Photoionization dynamics of H2 <i>C</i> 1Π<i>u</i>: Vibrational and rotational branching ratios

Abstract

Angle-integrated, rotationally resolved photoelectron spectra were determined for three-photon resonant, four-photon (3+1) ionization of H2 via the C 1Πu, v′=0–4 intermediate levels using a magnetic bottle electron spectrometer. The results confirm and extend our previous angle-resolved results obtained using a hemispherical electron energy analyzer. As predicted by both Franck–Condon factors and recent ab initio Hartree–Fock level calculations, ionizing transitions in which the vibrational level of the resonant intermediate state is preserved in the ion are most probable; however, large deviations from theoretical expectations are observed for photoionization of all intermediate levels. These deviations take the form of a progressive broadening of the ionic vibrational distribution as the vibrational level of the resonant intermediate state is increased. The rotational branching ratios also exhibit systematic variations with both the vibrational level of the C 1Πu state and the vibrational level of the ion. Photoelectron signal from ionization of electronically excited atomic hydrogen produced in the photodissociation of H2 C 1Πu is also observed.