Femtosecond time-resolved charged particle imaging studies of the ultraviolet photodissociation of the NO dimer

Abstract

Ultraviolet photodissociation dynamics of the NO dimer into NO(Α)+NO(Χ) in a supersonic molecular beam is studied by femtosecond pump-probe charged particle imaging. Time-resolved photoelectron imaging reveals that photoionization from the Franck–Condon region of the excited state(s) reaches vibrationally excited states of the NO dimer cation, resulting in spontaneous fragmentation into NO+NO+ even at the shortest pump-probe time delays. The corresponding photoelectron energy distribution extending up to more than 1 eV above the ionization threshold indicates a large structural difference between the dimer cation and the photoexcited neutral state. From the isotropic photoelectron distribution, this state is assigned to a valence state rather than a Rydberg state. The characteristic photoelectron distribution and also the small NO dimer ion signal vanish within 200 fs. In the same time the photoelectron image rapidly evolves from an almost isotropic distribution into an anisotropic one characteristic for ionization from the 3s Rydberg state of the NO dimer that is adiabatically correlated with the NO(Α)+NO(Χ) fragments. Time-resolved photoion imaging used to measure the translational energy release in the NO(Α)+NO(Χ) dissociation channel reveals that at least one of the fragments, most likely NO(Χ), has an inverted vibrational state distribution. The strong vibrational excitation is in qualitative agreement with a large structural change of the NO dimer in the excited state from the neutral ground state, which presumably involves excitation of the NO stretching vibration in the dimer.