Ion charge identification for spectral lines in nitrogen by beam foil light source techniqu

Bashkin, S.

Carriveau, G. W.

Hay, H. J.

English

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ANU -P/460
CHARGE IDENTIFICATION FOR SPECTRAL LINES IN NITROGEN
h	 ^-
S. Bashkin, G.W. Carriveau and H.J. Hay
Department of Nuclear Physics,
Research School of Physical Sciences,
The Australian National University, Canberra
N71-1 14N
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ANU-P/460
CHARGE IDENTIFICATION FOR SPECTRAL LINES IN NITROGEN
S. Bashkin + , G.W. Carriveau and H.J. Hay
Department of Nuclear Physics,
Research School of Physical Sciences,
The Australian National University, Canberra
Abstract: A modified technique has been applied to the
beam-foil light source to determine unambiguously and
correctly the charge of the ions from which spectral
lines are emitted. After transmission through the
thin exciter foil, the luminous beam of ions passes
through a transverse electric field of up to 70 kV/cm.
The field introduces a Doppler shift which depends in
magnitude on the charge state of the emitting ion and
in sign on the direction of the field. In the spectral
range of 3300 X to 6000 X, lines in nitrogen were
distinguished and assigned to various transition arrays.
1. Introduction
One generally unfortunate feature of every light
source which generates light from ionized emitters is that
more than one stage of ionization is present. Consequently
one must determine to which ion a given spectral line should
be assigned. Traditional spectroscopy has relied primarily
w
on three methods of achieving this identification - extra-
polation of a distinct pattern of spectral lines along an
i^	 isoelectronic sequence down to the neutral member (Edl6n,
1964), intensity measurements as a function of the voltage
This work is supported in part by Nas r rapt N^ GR
03-002-017 9 ONR Contract N00013-67-A-0209-0005, and
Aerospace Research Laboratories, Office of Aerospace
Research, U.S. Air Force, Contract F33615-70-C-1007.
This manuscript is submitted for publication with the
understanding that no limitation shall exist on the
reproduction and distribution of its published or
unpublished form in whole or in part for any purpose of
the U.S. Government.
+ Present address: Physics Department, University of
Arizona, Tucson, Arizona 85721
2applied in a spark discharge (Dieke et al. 1961, Bocka9ten et
al. 1963), or theoretical calculation (Edl6n, 196+) of the
level system which could give rise to the observed spectra.
The foregoing methods, which have produced a
wealth of unimpeachable identifications, are difficult to
apply. Moreover, those methods cannot be said to be prima
facie certain to lead to correct results. It is, therefore,
of interest that the beam-foil light source (Bashkin, 1968)
makes possible two additional techniques, one of which, like
the traditional cnes, is suggestive of the appropriate
ionization stage, and one of which is rigorously correct.
The fir"t of these techniques is to study the
intensity of a spectral line as a function of particle energy
(Kay 1965, Denis et al. 1969); some of the objections to
this procedure have been discussed previously (Bashkin and
Malmberg 1966). The second technique is to send the luminous
beam through an applied electric (or magnetic) field so as to
produce a separation of the particle paths according to the
particle charge. Spectroscopic studies of the light from the
separate paths can give a certain identification of the stage
of ionization with which the spectra lines are associated.
Several authors have described experiments of the
charge-splitting type. The first such paper (M alberg et al.
1965), in which observations were made normal to the plane of
the split beam, depended directly on the spatial
distribution of the trajectories of the ions of different
charge. The second (Fink 1968) employed the suggestion
(Traschlin 1968) that one take advantage of the differential
Doppler shifts acquired by spectral lines when their parent
ions are deflected in a transverse electric field; in that
experiment, observations were made in the plane of the split
	 j
