Oskam, H . J .
Jongerius, H . M .
1958
Physica XXIY
1092-1094
LETTER
TO THE
EDITOR
Helium-neon bands
F o r t h e first t i m e helium-neon was m e n t i o n e d b y D r u y v e s t e y n z) in a letter a b o u t
h e l i u m - n e o n bands in t h e violet region of a gas discharge spectrum. W e i z e l 2 ) has
d o u b t e d t h e existence of these bands. H o w e v e r , recent m e a s u r e m e n t s of a t o t a l l y
different c h a r a c t e r in gas discharges of a special kind done b y O s k a m 8) and b y P a h 1 4)
indicate t h e a p p e a r a n c e of t h e H e N e molecular ion. To confirm this we h a v e r e p e a t e d
t h e m e a s u r e m e n t s of D r u y v e s t e y n .
F o r this purpose we h a v e p h o t o g r a p h e d t h e 7000--3600 A range of t h e s p e c t r u m of
discharges in helium and neon separately and also in a m i x t u r e of the two gases. One
half of a double discharge t u b e (fig. 1) was filled w i t h h e l i u m of 10 m m .pressure, t h e
W
--ll
l
S ©m
a
W'
Fig. 1. T h e double discharge tube, in u p p e r view. A t first one half is filled w i t h
helium, t h e o t h e r w i t h neon. L a t e r on t h e t w o gases are m i x e d b y b r e a k i n g t h e glass
seal S b y m e a n s of t h e steel bali B. T h e n e g a t i v e glow is viewed v i a windows W, which
are n o t reached b y t h e c a t h o d e sputtering. Cathode C and anode A are fiat electrodes
of m o l y b d e n u m .
T u b e and electrodes are t h o u r o u g h l y degased before m e l t i n g off. T h e noble gases
used were v e r y p u r e (contamination less t h a n 1 to 107). I n t h e s p e c t r u m we found no
o t h e r lines t h a n those of t h e noble gas and of t h e electrode metal.
o t h e r half w i t h neon of t h e s a m e pressure; for t h ~ h e h u m - n e o n e x p e r i m e n t t h e gases
could be m i x e d b y b r e a k i n g a seal. B o t h before and after b r e a k i n g t h e dividing seal
exposures were m a d e of t h e positive and t h e n e g a t i v e glow b y m e a n s of a m e d i u m
size glass spectrograph. T h e t h r e e exposures of fig. 2 refer to t h e s a m e spot in t h e tube,
a b o u t 3 m m behind t h e cathode.
--
1092 - -
HELIUM-NEON BANDS
1093
Fig. 2. Spectrum of the negative glow of neon (a), helium (b), and the mixture (c).
Exposures on Ilford HP3 plate. Exposing time 2 hours. Medium size glass spectrograph.
Current l0 mA. Gas pressure l0 ram.
Fig. 3. Spectrum of the
negative glow of the
helium-neon mixture, between 4280 and 4020 A.
Exposure on Ilford HP8
plate. Exposure time 20
hours. Medium size glass
spectrograph. Enlargem e n t 4 times the original
exposure. Current 10 mA.
Gas pressure 10 mm.
H
il
i
,-
I
f
f
t
if
:,
,
Fig. 4. Micro-densitometer curve of the spectrum of a mixture of helium and neon. An
Ilford HP8 plate had been exposed during 2 hours. Current 10 mA. Gas pressure l0 ram.
1094
HELIUM-NEON BANDS
At this place the helium molecule spectrum (described b y W e i z e l 5)) has the
highest intensity. Obviously the helium molecules (ions ?) appear mainly at the edge
of the negative glow. I n the spectrum of the positive column in helium we have only
found atomic lines and also, though weak, the band at about 6400 A. I n neon only the
line spectrum was detected, both in the positive column and in the negative glow. I t
is remarkable t h a t in the negative glow of the neon discharge strong molybdenum lines
appeared (3903; 3864; 3798 A). This indicates electrode sputtering. I n the helium
discharge we have not detected these lines. They do appear in the spectrum of the
helium-neon mixture. This agrees with the results of sputtering measurements 6):
with helium ions the sputtering is much less t h a n with neon ions.
In the gas mixture the edge of the negative glow is violet. The spectrum contains
atomic lines of helium and neon, b u t not the helium molecule spectrum. Moreover we
have found two continuous parts (unresolved bands ?) in the spectrum, not appearing
in helium or neon. They extend from 4270 A to beyond 4235 A and from 4144 A to
beyond 3964 A (see fig. 3 and 4). A n u m b e r of lines is superposed on the continuum
4270--4235 A. They occur also in the negative glow of neon (with exception of 4261
and 4235 A which only appear in the mixture spectrum). I.n the continuum 4144--3964
A one can distinguish a few different parts (bands ?). Superposed on it are the helium
atomic lines 4121, 4026, 4024 and 4009 A. The strongest part of the continuum appears
between 4114 and 4096 A. This part contains the neon line 4098 A, occurring also in
the spectrum of the neon discharge. From 4096 to 4063 A about 13 lines can be observed in the continuum, which is weaker in this region. The most distinct line appears
at 4074 A; it was not possible to determine the origin of these lines (band structure ?).
I n the neon discharge spectrum only two are visible : 4070 A and 4063 A. From 4063 A
on the continuum becomes weaker in the direction of shorter wavelengths. However
it certainly extends beyond 3964 A. Increasing absorption in the spectrograph is a
cause of intensity loss in this region. Fig. 4 shows a record of the helium-neon negative
glow spectrum between about 4600 A and 3800 A. The two continuous parts of the
spectrum are clearly visible.
Summarizing we conclude t h a t the exposures confirm the existence of a helium-neon
compound at the edge of the negative glow.
Acknowledgement. The authors wish to t h a n k prof. dr. J. A. S m i t for stimulating
interest and mr. M. K t ~ p e r u s and mr. J. L. v a n K o e v e r i n g e for valuable help.
This investigation was supported by the N.V. Philips Gloeilampenfabrieken at
Eindhoven.
H. J. OSKAM
H. M. JONGERIUS.
Fysisch Laboratorium der Universiteit,
Utrecht, Nederland.
Received 30-8-58.
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