Photoabsorption in molecular nitrogen: A moment analysis of discretebasisset calculations in the staticexchange approximation
Abstract
Theoretical investigations of photoexcitation and ionization cross sections in molecular nitrogen are reported employing the recently devised StieltjesTchebycheff momenttheory technique in the staticexchange approximation. The coupledchannel 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 HartreeFock 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 StieltjesTchebycheff vertical photoionization cross section obtained from the discrete pseudospectra is in excellent agreement with recent electronion coincidence measurement of the cross section for parention 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 electronelectron coincidence and synchrotronradiation 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 StieltjesTchebycheff method in the staticexchange approximation under appropriate conditions.
 Publication:

Journal of Chemical Physics
 Pub Date:
 February 1978
 DOI:
 10.1063/1.435836
 Bibcode:
 1978JChPh..68..970R
 Keywords:

 Absorption Cross Sections;
 Ionization Cross Sections;
 Molecular Absorption;
 Nitrogen;
 Photoabsorption;
 Chebyshev Approximation;
 Molecular Excitation;
 Moments;
 Stieltjes Integral;
 Wave Equations;
 Atomic and Molecular Physics;
 33.80.b;
 Photon interactions with molecules