Correlation effects and resonances in molecular photoionization

Abstract

The photoionization cross sections of CS$sb2$ leading to the $Xsp2Pisb{g}$, $Asp2Pisb{u}$, and $Bsp2Sigmasb{u}sp+$, and $Csp2Sigmasb{g}sp+$ states of CS$sb2sp+$ have been computed in the multichannel frozen core Hartree-Fock approximation. The results were obtained using the iterative Schwinger variational method using a partial wave expansion up to l = 120. We have found shape resonances in the $kpisb{g}$ and $kpisb{u}$ continuum channels. In particular, the $kpisb{g}$ resonance is seen to affect dramatically both total cross sections and photoelectron asymmetry parameters in all other channels when interchannel coupling is included. The form of the resonant wave function indicates that this resonance is caused by low lying virtual d orbitals on the sulfur atoms. We have found the position of the corresponding poles of the single channel T matrix in the complex plane. We have determined that the position of the pole is relatively insensitive to the interatomic bond lengths. This is indicative of the atomic nature of the resonant state. Vibrationally resolved calculation for the various vibrational transitions show that both the $ksppi$ shape resonances located well above threshold and the $ksigma$ shape resonances below threshold are insensitive to bond length. We have also found near constant vibrational branching ratios in all channels and polarization components. These results indicate that caution must be applied when using bond length sensitivity as an exclusive means to determine the nature of the shape resonance. We have implemented a multichannel complete-active-space configuration-interaction (MCCASCI) approximation to study electron-correlation effects in molecular photoionization. This approach includes both target relaxation and correlation due to coupling between different asymptotic scattering channels. The method employs a numerical CASCI using a single center expansion to evaluate all integrals. We present results of a detailed MCCASCI-Schwinger study of the photoionization of molecular nitrogen in the photon energy region of 19-26 eV, including up to nine coupled electronic channels. The results are in good agreement with the available experimental data. In particular, we obtain for the first time the experimental structure in the (3$sigmasb{g})sp{-1}$ photoelectron asymmetry parameter near 22eV by including the $Csp2Sigmasb{u}sp+$ ion state of N$sb2sp+$.