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Excitation of neutral atomic nitrogen in a helium afterglow
Spectrochimica Acta,Vol.26B,pp.721to 731. Pergamon Press1971. Printedin Northern Ireland
Excitation of neutral atomic nitrogen in a helium afterglow* M. S. MANALIS and H. P. BROIDA Department
of Physics, University
of California, Santa Barbara, California 93106, U.S.A.
(Received 14 May
AbStract-A
1971)
light source of neutral atomic nitrogen has been developed by mixing traces of
molecular nitrogen with a flowing helium afterglow at pressures near one torr. This source produces radiation from energy levels near the ionization limit of the neutral atom but none from excited states of the nitrogen ion. The excited nitrogen is produced by recombination of electrons with ionized nitrogen atoms formed in dissociative charge transfer reactions of ionized helium with molecular nitrogen. Approximately 400 new atomic nitrogen lines were found. In addition, a second mechanism for exciting atomic nitrogen was isolated. It consists of a two step collisional process in which N, is dissociated by metastable molecular helium and the excitation is produced by collisions with energetic electrons. The characteristic temperature associated with this level population distribution was about lo4 K.
INTRODUCTION ADDITIONAL measurements of wavelengths and intensities of atomic nitrogen transitions are needed. It is known that the available experimental data suffer from uncertainties in the identification of emission lines and inaccurately measured intensities. Reported here is a new method for exciting atomic nitrogen by adding molecular nitrogen to a flowing helium afterglow. This source allows experimental control for identification and wavelength measurement, and is one of the “cleanest” and brightest sources of visible atomic nitrogen radiation. The experimental conditions necessary to produce this source and to obtain complete spectra over the visible and near i.r. spectral region have been described [l]. In order to learn which levels are populated and in what spectral region the various series limits may be found, energy level diagrams are given. Classification of all the spectral lines has not yet been made since sufficiently high spectral resolution was not available. Experimental evidence is given for two independent mechanisms for exciting nitrogen atoms. EXPERIMENTAL Excited atomic nitrogen was produced by injecting molecular nitrogen into a flowing helium afterglow (Fig. 1). A dc discharge operating near 1 kV and 500 mA was used with He at a pressure of one torr and with a flow rate of 300 atm-cc/see. Additional details on this source and information concerning the energetic species present in the afterglow can be found elsewhere [l-3]. Spectra were recorded with a Q-m Ebert scanning monochromator with photoelectric detection (Fig. 2) and with a
* Work supported in part by the Advanced Research Projects Agency and the Office of Naval Research and the U.S. Air Force Office of Scientific Research, Office ofAerospace Research, under Grant No. AFOSR-70-1851. M. S. MANALIB, Ph.D. Thesis, University of California at Santa Barbara (1970); M. S. MANALIS, Phys. Rev. 4A, 364 (1971). [2] J. L. DUNN, Ph.D. Thesis, University of California at Santa Barbara (1966). [3] E.E. FERQUSON,F.C.FERSEKFELD and A. L.~CHMELTEKOPF, in Advances in Atomic and Molecular Phg&x. Academic Press, New York (1969). [l]
1
721
Excitation
of neutral atomic nitrogen in a helium afterglow
725
Table 1 (continued) Id
N
Fe
Ne
1, A
Y, cm-’
6168.81 5167.82 5167.41 5165.90 5165.77 5165.66 5163.77 6162.91 5162.78 5162.61 6160.7 5168.49 5156.72 5155.87 5155.37 5155.26 5154.5 5153.76 5153.05 5151.33 5149.62 5146.6 5146.0 6141.93 5141.16 5136.31 5135.79 5134.39 6132.5 5120.3 5117.88 5116.6 5115.44 5110.52 5100.69 5100.45 5080.47 5037.68
