen.
In the case of
the 7 3 6 Ä
line the shape of
the
lines,
no.
1
2
3
4
5
p (10- 4 Torr)
0.3-0.8
3
4.5
19
27
respectively.
Thus
the 7 4 4 Ä
line is
mainly
excitation f u n c t i o n is also pressure dependent which
excited b y cascading a n d therefore does not
is demonstrated in F i g . 5 . T h i s dependence is, m o s t
the polarization typical f o r direct excitation of the
probably,
upper level.
due
to
the
which depends
on
the electron e n e r g y ,
fact
that
the
b y resonance radiation i m p r i s o n m e n t
polarization,
is
2.
reduced
Therefore
the shape b e c o m e s m o r e independent of the angle of
observation with increasing pressure,
tained f r o m
11
are a p p r o x i m a t e l y
6,10
- 1 8
cm-
and
2 . 5 • 1 0 ~ 1 8 c m 2 in the case of the 7 3 6 Ä and 7 4 4 A
11
P . V . FELTSAN,
I. P . ZAPESOCHNYI,
and
show
M. M.
POVCH.
This work was supported by the Deutsche Forschungsgemeinschaft. The author is indebted to Prof. Dr. H.
RAETHER for suggesting this investigation and to Dr.
W.
LEGLER
and Dr.
W.
SROKA
for discussions
perimental advice.
Ukrain. Fiz. Zh. 11. 1222 [1966].
Unauthenticated
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and
ex-