Theoretical Studies of Photoionization in Diatomic and Polyatomic Molecules

A review is given of recent theoretical studies of partial-channel photoionization cross sections in diatomic and polyatomic molecules. Results obtained from separated-channel static-exchange calculations and Stieltjes-Tchebycheff moment-theory techniques are compared with recent photoabsorption, electron-impact excitation, fluorescence production, photoelectron spectroscopy, and dipole (e, 2e) measurements. Various structures in the calculated and measured cross sections as functions of incident photon energy are attributed to final-state wavefunctions of either atomiclike or molecularlike composition. Specifically, σ-orbital cross sections in light diatomic and polyatomic molecules are found to be generally dominated by strong σ → σ^* photoionization resonances of molecularlike origin, whereas π-orbital cross sections in such molecules exhibit strong distinctly 2p → kd atomiclike features. These aspects of molecular photoionization are illustrated with the results of detailed theoretical and experimental studies of partial-channel cross sections in CO, CO_2, and H_2CO