19341.426 19345.131 19346.666 19352.321 19352.808 19353.183 19360.303 19363.528 19364.016 19364.654 19371.82 19380.12 19386.771 19389.968 19391.848 19392.262 19395.121 19397.906 19400.579 19407.056 19413.501 19424.89 19427.16 19438.754 19445.446 19463.807 19465.778 19471.086 19478.26 19524.665 19533.897 19538.555 19543.215 19562.030 19599.729 19600.651 19677.734 19844.874
I
Id
3.5 82.0 3.8 8.0 8.5 4.7 1.6 32.5 1.4 1.6 2.3? 5.2 4.3 3.9 2.6 5.3 2.1? 1.3 2.6 4.3 1.6 1.39 2.3? 5.4 2.0 4.1 3.1 6.1 1.4 1.6 1.6 1.4? 2.3 4.5 4.4 8.8 9.4 8.4
Fe
A, A
y, cm-’
5031.5 5009.2 5009.4 5007.6 4997.1 4993.3 4987.47 4987.3 4982.0 4976.31 4974.82 4973.37 4971.51 4968.31 4966.41 4965.54 4963.98 4962.29 495742 4955.74
19869.25 19957.7 19956.9 19964.08 20006.03 20021.25 20044.654 20045.34 20066.66 20089.606 20095.623 20101.482 20109.043 20121.95 20129.652 20133.179 20139.506 20146.771 20166156 20172.992 20182.440 20190.347 20195.446 20199.934 20205.609 20210.552 20213.126 20213.576 20222.489 20225.435 20247.51 20257.647 20263.724 20324.281 20335.733 20340.615 20344.34 20348.48
4953.42
N
N
4951.48 4950.23 4949.13 4947.74 4946.53 4945.90 4945.79 4943.61 4942.89 4937.5 4935.03 4933.55 4918.85 4916.08 [4914.9] 4914.0 4913.0
I 2.0 1.4 1.5 1.4 1.6 1.4 3.9 1.6 2.2 0.7 2.7 4.6 10.5 3.3 2.0 1.0 20.6 4.6 3.9 10.3 4.8 6.7 14.0 I.0 2.4 2.9 2.0 3.1 2.0 2.1 0.8 37.0 2.5 1.1 1.5 16.2 0.6 0.6
The purpose in constructing the expanded diagram, Fig. 5, was to list all states independent of their particular coupling schemes and to facilitate the recognition of electric dipole transitions. RACAH [9] states that there should be more of an effort to include these states along with Russell-Saunders states. This is difficult because the spectroscopic notation is different for different coupling schemes. But for nitrogen, the states are not all Russell-Saunders coupled and any complete discussion of the problem must include these other coupling schemes. Russell-Saunders states are located on the extreme left (quartet system) and right (doublet system) of the diagram. Each member of a major vertical group shares the same parity quantum This diagram incorporates more than Russell-Saunders states and also number. facilitates recognition of electric dipole transitions. Note the symmetry in energy between the terms of the quartet and doublet systems. The short dashed lines are terms which are predicted on the basis of this symmetry.
[9]
G. RACAH,
Trans.
J. Comm. Spectros.
50,
408
(1960).
-
Excitation
of neutral atomic nitrogen in a helium afterglow
727
For transitions which occur in the visible and near i.r., nearly all previously recorded levels were observed [l]. No levels above the ionization limit were detected. Also light from the high orbital angular momentum states (f) was not observed [lo]. From the energy level diagram, the series limits occur between 3000 and 3500 A for nitrogen. No clear limits were seen spectroscopically. However, in the green glow, energy levels near the ionization limit were substantially populated. Two
MECHANISMSFOR PRODUCTION OF EXCITED NITROGEN
When the amount of molecular nitrogen injected into the helium afterglow is increased from that necessary to obtain the green glow, a strong blue glow appears which is due predominately to the first negative system of N,+ [l, 21 Figs. 6 and 7
NI 3~~P-4p~P'
I _._“____~._
... .. __.&.
’
,
1
l._-,_.*._-
_w_--r~.^~~.“.-_~/--.-_-_.--.
4;75
42150
*
-.-
_____-__
42125
I_
ii
Fig. 6. Spectra taken during three different experimental conditions: blue glow (upper); green glow (middle); helium afterglow only (lower).
