ACCURATE PHOTOIONIZATION CROSS-SECTIONS OF DIATOMIC-MOLECULES BY MULTI-CONFIGURATION LINEAR-RESPONSE THEORY

Calculations of valence photoionization cross sections are performed for the diatomic molecules N2, O2, CO and NO, exploring the dependence of the low-energy resonances versus bond distance. Results are presented for calculations carried out in the random phase approximation (RPA) and with a multi-configuration linear response (MCLR) method employing a recently proposed algorithm to obtain pseudospectra from the iterative solutions of the linear response equations. The obtained pseudospectra are used to construct quadrature spectra, which in turn provide the total photoionization/photoabsorption cross sections by the Stieltjes imaging technique. We find that although RPA and MCLR can provide rather different excitation energies and oscillator strengths the integrated cross sections are much the same, reproducing also the same basic trends of cross section enhancements with respect to bond distance. The MCLR pseudospectrum technique is found to reproduce well both low and high spectral moments obtained from experimental data using semiempirical techniques.