Photochemistry of Hydrogen Halides on Water Clusters: Simulations of Electronic Spectra and Photodynamics, and Comparison with Photodissociation Experiments

The photochemistry of small HX center dot(H2O)(n), n = 4 and 5 and X = F, Cl, and Br, clusters has been modeled by means of ab initio-based molecular simulations. The theoretical results were utilized to support our interpretation of photodissociation experiments with hydrogen halides on ice nanoparticles HX center dot(H2O), n approximate to 10(2)-10(3). We have investigated the HX center dot(H2O) photochemistry for three structural types: covalently bound structures (CBS) and acidically dissociated structures in a form of contact ion pair (CIP) and solvent separated pair (SSP). For all structures, we have modeled the electronic absorption spectra using the reflection principle combined with a path integral molecular dynamics (PIMD) estimate of the ground state density. In addition, we have investigated the solvent effect of water on the absorption spectra within the nonequilibrium polarizable continuum model (PCM) scheme. The major conclusion from these calculations is that the spectra for ionic structures CIP and SSP are significantly red-shifted with respect to the spectra of CBS structures. We have also studied the photodynamics of HX center dot(H2O)(n) clusters using the Full Multiple Spawning method. In the CBS structures, the excitation led to almost immediate release of the hydrogen atom with high kinetic energy. The light absorption in ionically dissociated species generates the hydronium radical (H3O) and halogen radical (X) within a charge-transfer-to-solvent (CTTS) excitation process. The hydronium radical ultimately decays into a water molecule and hydrogen atom with a characteristic kinetic energy irrespective of the hydrogen halide. We have also investigated the dynamics of an isolated and water-solvated H3O radical that we view as a central species in water radiation chemistry. The theoretical findings support the following picture of the HX photochemistry on ice nanoparticles investigated in our molecular beam experiments: HX is acidically dissociated in the ground state on ice nanoparticles, generating the CIP structure, which is then excited by the UV laser light into the CTTS states, followed by the H3O radical formation.