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Nitrogen recombination continuum in the vacuum ultraviolet
1. Quant.
Specmsc.
Rodiat.
Transfer.
Vol.
6. pp.
899-902.
Pergamon
Press
Ltd.,
1966.
Printed
in Great
Britain
NOTE NITROGEN
RECOMBINATION CONTINUUM VACUUM ULTRAVIOLET* J. C.
Avco Corporation,
MORRIS
and R. L.
Space Systems
Division,
(Receioed25
April
IN THE
GARRISON Wilmington,
Massachusetts
1966)
THIS NOTE describes preliminary experimental results that have been obtained for the electron-1st ion recombination radiation of nitrogen, to the 'P and 'D NI excited levels in the vacuum ultraviolet. These data have been obtained using a mechanically constricted arc as a plasma source. The geometry and operational characteristics of this generator are described in detail elsewhere in the literature;“,2) however, for the sake of clarity and continuity, a brief outline of the experiment follows. Figure 1 shows a sketch of the plasma generator. The arc is run on argon with a controlled percentage of nitrogen added to the center of the column. Gas flows are adjusted so that nitrogen cannot escape to the electrode regions of the arc. A half-meter Seya-Namioka spectrometer is connected to the end of the generator through a set of pin hole aperatures that limit the field of view to the axis of the discharge. These baffles also serve as part of a differential pumping system that permits the arc to run at atmospheric pressure while the spectrometer operates at 5 x 10m6 mm Hg. With this experimental arrangement and using argon as the parent gas, it is possible to observe the nitrogen emission spectrum from an isothermal pla;sma to a wavelength of approximately 9OOA, at which point strong absorption due to the photo-ionization of the ground state of argon begins. The two photo-ionization edges for nitrogen that are most prominent in this region are from the 2P and 2D NI levels. Experimental measurements of the recombination continuum for these levels were made at wavelengths which were as free as possible from interfering line radiation. The calibration of the detecting system for absolute intensity measurements was made using NI and hydrogen lines which were made to radiate like a black body by increasing the test-gas concentration until the lines became optically thick. Intensity calibrations for the wavelengths at which the continuum was measured were taken froman interpolation of these line data. The temperature of the gas under observation was measured by introducing a trace amount of hydrogen into the center of the arc and comparing the electron concentration
* Research reported herein is part of the research program on Radiation from Arc Heated Plasma Aeronautical Research Laboratories, Office of Aerospace Research, U.S. Air Force, under the technical zance of E. Soehngen and P. Schreiber. AF33(615)-2976. 899
of the cogni-
900
J.C.
MORRIS and R. L. GARRISON
k-Ix. "t-~ ¢.9 n,.
-ill
I
I I,-
I I
_J O
J ~
0 Z
O
I I ,0 Z
¢D e~ O
X o
,-t
~t
~
~z l--j-~m ~o I-'-Z
W er" "'1
,o
o~
i" =,
mO
E
6 k~ taJ
JZ
.
0 Z
IE L t.
o w
~ ~* ' Z k-o
901
Nitrogen recombination continuum in the vacuum ultraviolet
determined from the half width of Hp (cf. GRIEM (3)) and using the equilibrium composition of DRELLISHAK.(4) The nitrogen continuum intensities were measured with a sufficiently low concentration of nitrogen that the gas was optically thin, i.e. aL ,~ 1. The data are reported in the form of absorption coefficients which were calculated from the relation
w~ L WBB where W~ = signal from the nitrogen radiation, WsB signal from the blackbody, and L = the length of the nitrogen column. An adjustment to a one atmosphere nitrogen plasma was made by multiplying this ~ by the ratio of the nitrogen number density at atmospheric pressure to that at which the measurements were taken. These data are shown in Fig. 2. Also included in this figure is the theoretical estimate of ~ using the cross sections calculated by using the method of BURGESSand SEATON.(5)
1.0,
I 51 20 2p (D.) P
Q8
2p3(3p)2D0
I
0.6
0
Zl
¢J
0.4
U
i1Z i,i
zl
U_ b. W
o u z
o
0.2 2 3(3p )2 pO
D. llg
o T =13,150 *K P= I ATM = THEORY O=RUN I
111
0.1
0
O
Z~ = RUN 2
Zl
A
0
0.08
O.OE 850
LI
900
950
I000
1050
I100
1150
WAVELENGTH, angstroms FIG. 2. Nitrogen continuum absorption coefficient, and theoretical calculation using Burgess and Seaton's cross sections compared with experiment, as a function of wavelength.