beams. Other suggestions for charge-splitting experiments
have also been advanced (Fink 1968). 	 ^1
The cited experiments used photographic plates as
	 ;A
the ultimate photon detectors. There are various reasons
linearity of response vs intensity, direct-reading output,
application in the vacuum ultraviolet - why photoinultiplier
detectors are to be preferred. In the present paper, we
describe the use of photoelectric detection in charge-
splitting research of the second type.
6
6- 3 -
2. Experiment
A horizontal beam of N +
 ions, with an energy of
850 keV and an intensity of --- 0.7 µamp, was sent through a
carbon foil, the thickness of which was estimated to be 6
µgm/cm2 . The pressure in the target chamber was
	 3 x 10 6
Torr. After passage through the foil, the beam was
collimated by a 2 mm, circular aperture in a thin grounded
plate; it then entered the 5 mm gap between two vertical,
parallel, metal plates. The plates had 3 mm-high slots
centered at the elevation of the beam. A deflecting
electrostatic field of up to 80 kV/cm, which could be
reversed in sign, gave the ions a transverse velocity
component which varied linearly with the ionic charge and
the distance travelled in the field. Consequently the
emitted spectral lines, when observed in the horizontal
plane, were Doppler-shifted either to the red or the blue,
depending on the sign of the field. The magnitude of the
shift was a simple function of the wavelength, as measured
in the rest system of the emitter, and the transverse
velocity at the point of observation.
Although the Doppler shift is enhanced by looking
as far downstream as possible, practical limitations are
imposed by the finite mean lives of the radiating levels.
Thus, intensity considerations militate in favour of early
observation, and a compromise is demanded by the conflicting
requirements of signal-to-background (enhanced by working
near the foil) and spectral resolving power (which, aside
from the matter of line blending, can be reduced by	 or
increasing the distance to the point of observation). In
some cases, increasing the distance from the foil at which
the data were taken reduced the contributions from short-
lived levels and simplified the spectrum. Again, intensity
problems limited how far one coiild go in this direction.
The specific distances we used are indicated in fig. 1.
It is also apparent that the effects are
strengthened by increasing the applied field. We were
limited to 80 kV/cm because electrons, released at the foil,
at the c:,^limating aperture, and where positive ions struck
various surfaces, caused excessive loading of the power
supplies. They also triggered sparks at fields above
beam-foil spec
B. Foil-wheel
plate. D1. D29
and 27 mm from
mm separation.
troscopy chamber.
holding 20 foils.
D3. Points of
foil.
F. Glass window.
- 4 -
80 kV/cm. We might note that fields of this order introduce
Stark perturbations of the energy levels (Bashkir. and
Carriveau 1970). Further remarks on this matter appear in
the section on Discussion.
6
Fig. 1: Diagram of the
A. 6 mm pre-collimator.
C. Grounded collimator
observation 7 mm, 17 mm
E. Deflecting plates, 5
G. O-ring seal.
As shown in fig. 2 observations were made through
a glass window in the target chamber, the angle of	 •^
observation being a nominal 90 0 to the particle velocity.
A glass lens (focal length 10 cm) focussed and slightly
de-magnified the light onto the 100-micron-wide entrance
;slit of the monochromator. The available monochromator, a
Jarrell-Ash 1/4 metre, Ebert type, was fast (f/3.7) but
gave an instrumental line width of 6 A. Because of this
width many observed lines would not be properly resolved.
Tha dispersed light which passed through the exit slit
(width 100 microns) was detected with a selected 1P21 photo-
multiplier which was radiatively cooled from a closely
- 5 -
fitting brass cylinder at the temperature of liquid nitrogen.
To keep frost from condensing on the photomultiplier, a
steady steam of dry nitrogen, released from a reservoir of
liquid nitrogen and warmed to room temperature, blew over the
photomultiplier and through the monochromator.
FOIL
WHEEL (APERTURE
FARADAY
- --- --- ------ CUPDEFLECTINO
PLATES
	
