Two-color studies of CH3Br excitation dynamics with MPI and slice imaging

Abstract

Two-color pump–probe experiments were performed to explore the multiphoton dynamics of CH₃Br at high excitation energies of 8–10 eV, involving two-photon resonant excitations to a number of np and nd Rydberg states (pump) followed by REMPI detection (probe) of the Br, Br* and CH₃(X) photoproducts. Slice images of Br⁺ and CH₃⁺ ions were recorded in pump-only, probe-only and pump and probe experiments. Kinetic-energy release spectra (KERs), as well as spatial anisotropy parameters, were extracted from the images to identify the processes and the dynamics involved. Predissociation channels, following the two-photon resonant excitations and non-resonant photodissociation forming CH₃(X) and Br/Br*, were identified and characterized. Furthermore, the probe excitations for CH₃(X) involved near-resonant excitations to lower energy 5s Rydberg states of CH₃Br. In three-photon excitation processes, a striking contrast is seen between excitations via the p/d and the s Rydberg states. Involvement of high-energy interactions between Rydberg and ion-pair states is identified.