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, s̀‐orbital cross sections in light diatomic and polyatomic molecules are found to be generally dominated by strong s̀ → s̀* 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, CO2, and H2CO.