Non-Franck-Condon effects induced by shape resonances and orbital evolution in resonance enhanced multiphoton ionization of small molecules

We review and discuss some recent resonance enhanced multiphoton ionization (REMPI) studies of small molecules, emphasizing the role that shape resonances and Rydberg orbital evolution play in determining vibrational and rotational ion distributions. Strong non-Franck-Condon effects observed in ion vibrational distributions for REMPI of O₂ via the C³Π_g and d¹Π_g Rydberg states are partially attributed to a σ_u shape resonance, previously observed in ground state photoionization of diatomic molecules. Autoionization of repulsive valence states also induces non-Franck-Condon effects in these REMPI spectra. Significant non-Franck-Condon effects in molecular REMPI spectra also arise from a mechanism associated with rapid evolution of the resonantly populated Rydberg orbital with changing inter-nuclear distance. These effects should be most pronounced in diatomic hydrides, and are illustrated by theoretical predictions of vibrational and rotational ion distributions for REMPI of OH and CH via the D²Σ⁻ and E'²Σ⁺ Rydberg states, respectively.