GLASS
	
SEAM
	
WINDOW	 CURRENT
INDICATOR
STRIP
	
GLASS
	 C HART
	
LENS	 I RECORDER
0IDA
EVZNT
MARKER
MONOCHROMATOR
PI COAMM ETE R
OR
I P21	 E LECTROMETEJ
Fig. 2: Schematic of charge assignment apparatus.
6
i
1
The 1P21 was operated with -900 volts applied to
the photocathode; the output of the photomultiplier was
amplified with a General Radio Type 1230 -A electrometer was
used in its E-.mode with an input resistance of 10 10
 ohm, or
with a Keithley picoammeter. A permanent record waP
obtained with a strip chart recorder. The photomultiplier
currents were usually of the order of 5 x 10-12 amp, but
occasionally reached the level of 10-10 amp; thus weak
signals were sometimes obscured by noise, and repetitive
6 -
runs were made to eliminate uncertainties. No beam monitor
was used, and beam fluctuations were troublesome for very
weak lines; however, many lines were seen without difficulty.
Since the basic data ac.luired in this experiment
were on line intensity vs wavelength, wavelength calibration
of the spectrograph was required. Standard lamps provided
adequate calibration lines. As the grating was rotated by a
synchronous motor geared to an appropriate speed, generally
	 j
50 X/min, a marker pulse was manually triggered every 10 X.
The combination of instrumental line width, uncertainty in
marker position, and other factors produced a final
uncertainty of ±2 R  in the wavelength of the centre of an
isolated line; for other lines, the uncertainty could be
twice as great. .neither the 90 0 Doppler shift nor the
Doppler broadening which arises from the finite aperture of
the viewing system was significant in the present experiment
for the particle speed was low (3 =1.14 x 10 -2
 corre3ponding
to a shift of 0.36 R at 4000 R) and the monochromator
produced a large intrinsic line width.
3. Results
Figure 3b illustrates part of the spectral scan in
the absence of any applied field; figs. 3a and 3c show the
same spectral region when fields of +70 kV/cm and -50 kV/cm,
respectively, were present. Table 1 lists in column 1 the
wavelengths of the observed lines in the observed field and
in column 2 the assignment permitted by wavelength alone.
Column 3 gives the charge as determined from the present
study. 1
4. Discussion
Three separate points merit mention. In the first
place, it is clear that the method we used cannot
distinguish between one level and another in the same
ionization stage, when the spectral lines have the same
wavelength. Thus the first entry on Table 1 could arise
from either (or both) of two transitions in N III. Only
better resolving power could help decide the relative
contributions of such transitions.
6
YMCOOM,ra x W
t
- 7 -
Fig. 3: Partial spectra
showing Doppler-shift when:
a. +70 kV/cm deflecting
field applied.
b. 0 deflecting field applied.
c. -50 kV/cm deflecting field
applied.
Secondly, when blends from different charge states
are possible within the wavelength unc6rtainty, as occurs for
the entries marked with *, the field should clarify which
charge states are actually present. Thus 13748 is definitely
j	 broadened by the field. Although we could not detect two
distinct lines we conclude from the broadening that there is
•	 a small contribution from N IV (8). In measurements at
higher particle energy, any N IV (8) Axcited in the inter-
action would probably become stronger relative to the N III
levels.
Thirdly, the Stark effect can produce some
broadening and displacing of the spectral lines. However,
the Stark effect is independont of the direction of the
applied field so that it can, at least in principle, be
t6
8 -
Table 1
Results of charge determinations.
Entries with * are discussed in the text.
Observed	 Identification from 	 Present
Wavelength	 Moore (number) or Striganov 	 value of
(R)	 and Sventitskii (+)	 charge
3355 III (597) 2
3368 III (5) 2
3374 III (5) 2
3480 Iv (1) 3
3484 Iv (1) 3
3748* III (4 9 11)	 IV	 (8) 2
3756 III (4 9 11) 2
3772 III (4,11) 2
3794 III (11) 2
3940* II (+)	 III	 (8) 1
3957 II ( 6) 1