(upper) show parts of the spectral region associated with the blue glow. There also is radiation from atomic nitrogen in this glow, but with a different population distribution than in the green glow (Fig. 8). This blue glow always spatially precedes The N lines from the blue were not sensitive to the the green in the afterglow. application of an rf field, while the lines from the green glow were extremely sensitive to the field (Fig. 9). This information suggests two mechanisms for proThe blue glow population distribution was determined ducing excited nitrogen. by using a rf field to quench contributions from the recombination mechanism. The results was a linear distribution corresponding to a temperature of lo4 K (Fig. 8). [lo]
J. W. MCCONKEY,D.J.BURNS 8, 823 (1968).
and J. A.KERNAEAN, J. Quant. Spectrosc.
Radiat.
Transfer
728
M. S. MANALIS
and H.
P. BROIDA
(a)
NI
3s2P-5p2Do
3
5
-Z’2;,;
I
(b)
Fig.
I
7. The upper
spectrum was observed from the blue glow spectrum was observed from the green glow.
NI
population
and the lower
distribution
._
._ En
s
eV
Fig. 8. The relative population per unit statistical weight (arbitrary units) as a function of the electron excitation energy in atomic nitrogen.
Excitation
of neutral
atomic
nitrogen
in a helium afterglow
729
The mechanism for the excitation of atomic nitrogen in the blue region was determined from energetics and qualitative observations. It takes about 10 eV to dissociate the nitrogen molecule, and since N transitions from energy levels of 14 eV were observed in the blue region, a minimum of 24 eV is required. Since there is no long-lived helium species remaining which has that much energy, a two step process may be involved. Qualitative experiments using argon to preferentially remove metastable helium atoms demonstrated that atomic nitrogen was not excited by these atoms [l, 21. Thus, the only species left with sufficient energy are metastable helium molecules, From energetics and the distribution of atomic Hean, and energetic electrons [l]. levels, the nitrogen molecule was most likely dissociated by HeaM and the nitrogen atoms were excited by collisions with electrons. The two step process in which electronically excited atoms are produced in the blue glow is referred to as the collisional mechanism and is written as: HeaM + N, -+ 2N + 2He N+e--+N*-+e. The observation that the metastable helium molecule, instead of the metastable helium atom, dissociates the nitrogen molecule was not expected [ 111. No experimental evidence was found to connect either of the two mechanisms for formation of N* with N,+. The energy of N,+ is most likely dissipated at the walls [la]. Figure 9 is an example of photoelectric data which formed the basis for postulating these two independent mechanisms for exciting atomic nitrogen lines. The top left spectrum shows atomic lines excited in the blue glow (collisional), while the right spectrum shows the same lines excited in the green glow (recombination). Notice that the relative intensity of the nitrogen multiplets has changed. The lower spectra were taken under identical experimental conditions except that a rf field (2450 MHz) was directed towards the plasma. Notice that the He and N lines which were produced by electron recombination have vanished, while the N lines produced by the collisional mechanism still remain (lower left spectrum). The weak N emission seen in the lower right spectrum is due to the collisional mechanism. Identifying atomic lines in the blue glow is very difficult because of the many molecular bands which occur throughout the entire spectral region. Since the green glow emission was predominately from atomic nitrogen, this source is helpful in finding particular lines in the blue glow. The line could be observed under the green condition and then a small decrease of the molecular nitrogen would allow the line to be observed under the blue condition. Adjustment of the flow rate moved different regions of the nitrogen plasma in front of the spectrometer slits. Figures 6 and 7 compare the spectra from the two conditions. POPULATIONS
The populations of levels excited by the two mechanisms, shown in Fig. 8, were obtained using measured intensities and calculated transition probabilities [ll] 0. S. DUFFENDACK, and R. A. WOLFE, Phys. Rev. 34,409 (1929). [12] S. N. GHOSH and S. K. JAIN, Brit. J. app1. Phys. 17,765(1966).
M. S. MANALIS and H. I?. BROIDA
730
(0)
No RF field
.1
(b)
RF
+.
