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Linear Momentum Transfer Effects in Molecular Dissociation Produced by Electron Impact
1.G. H. Dunn, Phys. Rev. Letters 8, 62 (1962).
2.R. N. Zare and D. R. Herschbach, Proc. IEEE 51, 173 (1963).
3.J. Solomon, J. Chem. Phys. 47, 889 (1967).
4.J. Solomon, C. Jonah, P. Chandra, and R. Bersohn, J. Chem. Phys. 55, 1908 (1971).
5.G. E. Busch, R. T. Mahoney, R. I. Morse, and K. R. Wilson, J. Chem. Phys. 51, 449, 837 (1969);
5.R. J. Oldman, R. K. Sander, and K. R. Wilson, J. Chem. Phys. 54, 4127 (1971)., J. Chem. Phys.
6.R. W. Diesen, J. C. Wahr and S. E. Adler, J. Chem. Phys. 50, 3635 (1969).
7.G. H. Dunn and L. J. Kieffer, Phys. Rev. 132, 2109 (1963).
8.R. J. Van Brunt and L. J. Kieffer, Phys. Rev. A2, 1293 (1970).
9.M. Misakian and J. C. Zorn, Phys. Rev. Letters 27, 174 (1971).
9.M. Misakian, Dissertation, University of Michigan, 1971 (unpublished).
10.C. Jonah, J. Chem. Phys. 55, 1915 (1971).
11.We assume that the fragment kinetic energy will be large enough so that effects of rotational motion can be neglected (see Refs. 2 and 10). We further note that while the effects of momentum transfer on fragment trajectories can be ignored in photodissociation, a measureable effect is readily observed for molecular dissociation by electron impact (see Refs. 8 and 9).
12.Chantry and Schulz [P. J. Chantry and G. J. Schulz, Phys. Rev. Letters 12, 449 (1964);
12.P. J. Chantry and G. J. Schulz, Phys. Rev. 156, 134 (1967)] have evaluated and discussed the effects of thermal spreading of fragment velocities in dissociative attachment experiments and the implications for measurements of electron affinity. When momentum transfer effects are also included, the velocity distribution acquires an angular dependence.
13.For a diatomic molecule with atoms 1 and 2 and masses and the “recoil” velocity of is given by where μ is the reduced mass and
14.See, for example, G. Herzberg, Molecular Spectra and Molecular Structure, I. Spectra of Diatomic Molecules (Van Nostrand‐Reinhold, New York, 1950).
15.R. Stephan Berry and Svena Erik Nielson, Phys. Rev. A 1, 395 (1970);
15.Paul S. Julienne, Chem. Phys. Letters 8, 27 (1971).
16.Several calculations, both quantum mechanical (R. N. Zare, Ph.D. thesis, Harvard University, 1964; Ref. 10)
16.and classical [Ref. 10 and G. E. Busch and K. R. Wilson, J. Chem. Phys. 56, 3638 (1972)] have been made describing fragment angular distributions from predissociating states following photoexcitation. The results of these calculations indicate a diminished anisotropy for long lived predissociating states; this effect can be expected to carry over to predissociation produced by electron impact.
17.J. C. Pearl, dissertation, University of Michigan, 1970 (unpublished).
18.Preliminary calculations (Pearl) show that if pure etc., wave scattering occurs near threshold, the predicted velocity distribution [Eq. (1)] is affected by only a few percent. However, if there are interference effects, the predictions of Eq. (1) may undergo much greater changes, particularly for large and small angles. Thus a more exact treatment of the problem requires knowledge of the electron resonances near threshold Such considerations are omitted from the present calculation for simplicity.
19.Equation (1) has been verified experimentally for several gases including neon by one of the authors (Misakian) as part of a study of dissociative excitation of A preliminary report has been published (Ref. 9) and a more complete account is now being prepared for publication.
20.Because of “thermal spreading” and momentum transfer in the collision process, contributions to volume element can occur from ’s of different magnitudes if they are close enough to in velocity space. We also make the simplifying assumption that will be the same for ’s of different magnitude while in actuality this is not the case (see Ref. 7).
21.The case of predissociation may be treated in the following approximate fashion. Because the excited rovibronic levels lie within about 0.02 eV of the rovibronic energy, the contributing may be approximated with single ’s associated with the level of each vibrational state.
22.S. Geltman, Phys. Rev. 102, 171 (1956).
22.Dunn and Kieffer have verified (Ref. 7) that the cross section for dissociative ionization of varies near threshold as times’a Franck‐Condon factor. This linear dependence on excess energy has also been observed for dissociative excitation; for exchange processes, the linear range can be limited to (Ref. 9). A further note is that for predissociation between two excited states, the contributing must also be weighted by a factor proportional to the probability for mixing.
23.N. Sasaki and T. Nakao, Proc. Imp. Acad. (Tokyo) 11, 138 (1935);
23.N. Sasaki and T. Nakao, 17, 75 (1941)., Proc. Imp. Acad. (Tokyo)
24.L. J. Kieffer and Gordon H. Dunn, Phys. Rev. 158, 61 (1967).
25.R. N. Zare, J. Chem. Phys. 47, 204 (1967).
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