1200
902
J. C
MORRIS and
R. L. GARRISON
The use of the above methods of analysis requires of course that the plasma be in equilibrium. Applying the equilibrium criteria of GRIP@’ leaves some doubt about whether an over-population of the ground state and of the first two metastable levels exists. The use of the theoretical f values, given by Griem, in the rate equations would predict an over-population of these levels. The j’values, however, are open to questions.‘627) If experimental f valuesY or the theoretical values of BURGES and SEATON are used, the criteria for LTE are satisfied. Acknowfedgmm-We are grateful to R. U. Krey and R P. Rudis for their help in makmg the temperaturd measurements and to R W. Liebxmann for his calculatmns of the theoretical absorption coefficient.
REFERENCES I. 3. C. MORRIS and G. R. BACH, NSll(109) IEEE Trans. Nucl. SCI. (1964). 2. J. C. MORRIS, R. U. KREY and R. W. LLEBBRMANN,ARL Report on Radiation 65-164, August 1965, Aerospace Research Laboratories Oflice of Aerospace Research, USAF (AF33(615)-2976). 3. H. R. GRIEM, Plasma Spectroscopy, McGraw-Hill, New York (1964). 4. K. S. DRELLISHAK, C. F. KNOPP and ALI-BULENT CAMBEL, Report AEDC-TDR-63-146; Arnold Engineering Development Center AFSC USAF (1963). 5. A. BURG= and M. F. SEATON, Mon. Not. R. Asrr. Sm., 120, 121 (1960). 6. F. LABIJHN, Max Planck Institute of Physics, Munich, Germany, private communication. 7. I. C. MORRIS, Quarterly Progress Report (2), Aerospace Research Laboratories Office of Aerospace Research. USAF (1965).
Specmsc.
Rodiat.
Transfer.
Vol.
6. pp.
899-902.
Pergamon
Press
Ltd.,
1966.
Printed
in Great
Britain
NOTE NITROGEN
RECOMBINATION CONTINUUM VACUUM ULTRAVIOLET* J. C.
Avco Corporation,
MORRIS
and R. L.
Space Systems
Division,
(Receioed25
April
IN THE
GARRISON Wilmington,
Massachusetts
1966)
THIS NOTE describes preliminary experimental results that have been obtained for the electron-1st ion recombination radiation of nitrogen, to the 'P and 'D NI excited levels in the vacuum ultraviolet. These data have been obtained using a mechanically constricted arc as a plasma source. The geometry and operational characteristics of this generator are described in detail elsewhere in the literature;“,2) however, for the sake of clarity and continuity, a brief outline of the experiment follows. Figure 1 shows a sketch of the plasma generator. The arc is run on argon with a controlled percentage of nitrogen added to the center of the column. Gas flows are adjusted so that nitrogen cannot escape to the electrode regions of the arc. A half-meter Seya-Namioka spectrometer is connected to the end of the generator through a set of pin hole aperatures that limit the field of view to the axis of the discharge. These baffles also serve as part of a differential pumping system that permits the arc to run at atmospheric pressure while the spectrometer operates at 5 x 10m6 mm Hg. With this experimental arrangement and using argon as the parent gas, it is possible to observe the nitrogen emission spectrum from an isothermal pla;sma to a wavelength of approximately 9OOA, at which point strong absorption due to the photo-ionization of the ground state of argon begins. The two photo-ionization edges for nitrogen that are most prominent in this region are from the 2P and 2D NI levels. Experimental measurements of the recombination continuum for these levels were made at wavelengths which were as free as possible from interfering line radiation. The calibration of the detecting system for absolute intensity measurements was made using NI and hydrogen lines which were made to radiate like a black body by increasing the test-gas concentration until the lines became optically thick. Intensity calibrations for the wavelengths at which the continuum was measured were taken froman interpolation of these line data. The temperature of the gas under observation was measured by introducing a trace amount of hydrogen into the center of the arc and comparing the electron concentration
* Research reported herein is part of the research program on Radiation from Arc Heated Plasma Aeronautical Research Laboratories, Office of Aerospace Research, U.S. Air Force, under the technical zance of E. Soehngen and P. Schreiber. AF33(615)-2976. 899
of the cogni-
900
J.C.