NI a i
NIP
-
field
Collision
Recombination
Fig. 9. Photoelectric data used as a partial basis for postulating two independent mechanisms for producing N*. Blue glow (left) and green glow (right). Lower spectra with rf field but otherwise identical conditions to upper spectra. NI a refers to 2p3s’2D
-
NI fi refers to 2pz3p2S” -
2p2(1D)3p’2P” 2p2(3P)3d2P
[13, 141. As the levels approach the ionization limit for nitrogen (14.53 eV), the relative population due to the recombination mechanism is about one order of
magnitude greater than that due to the collisional mechanism. The two distributions were normalized around 12 eV. Cascading is a probable cause of the structure in the recombination distribution around 13.2 eV. Two separate mechanisms were found to excite atomic nitrogen. He+ was the precursor of the atomic emission in the green glow and HeZM was the precursor in the blue glow. The green glow was found to be an extremely clean and relatively bright source of excited atomic nitrogen. It should make an excellent source for accurate wavelength measurements which in turn would allow precise determination of [13] H. R. GRIEM, PZcuwa Spectroscopy. McGraw-Hill, Now York (1964). [14] TV. L. WIESE, M. W. SMITHand B. M. GLENNON,Atomic Transition Probubilities. National Bureau of Standards. NSRDS-NBS 4, Vol. One, U.S. Dept. Commerce (1966). [ 1 :i] C. DE WITT COLEMAN,W. R. BOZ~IAN and XV. P. MEGGEBS,Table of Wavenumbers. Vol. I and II. Nat. Bur. Stand. Mono. 3 (1960).
Excitation
of neutral atomic nitrogen in a helium afterglow
731
many new energy levels near the ionization limit. Since the manner in which these levels are populated is not understood quantitatively, intensity measurements could not be used to determine transition probabilities. However, the collisional mechanism of the blue glow results in a level distribution which appears to be in equilibrium and thus could be used to measure transition probabilities. With this latter source, atomic emission often is obscured by strong molecular emission and intensities of transitions near the ionization limit are very weak (Fig. 8). A combination of the best features of both sources might be used to measure transition probabilities.
Excitation of neutral atomic nitrogen in a helium afterglow* M. S. MANALIS and H. P. BROIDA Department
of Physics, University
of California, Santa Barbara, California 93106, U.S.A.
(Received 14 May
AbStract-A
1971)
light source of neutral atomic nitrogen has been developed by mixing traces of
molecular nitrogen with a flowing helium afterglow at pressures near one torr. This source produces radiation from energy levels near the ionization limit of the neutral atom but none from excited states of the nitrogen ion. The excited nitrogen is produced by recombination of electrons with ionized nitrogen atoms formed in dissociative charge transfer reactions of ionized helium with molecular nitrogen. Approximately 400 new atomic nitrogen lines were found. In addition, a second mechanism for exciting atomic nitrogen was isolated. It consists of a two step collisional process in which N, is dissociated by metastable molecular helium and the excitation is produced by collisions with energetic electrons. The characteristic temperature associated with this level population distribution was about lo4 K.
