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Photoabsorption in molecular nitrogen: A moment analysis of discrete-basis-set calculations in the static-exchange approximation

Rescigno, T. N. and Bender, C. F. and McKoy, B. V. and Langhoff, P. W. (1978) Photoabsorption in molecular nitrogen: A moment analysis of discrete-basis-set calculations in the static-exchange approximation. Journal of Chemical Physics, 68 (3). pp. 970-982. ISSN 0021-9606. doi:10.1063/1.435836.

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Theoretical investigations of photoexcitation and ionization cross sections in molecular nitrogen are reported employing the recently devised Stieltjes–Tchebycheff moment-theory technique in the static-exchange approximation. The coupled-channel equations for photoabsorption are separated approximately by identifying the important physically distinct excitation processes associated with formation of the three lowest electronic states of the parent molecular ion. Approximate Rydberg series and pseudospectra of transition frequencies and oscillator strengths are constructed for the seven individual channel components identified using Hartree–Fock ionic core functions and normalizable Gaussian orbitals to describe the photoexcited and ejected electrons. Detailed comparisons of the theoretically determined discrete excitation series with available spectral data indicate general accord between the calculated and observed excitation frequencies and oscillator strengths, although there are some discrepancies and certain Rydberg series have apparently not yet been identified in the measured spectra. The total Stieltjes–Tchebycheff vertical photoionization cross section obtained from the discrete pseudospectra is in excellent agreement with recent electron–ion coincidence measurement of the cross section for parent–ion production from threshold to 50 eV excitation energy. Similarly, the calculated vertical partial cross sections for the production of the three lowest electronic states in the parent molecular ion are in excellent accord with the results of recent electron–electron coincidence and synchrotron–radiation branching ratio measurements. The origins of particularly intense resonancelike features in the discrete and continuum portions of the photoabsorption cross sections are discussed in terms of excitations into valencelike molecular orbitals. Small discrepancies between theory and experiment are attributed to specific autoionization processes and channel couplings not included in the calculations. In contrast to previously reported model or local potential studies, the present results employ the full nonlocal and nonspherical molecular Fock potential in ab initio photoabsorption calculations. The excellent agreement obtained between theory and experiment in molecular nitrogen suggests that highly reliable photoabsorption cross sections for diatomic molecules can be obtained from Hilbert space calculations and the Stieltjes–Tchebycheff method in the static-exchange approximation under appropriate conditions.

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Additional Information:Copyright © 1978 American Institute of Physics. Received 3 October 1977. The authors acknowledge helpful discussions and correspondence with R. A. Bonham, C. E. Brion, D. C. Cartwright, D. E. Eastman, T. Gustafson, A. U. Hazi, Y.-K. Kim, E. W. Plummer, M. J. van del' Wiel, and N. Winter. We also thank W. P. Reinhardt and the JILA Fellows for their kind hospitality to PWL, C. T. Corcoran for computational assistance, and R. H. Garstang and G. B. Ellison for bringing the review of Lofthus and Krupenie to our attention. Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society for partial support of these studies. This work was performed under the auspices of the U.S. Energy Research and Development Administration under contract No. W-7405-Eng-48. [B.V.M. was] supported by a grant from the National Science Foundation. [P.W.L. was a] JILA Visting Fellow, 1976. Supported in part by the Donors of the Petroleum Research Fund, administered by the American Chemical Society.
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Energy Research and Development AdministrationW-7405-Eng-48
American Chemical Society Petroleum Research FundUNSPECIFIED
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Deposited On:30 Jul 2008 20:35
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