An experimental survey of the low energy electron scattering spectrum of nitrogen

Planet. Space Sci. 1969, Vol. 17, pp. 1527 to 1537.

Permnon

Press. Printed in Northern Ireland

AN EXPERIMENTAL SURVEY OF THE LOW ENERGY ELECTRON SCATTERING SPECTRUM OF NITROGEN Department

ALBERT J. WILLIAMS III* and JOHN P. DOERING? of Chemistry, The Johns Hopkins University, Baltimore, Md. 21218, U.S.A. (Received

25 March 1969)

Abstract-The electron energy loss spectrum of nitrogen has been studied at incident energies from 9 to 50 eV with an electron spectrometer. Scattering angles from 4” to 30” were used. Energy resolution varied from 35 to 100 meV (FWHM). The spectra show that at incident energies less than 20 eV, which are of greatest relevance to atmospheric processes, the energy loss spectrum is dominated by the electric quadrupole transition X1x:,’ - allIl, and the spin-forbidden singlet-triplet transitions X1&+ --, A%,+, X’C,+ - 8Q, and X1&,+ + C%. These electron excitations are responsible for the first and second positive groups and LymanBirge-Hopfield bands which are the most prominent of the atmospheric N, emissions. The electric dipole transition X1&+ --, &III, which is one of the strongest features of the 50 eV electron energy loss spectrum is shown to be extremely weak at an incident energy of 15 eV. The results suggest that the observed intensities of atmospheric N, emissions in the aurora are entirely compatible with low energy electron impact excitation mechanisms. INTRODUCTION

knowledge of electron inelastic collision cross sections in nitrogen is of great importance to the understanding of many different aspects of planetary atmospheres, A variety of measurements (1-5) of nonthermal electron energy distributions in the aurora and airglow have shown that the differential electron flux decreases rapidly above 5 or IO eV. Since the thresholds for excitation of the important N, molecular emissions observed in the atmosphere are between 6 and 13 eV, it follows that the electron excitation cross sections of greatest interest to aeronomy are those for the excitation of N, by electrons of energy from threshold to perhaps 50 eV. Unfortunately, the 5-50 eV energy region has not been well investigated since it lies between the range covered by threshold experiments such as the trapped electron method of Schulz(6’ and its variations(7s8) and the higher energy inelastic scattering work of Lassettre and his collaborators.(g*lO) Some work has been done on the use of transport coefficients for the determination of inelastic electron collision cross sections from 0.03 to 30 eV by Englehardt ef al., (W but at the higher energies, such a method does not give detailed information due to its inherent lack of energy resolution. A number of investigators(l~,l3) have used the optical emissions from a given excited state to determine the electron excitation cross section for the state; but measurements of this sort cannot follow the excitation cross section more than a few volts above threshold because of cascading and the onset of other processes. Perhaps the most widely used excitation cross sections have been the phenomenological calculations of Green and Barth.(14) The most direct method for studying electron inelastic collision cross sections is by means of an electron scattering apparatus in which a beam of electrons of known current and geometry is scattered by a gas of known pressure and the resulting momentum distribution of the scattered electrons is determined by an anaIyzer of known transmission. Accurate

* Present address: Woods Hole Oceanographic t Alfred P. Sloan Fellow.

I~titution, 1527

Woods Hole, Massachusetts,

U.S.A.

1528

ALBERT

J. ALLIES

III

and JOHN P. DOERING

Such an ideal apparatus has yet to be constructed ; however, advances in low energy electron energy analyzers by Kuyatt and Simpson (15~r6)have made possible substantial improvements over previous methods. Electron spectrometers of the Kuyatt and Simpson type have been used by ourselves, Kuppermann and his collaborators,tls) as well as by Heideman er ~1.‘~~’to study inelastic scattering processes in nitrogen at energies from 15 eV to 50 eV. The present paper reports a survey of the electron energy loss spectrum of nitrogen at energies near threshold. We regard this work as a necessary first step in the eventual measurement of these important inelastic collision cross sections, EXPERIMENTAL

