Long time scale dynamics of vibrationally excited (HBr)n clusters.

We investigated the photodissociation dynamics of vibrationally excited HBr molecules and clusters. The species were generated in a molecular beam and excited with an IR laser to a v = 1 vibrational state. A subsequent ultraviolet (UV)-pulse with 243 nm radiation photolysed the molecules to yield H-fragments, which were resonantly ionized by the same UV-pulse (2 + 1 REMPI) and detected in a velocity map imaging (VMI) experiment. We performed action spectroscopy to distinguish between two expansion regimes: (i) expansion leading to isolated HBr molecules and (ii) generation of large (HBr)n clusters. Photodissociation of isolated HBr ( v = 1) molecules in particular J ro-vibrational states yielded faster H-fragments (by approximately 0.3 eV) with respect to the photodissociation of the ground state HBr ( v = 0). On the contrary, the IR excitation of molecules in (HBr) n clusters enhanced the yield of the H-fragments UV-photodissociated from the ground-state HBr ( v = 0) molecules. Our findings show that these molecules are photodissociated within clusters, and they are not free molecules evaporated from clusters after the IR excitation. Nanosecond IR-UV pump-probe experiments show that the IR-excitation enhances the H-fragment UV-photodissociation yield up to ∼100 ns after the IR excitation. After these long IR-UV delays, excitation of HBr molecules in clusters does not originate from the IR-excitation but from the UV-photodissociation and subsequent caging of HBr molecules in v > 0 states. We show that even after ∼100 ns the IR-excited larger (HBr) n clusters do not decay to individual molecules, and the excitation is still present in some form within these clusters enhancing their UV-photodissociation.