MORRIS and R. L. GARRISON
k-Ix. "t-~ ¢.9 n,.
-ill
I
I I,-
I I
_J O
J ~
0 Z
O
I I ,0 Z
¢D e~ O
X o
,-t
~t
~
~z l--j-~m ~o I-'-Z
W er" "'1
,o
o~
i" =,
mO
E
6 k~ taJ
JZ
.
0 Z
IE L t.
o w
~ ~* ' Z k-o
901
Nitrogen recombination continuum in the vacuum ultraviolet
determined from the half width of Hp (cf. GRIEM (3)) and using the equilibrium composition of DRELLISHAK.(4) The nitrogen continuum intensities were measured with a sufficiently low concentration of nitrogen that the gas was optically thin, i.e. aL ,~ 1. The data are reported in the form of absorption coefficients which were calculated from the relation
w~ L WBB where W~ = signal from the nitrogen radiation, WsB signal from the blackbody, and L = the length of the nitrogen column. An adjustment to a one atmosphere nitrogen plasma was made by multiplying this ~ by the ratio of the nitrogen number density at atmospheric pressure to that at which the measurements were taken. These data are shown in Fig. 2. Also included in this figure is the theoretical estimate of ~ using the cross sections calculated by using the method of BURGESSand SEATON.(5)
1.0,
I 51 20 2p (D.) P
Q8
2p3(3p)2D0
I
0.6
0
Zl
¢J
0.4
U
i1Z i,i
zl
U_ b. W
o u z
o
0.2 2 3(3p )2 pO
D. llg
o T =13,150 *K P= I ATM = THEORY O=RUN I
111
0.1
0
O
Z~ = RUN 2
Zl
A
0
0.08
O.OE 850
LI
900
950
I000
1050
I100
1150
WAVELENGTH, angstroms FIG. 2. Nitrogen continuum absorption coefficient, and theoretical calculation using Burgess and Seaton's cross sections compared with experiment, as a function of wavelength.
1200
902
J. C
MORRIS and
R. L. GARRISON
The use of the above methods of analysis requires of course that the plasma be in equilibrium. Applying the equilibrium criteria of GRIP@’ leaves some doubt about whether an over-population of the ground state and of the first two metastable levels exists. The use of the theoretical f values, given by Griem, in the rate equations would predict an over-population of these levels. The j’values, however, are open to questions.‘627) If experimental f valuesY or the theoretical values of BURGES and SEATON are used, the criteria for LTE are satisfied. Acknowfedgmm-We are grateful to R. U. Krey and R P. Rudis for their help in makmg the temperaturd measurements and to R W. Liebxmann for his calculatmns of the theoretical absorption coefficient.
REFERENCES I. 3. C. MORRIS and G. R. BACH, NSll(109) IEEE Trans. Nucl. SCI. (1964). 2. J. C. MORRIS, R. U. KREY and R. W. LLEBBRMANN,ARL Report on Radiation 65-164, August 1965, Aerospace Research Laboratories Oflice of Aerospace Research, USAF (AF33(615)-2976). 3. H. R. GRIEM, Plasma Spectroscopy, McGraw-Hill, New York (1964). 4. K. S. DRELLISHAK, C. F. KNOPP and ALI-BULENT CAMBEL, Report AEDC-TDR-63-146; Arnold Engineering Development Center AFSC USAF (1963). 5. A. BURG= and M. F. SEATON, Mon. Not. R. Asrr. Sm., 120, 121 (1960). 6. F. LABIJHN, Max Planck Institute of Physics, Munich, Germany, private communication. 7. I. C. MORRIS, Quarterly Progress Report (2), Aerospace Research Laboratories Office of Aerospace Research. USAF (1965).