4026 II (40) 1
4042 II (39) 1
4097* II (38)	 III	 (1) 2
4102* I (10)	 III	 (1) 2
4133* I (+)	 II	 (65) 1
4378* II (16)	 III	 (17) 2
4535 III (39 13) 2
4605* II (5)	 Iv	 (+)	 v (1) 1094
4620* II ( 5)	 v(1) 19 4
4628* I (+)	 II	 ( 5)	 III	 (2) 1
4641* II (5)	 III	 (2) 1
5004 II (4 9 16,24 9 64) 1
5012 II ( +) 1
5534* I (+)	 II	 (63) 1
5 679 II (3) 1
recognized and not confuse the charge state determinations.
In the present case, the low resolution of the spectrograph
and the instability of the beam made the record too
uncertain for Stark effects to be identified reliably.
Returning to table 1, we consider the remaining
entries which indicate the possibility of blends _rom
1
S^
11
9 -
different charge states. At X3940 R there is no evidence
for charge 2, although our other results (e.g. 0748
discussed above) show that N III levels were populated in
this experiment. We are forced to conclude that this line
does not belong to N III, and that its identification with
any of the N III, multiplet 8, lines in Moore's tables
(Moore 1959) and Striganov and Sventitskii (1968) is
incorrect. This agrees with similar work by Fink (1968)
and Fink, McIntire and Hashkin (1968).
The pair of lines at X4097, 4102 clearly belong
to the same transition array in N III; there is no sign of
blending with lines from N I or N II. Again at X4378, the
evidence favours N III, with no sign of N II.
At X4605, the line is quite broad, and N II, N IV
and N V are probably all present. Also, ;4620 clearly
divides into two lines, corresponding to N II and N V
constituents. At X4628, only the N II line is present.
Only N II is seen at X4641. Finally, at ;5534, only N II
is present.
In assessing the value of the present technique,
there is the obvious advantage that isolated lines can
definitely be given their correct origins. Moreover, the
poor line shapes which are characteristic of beam-foil
spectra do not preclude charge identification. Perhaps the
most serious drawback is that line intensities vary with
particle energy in a complicated, and presently
unpredictable way. Thus, while it is correct to conclude
that 0940 is part of the N II spectrum, it is somewhat
speculative to say that it is not present in the N III
spectrum - it might be. Perhaps a different particle
energy would give rise to such a line that would indeed
originate in N III. There is even the possibility that
another type of light source could generate an N I TJ line
which the beam-foil source could not. However, this is a
handicap of the source and not the method we have used.
The method does permit the charge classification of what-
ever spectral lines can be seen.
6
ti
- 10 -
References
Bashkin, S., 1968, Beam-Foil Spectroscopy,  (New York:
Gordon and Breach
Bashkin, S. and Carriveava, G.W., 1970, Phys. Rev. Al,
269-273
Bashkin, S. and Malmberg, P.R., 1966, Proc. Phys. Soc.
(London), 87, 589-590
Bockhasten, K., Hallin, R. and Hughes, T.P., 19639
Proc. Phys. Soc. (London), 81, 522-530
Denis, A., Desequelles, J. and Dufay, M., 1969 9 J. Opt.
Soc. Amer., 59, 976-980
Dieke, G.H., Crosswhite, J.M. and Dann, B., 1961, J. Opt.
Soc. Amer., 51, 820-833
Edl4n, P.., 1964, Handbuck der Physik, Vol. 27, (Berlin:
Springer-Verlag)
Fink, U., 1968 9 Appl. Opt., Z, 2373-2375
Fink, U., 1968 9 J. Opt, Suc. Amer., 5-8, 937-940
Fink, IT., McIntire, G.N. and Baskin, S., 1968, J. Opt.
Soc. Amer., L8, 475-479
Kay, L., 1965, Proc. Phys. Soc. (London), §J, 163-166
Malmberg, P.R., Bashkin, S. and Tilford, S.G., 1965,
Phys. Rev. Lett., 15, 98 -100
Moore, C.E., 1959, A Multiplet Table of Astrophysical
Interest, NBS Technical Note 3 , Washington D.C.:
U.S. Government Printing Office)
Striganov, A.R. and SventItskii, N.S., 1968, Tables of
Spectral Lines of Neutral and Ionized Atoms, New York:
IFI Plenum
Traschlin, W., 1968, Beam-Foil Spectroscopy,  (New York:
Gordon and Breach
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Charge identification for spectral lines in nitrogen

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