INTRODUCTION ADDITIONAL measurements of wavelengths and intensities of atomic nitrogen transitions are needed. It is known that the available experimental data suffer from uncertainties in the identification of emission lines and inaccurately measured intensities. Reported here is a new method for exciting atomic nitrogen by adding molecular nitrogen to a flowing helium afterglow. This source allows experimental control for identification and wavelength measurement, and is one of the “cleanest” and brightest sources of visible atomic nitrogen radiation. The experimental conditions necessary to produce this source and to obtain complete spectra over the visible and near i.r. spectral region have been described [l]. In order to learn which levels are populated and in what spectral region the various series limits may be found, energy level diagrams are given. Classification of all the spectral lines has not yet been made since sufficiently high spectral resolution was not available. Experimental evidence is given for two independent mechanisms for exciting nitrogen atoms. EXPERIMENTAL Excited atomic nitrogen was produced by injecting molecular nitrogen into a flowing helium afterglow (Fig. 1). A dc discharge operating near 1 kV and 500 mA was used with He at a pressure of one torr and with a flow rate of 300 atm-cc/see. Additional details on this source and information concerning the energetic species present in the afterglow can be found elsewhere [l-3]. Spectra were recorded with a Q-m Ebert scanning monochromator with photoelectric detection (Fig. 2) and with a
* Work supported in part by the Advanced Research Projects Agency and the Office of Naval Research and the U.S. Air Force Office of Scientific Research, Office ofAerospace Research, under Grant No. AFOSR-70-1851. M. S. MANALIB, Ph.D. Thesis, University of California at Santa Barbara (1970); M. S. MANALIS, Phys. Rev. 4A, 364 (1971). [2] J. L. DUNN, Ph.D. Thesis, University of California at Santa Barbara (1966). [3] E.E. FERQUSON,F.C.FERSEKFELD and A. L.~CHMELTEKOPF, in Advances in Atomic and Molecular Phg&x. Academic Press, New York (1969). [l]
1
721
Excitation
of neutral atomic nitrogen in a helium afterglow
725
Table 1 (continued) Id
N
Fe
Ne
1, A
Y, cm-’
6168.81 5167.82 5167.41 5165.90 5165.77 5165.66 5163.77 6162.91 5162.78 5162.61 6160.7 5168.49 5156.72 5155.87 5155.37 5155.26 5154.5 5153.76 5153.05 5151.33 5149.62 5146.6 5146.0 6141.93 5141.16 5136.31 5135.79 5134.39 6132.5 5120.3 5117.88 5116.6 5115.44 5110.52 5100.69 5100.45 5080.47 5037.68
19341.426 19345.131 19346.666 19352.321 19352.808 19353.183 19360.303 19363.528 19364.016 19364.654 19371.82 19380.12 19386.771 19389.968 19391.848 19392.262 19395.121 19397.906 19400.579 19407.056 19413.501 19424.89 19427.16 19438.754 19445.446 19463.807 19465.778 19471.086 19478.26 19524.665 19533.897 19538.555 19543.215 19562.030 19599.729 19600.651 19677.734 19844.874
I
Id
3.5 82.0 3.8 8.0 8.5 4.7 1.6 32.5 1.4 1.6 2.3? 5.2 4.3 3.9 2.6 5.3 2.1? 1.3 2.6 4.3 1.6 1.39 2.3? 5.4 2.0 4.1 3.1 6.1 1.4 1.6 1.6 1.4? 2.3 4.5 4.4 8.8 9.4 8.4
Fe
A, A
y, cm-’
5031.5 5009.2 5009.4 5007.6 4997.1 4993.3 4987.47 4987.3 4982.0 4976.31 4974.82 4973.37 4971.51 4968.31 4966.41 4965.54 4963.98 4962.29 495742 4955.74
19869.25 19957.7 19956.9 19964.08 20006.03 20021.25 20044.654 20045.34 20066.66 20089.606 20095.623 20101.482 20109.043 20121.95 20129.652 20133.179 20139.506 20146.771 20166156 20172.992 20182.440 20190.347 20195.446 20199.934 20205.609 20210.552 20213.126 20213.576 20222.489 20225.435 20247.51 20257.647 20263.724 20324.281 20335.733 20340.615 20344.34 20348.48
4953.42
N
N
4951.48 4950.23 4949.13 4947.74 4946.53 4945.90 4945.79 4943.61 4942.89 4937.5 4935.03 4933.55 4918.85 4916.08 [4914.9] 4914.0 4913.0
I 2.0 1.4 1.5 1.4 1.6 1.4 3.9 1.6 2.2 0.7 2.7 4.6 10.5 3.3 2.0 1.0 20.6 4.6 3.9 10.3 4.8 6.7 14.0 I.0 2.4 2.9 2.0 3.1 2.0 2.1 0.8 37.0 2.5 1.1 1.5 16.2 0.6 0.6
The purpose in constructing the expanded diagram, Fig. 5, was to list all states independent of their particular coupling schemes and to facilitate the recognition of electric dipole transitions. RACAH [9] states that there should be more of an effort to include these states along with Russell-Saunders states. This is difficult because the spectroscopic notation is different for different coupling schemes. But for nitrogen, the states are not all Russell-Saunders coupled and any complete discussion of the problem must include these other coupling schemes. Russell-Saunders states are located on the extreme left (quartet system) and right (doublet system) of the diagram. Each member of a major vertical group shares the same parity quantum This diagram incorporates more than Russell-Saunders states and also number. facilitates recognition of electric dipole transitions. Note the symmetry in energy between the terms of the quartet and doublet systems. The short dashed lines are terms which are predicted on the basis of this symmetry.