The apparatus used for the present work will be described in detail elsewhere.(20) Briefly, it consists of two similar electron energy analyzers used as an electron monochromator and scattered electron analyzer. These analyzers were constructed following the recent advances in low-energy electron spectrometer design made by Simpson and Kuyatt.(1.S*1s*21) The analyzers are mounted about a collision chamber which consists of a cylinder split in a plane perpendicular to the cylindrical axis. The upper part of the cylinder which carries the monoc~omator rotates with respect to the lower part which contains the analyzer. The electron-optic axis is inclined 20” to the dividing plane. The scattering angle can be varied from -2” to over 100”. This design of the collision chamber provides a direct optical path from the scattering center to outside the vacuum system and will eventually allow the system to be used for simultaneous optical excitation and electron scattering measurements. An electron multiplier detector was used. Pulses were amplified and discriminated. The discriminator output was fed to either a ratemeter or a 128 channel multiscaler. Both types of analysis were used for the spectra in this paper. The spectra presented in this paper were taken under single collision conditions so that multiple scattering processes were not important. Energy resolutions from 35 to 100 meV (full width at half maximum) were used. Severe differences in transmission due to the electron-optical analogue of chromatic aberration have been largely eliminated in modern electron spectrometers. However, small effects undoubtedly remain. We believe that the relative peak heights in our spectra are accurate to within 10 per cent except where the difference between the energy loss and impact energy is less than 1.5 eV, i.e. very near threshold. RESULTS

An energy level diagram of N, which includes the relative Franck-Condon factors for the overlap of the various vibrational states with the VII= 0 level of the ground state where these are known is shown in Fig. 1. Figure 2 shows a survey of the 50 eV electron impact spectrum taken at a scattering angle of 4”. This spectrum agrees closely with a similar one taken at about the same resolution at 50 eV by Lassettre et al. (lo) Visible in this spectrum are the transitions to the all&, state between 8.5 and 10 eV, the u”lzg+ state at 12.28 eV, and the blff, states from 12.5 to 12.8 eV. The strong peak at 12.93 eV is an unresolved combination of the states p’lZL+ and IllI, as has been recently shown by Geiger and SchrGder.(2s) A number of higher energy states including many Rydberg transitions are visible above 13 eV. In Fig. 3 is shown a detailed spectrum also taken at 50 eV and 4” of the energy loss region between 9 and 105 eV. A number of states in this region have been observed spectroscopically in emission to lower lying excited states. It was of interest to us to determine whether any of these states could be seen in transitions directly from the ground state

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FIG. 1. ENERGY LEVEL DIAGRAM OF N,. The relative Franck-Condon factors for the overlap of the various vibrational states with the # zzz0 level of the ground state are shown increasing to the right next to the levels in those cases where they are availabie. Levels and Franck-Condon factors for states A, B,B',a',a,w, and C from Ref. (22). Levels for states C”, D, and E from Ref. (23). Levels for allowed singlets from Ref. (24). Terms for singIets from Ref. (25). Vibrational assignments for singlets from Ref. (26). Assignment of the a” state from Ref. (27). 1529

1530

ALBERT J. WLLfAMS

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FIG. 3. DETAILED ENERGY-LOSS SPECTRUM OF THE 9*0-10.5 eV REGION.

Primary energy 50 eV, scattering angle 4”, multiscaler

mode.

excited by electron impact. The &II, state is certainly excited much more strongly under these conditions than anything else. However, significant intensity can be observed between the t” = 6,7 and 8 Ievels of the allYI% state which apparently corresponds to a weak transition to the dAll state. Transitions to the u’lI& and X32%- states were not observed. A survey spectrum taken at 2” and 15.0 eV impact energy is shown in Fig. 4. In contrast to the 50 eV spectrum of Fig. 2, a number of spin-forbidden singlet-triplet transitions excited by electron exchange appear at this much lower impact energy. Both the A3xU+ and PII, triplet states can be seen although it is difficult to appreciate the relative strength of these excitations since the Franck-Condon factors for transitions to the various vibrational levels of these states from the U” = 0 level of the ground state are of about the same size for a number of levels. The C31f, state is much more easily observed since the equilibrium i~ternucIear separation of this state is almost the same as the ground state so only four significant bands are produced. Also by comparison with Fig. 2, it can be seen that the electric quadrupole transition to the u”~~~+ state(27) is fairly strongly excited at this low energy, but the electric dipole transitions to the 12.5-13 eV states which are so prominent in the 50 eV spectrum are barely detectable at 15 eV. Figure 5 shows a 15 eV spectrum taken at a scattering angle of 20”. Since the scattering volume is much smaller at this large angle, the signal to noise ratio in this spectrum is not as good as for the previous ones. There are no dramatic changes in the spectrum at the two angles. The C3D, excitation cross section does, however, increase somewhat with respect to the &‘I, cross section as would be expected.

