Pickup and photodissociation of hydrogen halides in floppy neon clusters

Neon clusters with an average size in the range = 100-1600 atoms are generated in an adiabatic expansion and are doped with a single HBr or HCl molecule in a pickup process. The hydrogen halide molecule is photodissociated by a UV laser, and the outgoing H fragment is ionized by resonance enhanced multiphoton ionization (REMPI) in a (2 + 1) excitation scheme at a wavelength of 243 nm. The H ions are extracted in a Wiley-McLaren time-of-flight mass spectrometer operating in the low-field mode to be sensitive to low velocities. The measured kinetic energy distributions are compared with quasi-classical molecular dynamics simulations, which allow for a detailed analysis of the underlying processes. Simultaneously, the neon cluster phase behavior, the pickup procedure, and the photodissociation dynamics are investigated theoretically. The phase behavior is studied by means of the instantaneous normal modes approach with a newly introduced projection technique, which allows us to disentangle the phases of the different cluster shells. For the cluster sizes investigated, the cluster core is basically solid or semiliquid, while the outer shell is always liquid. Correspondingly, during the semiclassical pickup simulation most of the HBr dopants stay in the surface of the cluster. Finally, the photodissociation simulation is performed starting either from the quantum mechanical ground state at T = 0 K or from a distribution at T = 10 K. It is demonstrated that the inclusion of the temperature effects is necessary to reproduce the experimental data and, therefore, plays a crucial role in the interpretation of the floppy neon clusters.