Photodissociation–ionization dynamics of molecular chlorine Rydberg states using velocity map imaging

Abstract

Velocity map images are reported for photoelectrons and atomic chlorine ions produced during one-color REMPI (resonance enhanced multiphoton ionization) of molecular chlorine via the v=0–15 vibrational levels of the 2 1Πg(4s) Rydberg state. Previous magnetic bottle photoelectron studies by Koenders et al. of the same process have shown that REMPI proceeds by two-photon resonant excitation, followed by core-excitation to a super-excited Rydberg state at the three-photon level. This state undergoes (auto)-ionization, resulting in the formation of ground-state molecular chlorine ions, and/or dissociation to neutral products, resulting in the production of electronically excited neutral chlorine atoms. Photoelectrons arise from ionization of Cl2 and electronically excited Cl atoms, while Cl+ ions arise from the dissociation of Cl2+ and from ionization of the excited Cl atoms. The chlorine ion velocity map images reveal new information on the dissociation–ionization dynamics of superexcited Cl2 and the dissociation dynamics of the subsequently formed Cl2+ ions. In the latter case an unexpected low-energy Σ←Π (perpendicular) dissociation pathway to Cl+(1D)+Cl(3P) product atoms is observed. Results from the photoelectron images are compared with those from the magnetic bottle studies. While the imaging kinetic-energy resolution is less than that of the magnetic bottle spectrometer, the angular distribution information and lack of velocity bias of imaging is advantageous. Most of the trends observed in the electron and Cl+ images can be rationalized in terms of single-electron excitation processes and the known molecular orbital structure of the electronic states involved.