Vibrational state dependence of ionic rotational branching ratios in resonance enhanced multiphoton ionization of CH

Abstract
We show that rapid evolution of a Rydberg orbital with internuclear distance in a resonance enhanced multiphoton ionization (REMPI) process can have a profound influence on the production of molecular ions in alternative rotational states. This is illustrated by calculations of ionic rotational branching ratios for (2+1’) REMPI via the O11 (20.5) branch of the E2Σ+(3pσ) Rydberg state of CH. The rotational propensity rule for ionization changes from ΔN=oddN=N+Ni) at lower vibrational excitation, as expected from the ΔN+l=odd selection rule, to ΔN=even at higher vibrational levels. This effect is expected to be quite general and should be most readily observable in diatomic hydrides.