[9]
G. RACAH,
Trans.
J. Comm. Spectros.
50,
408
(1960).
-
Excitation
of neutral atomic nitrogen in a helium afterglow
727
For transitions which occur in the visible and near i.r., nearly all previously recorded levels were observed [l]. No levels above the ionization limit were detected. Also light from the high orbital angular momentum states (f) was not observed [lo]. From the energy level diagram, the series limits occur between 3000 and 3500 A for nitrogen. No clear limits were seen spectroscopically. However, in the green glow, energy levels near the ionization limit were substantially populated. Two
MECHANISMSFOR PRODUCTION OF EXCITED NITROGEN
When the amount of molecular nitrogen injected into the helium afterglow is increased from that necessary to obtain the green glow, a strong blue glow appears which is due predominately to the first negative system of N,+ [l, 21 Figs. 6 and 7
NI 3~~P-4p~P'
I _._“____~._
... .. __.&.
’
,
1
l._-,_.*._-
_w_--r~.^~~.“.-_~/--.-_-_.--.
4;75
42150
*
-.-
_____-__
42125
I_
ii
Fig. 6. Spectra taken during three different experimental conditions: blue glow (upper); green glow (middle); helium afterglow only (lower).
(upper) show parts of the spectral region associated with the blue glow. There also is radiation from atomic nitrogen in this glow, but with a different population distribution than in the green glow (Fig. 8). This blue glow always spatially precedes The N lines from the blue were not sensitive to the the green in the afterglow. application of an rf field, while the lines from the green glow were extremely sensitive to the field (Fig. 9). This information suggests two mechanisms for proThe blue glow population distribution was determined ducing excited nitrogen. by using a rf field to quench contributions from the recombination mechanism. The results was a linear distribution corresponding to a temperature of lo4 K (Fig. 8). [lo]
J. W. MCCONKEY,D.J.BURNS 8, 823 (1968).
and J. A.KERNAEAN, J. Quant. Spectrosc.
Radiat.
Transfer
728
M. S. MANALIS
and H.
P. BROIDA
(a)
NI
3s2P-5p2Do
3
5
-Z’2;,;
I
(b)
Fig.
I
7. The upper
spectrum was observed from the blue glow spectrum was observed from the green glow.
NI
population
and the lower
distribution
._
._ En
s
eV
Fig. 8. The relative population per unit statistical weight (arbitrary units) as a function of the electron excitation energy in atomic nitrogen.