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FIG. 4. ENERGY LOSS SPECTRUM FROM 7-O TO 13.OeV. Primary energy 15-O eV, scattering angle 2”. Note change in accumulation 1532

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OF NITROGEN 0. 20.

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FIG.5. ENERGY LOSSSPECTRUMFROM 6.0TO 14.0eV. Primary energy 15.0 eV, scattering angle 20”. 7

1533

FIG. 6. ENERGY LOSS SPECTRUM FROM 7-O TO !+OeY. Primary energy lO*OeV. Scattering angles lo”, 20”, and 30”. Note different accumulation times per channel at the different angles. 1534

ELECTRON

SCA~ERING

SPECTRUM

OF ~TROGE~

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1536

ALBERT

J. WILLIAMS

III and JOHN

P. DOERING

Figure 6 shows a IO.0 eV spectrum taken at scattering angles of IO’, 20” and 30”. The spectrum remains relatively unchanged at the three angles. The strongest features are the and B311, states. It appears that the two states are excited about equally since the A3Z;\U3. strongest A state band at u’ = 8 is about 3 the size of the composite A(v’ = 10) and B(u’ = 2) band. The allIu state v’ = 0, 1 and 2 bands can be seen in this spectrum but these features are weak compared to the singlet-triplet transitions. This is in contrast to the situation at 15 eV. Figure 7 shows 9 eV spectra taken at 10” and 30”. Here, the allI, state can no longer be identified with certainty. As was the case at 10 eV, both the A”&+ and B3111,states are excited although the 3 state excitation is somewhat weaker at this energy. Figures 6 and 7 show that the only inelastic processes available to a P-10 eV electron in the energy loss region above 7 eV are the excitation of the A3zu+ and B311, states through singlet-triplet transitions from the ground state. CONCLUSIONS

The data presented in this paper do not allow us to make quantitative measurements of the relative excitation cross sections at different impact energies. However, we can come to a number of interesting conclusions about the relative importance of various electron impact processes in the atmosphere. It is obvious that the most important atmospheric emissions from molecular nitrogen, the Lyman-Birge-Hopfield (LBH) (al& + X1zg+) system and first and second positive groups (B311, + A3&+, C31’I, -+ B311,), are strongly excited by electrons in the 15 eV energy range. The observation of Miller et al. w that the Birge-Hopfield (BH) system (NI, -+ X1zg+) is weak compared to the LBH system in auroras can be understood on the basis of the small relative excitation cross section for the blII, state at 15 eV and the rapidly decreasing differential non-thermal electron energy spectrum. In auroras, there are apparently not sufficient numbers of high energy non-thermal efectrons to give strong excitation of the BH system. Heideman et ~1.(‘~) have shown that the excitation cross section for the 11.87 eV E”z,+ state exhibits a sharp resonance-like onset near 12 eV. The sharpness of this cross section is demonstrated by Figs. 4 and 5 at 15 eV in which the E state cannot be detected with certainty. We have, however, detected the E state at lower impact energies. Since this excitation function is large over such a small energy range, the Estate is probably not very important in atmospheric processes. These results show that the only electronic transitions available below 10 eV are the A and B triplet states. Vibrational excitation becomes important at about this point, however. Finally, it is gratifying to note that the important amoral emissions from molecular nitrogen are just those which are found to be excited strongly in the low energy electron impact spectrum. The electron impact spectrum at primary energies greater than 25 eV does not appear to be relevant to atmospheric processes. Acknowledgement-This tration.

work was supported by a grant from the National Aeronautics REFERENCES

1. 2. 3. 4. 5. 6.

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SCATTERING

SPECTRUM

OF

NITROGEN

1537

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