Clustering and multiphoton effects in velocity map imaging of methyl chloride

Abstract

We have investigated different effects caused by clustering and multiphoton processes in the photodissociation of CHCl using velocity map imaging (VMI). Single-photon dissociation of CHCl at 193.3 nm yields CH and Cl fragments probed subsequently by resonance enhanced multiphoton ionisation (REMPI) at 333.52 nm and 235.31 nm, respectively. In the clusters, the CH fragments are strongly ‘caged’ losing most of their kinetic energy. Consequently, the VMI is dominated by a sharp central peak. On the other hand, the Cl fragments lose a broad range of energies in the cluster, yielding a blurred isotropic ‘central blob’ in VMI. At an increased 193 nm photon flux, another central feature appears in the CH images for isolated CHCl molecules. This ‘hour-glass’ shaped image corresponds to the CHCl multiphoton ionisation at 193 nm and subsequent CHCl ion photodissociation at 333 nm. The ion photodissociation is a parallel process with the anisotropy β-parameter values between +1.3 and +2.0. The total kinetic energy release (TKER) distribution suggests a high CH vibrational excitation. The CHCl ion photodissociation is discussed and compared to the photodissociation of similar CHBr ion.