Excitation
of neutral
atomic
nitrogen
in a helium afterglow
729
The mechanism for the excitation of atomic nitrogen in the blue region was determined from energetics and qualitative observations. It takes about 10 eV to dissociate the nitrogen molecule, and since N transitions from energy levels of 14 eV were observed in the blue region, a minimum of 24 eV is required. Since there is no long-lived helium species remaining which has that much energy, a two step process may be involved. Qualitative experiments using argon to preferentially remove metastable helium atoms demonstrated that atomic nitrogen was not excited by these atoms [l, 21. Thus, the only species left with sufficient energy are metastable helium molecules, From energetics and the distribution of atomic Hean, and energetic electrons [l]. levels, the nitrogen molecule was most likely dissociated by HeaM and the nitrogen atoms were excited by collisions with electrons. The two step process in which electronically excited atoms are produced in the blue glow is referred to as the collisional mechanism and is written as: HeaM + N, -+ 2N + 2He N+e--+N*-+e. The observation that the metastable helium molecule, instead of the metastable helium atom, dissociates the nitrogen molecule was not expected [ 111. No experimental evidence was found to connect either of the two mechanisms for formation of N* with N,+. The energy of N,+ is most likely dissipated at the walls [la]. Figure 9 is an example of photoelectric data which formed the basis for postulating these two independent mechanisms for exciting atomic nitrogen lines. The top left spectrum shows atomic lines excited in the blue glow (collisional), while the right spectrum shows the same lines excited in the green glow (recombination). Notice that the relative intensity of the nitrogen multiplets has changed. The lower spectra were taken under identical experimental conditions except that a rf field (2450 MHz) was directed towards the plasma. Notice that the He and N lines which were produced by electron recombination have vanished, while the N lines produced by the collisional mechanism still remain (lower left spectrum). The weak N emission seen in the lower right spectrum is due to the collisional mechanism. Identifying atomic lines in the blue glow is very difficult because of the many molecular bands which occur throughout the entire spectral region. Since the green glow emission was predominately from atomic nitrogen, this source is helpful in finding particular lines in the blue glow. The line could be observed under the green condition and then a small decrease of the molecular nitrogen would allow the line to be observed under the blue condition. Adjustment of the flow rate moved different regions of the nitrogen plasma in front of the spectrometer slits. Figures 6 and 7 compare the spectra from the two conditions. POPULATIONS
The populations of levels excited by the two mechanisms, shown in Fig. 8, were obtained using measured intensities and calculated transition probabilities [ll] 0. S. DUFFENDACK, and R. A. WOLFE, Phys. Rev. 34,409 (1929). [12] S. N. GHOSH and S. K. JAIN, Brit. J. app1. Phys. 17,765(1966).
M. S. MANALIS and H. I?. BROIDA
730
(0)
No RF field
.1
(b)
RF
+.
NI a i
NIP
-
field
Collision
Recombination
Fig. 9. Photoelectric data used as a partial basis for postulating two independent mechanisms for producing N*. Blue glow (left) and green glow (right). Lower spectra with rf field but otherwise identical conditions to upper spectra. NI a refers to 2p3s’2D
-
NI fi refers to 2pz3p2S” -
2p2(1D)3p’2P” 2p2(3P)3d2P
[13, 141. As the levels approach the ionization limit for nitrogen (14.53 eV), the relative population due to the recombination mechanism is about one order of
magnitude greater than that due to the collisional mechanism. The two distributions were normalized around 12 eV. Cascading is a probable cause of the structure in the recombination distribution around 13.2 eV. Two separate mechanisms were found to excite atomic nitrogen. He+ was the precursor of the atomic emission in the green glow and HeZM was the precursor in the blue glow. The green glow was found to be an extremely clean and relatively bright source of excited atomic nitrogen. It should make an excellent source for accurate wavelength measurements which in turn would allow precise determination of [13] H. R. GRIEM, PZcuwa Spectroscopy. McGraw-Hill, Now York (1964). [14] TV. L. WIESE, M. W. SMITHand B. M. GLENNON,Atomic Transition Probubilities. National Bureau of Standards. NSRDS-NBS 4, Vol. One, U.S. Dept. Commerce (1966). [ 1 :i] C. DE WITT COLEMAN,W. R. BOZ~IAN and XV. P. MEGGEBS,Table of Wavenumbers. Vol. I and II. Nat. Bur. Stand. Mono. 3 (1960).
Excitation
of neutral atomic nitrogen in a helium afterglow
731
many new energy levels near the ionization limit. Since the manner in which these levels are populated is not understood quantitatively, intensity measurements could not be used to determine transition probabilities. However, the collisional mechanism of the blue glow results in a level distribution which appears to be in equilibrium and thus could be used to measure transition probabilities. With this latter source, atomic emission often is obscured by strong molecular emission and intensities of transitions near the ionization limit are very weak (Fig. 8). A combination of the best features of both sources might be used to measure transition probabilities.