Influences of cluster solvation on excited state energy transfer processes.

摘要

The influences of cluster solvation on photo-induced chemical processes are investigated using the ultrafast pump-probe technique and time-of-flight mass spectrometer. Particular interest is focused on the solvation of atmospheric molecules and the photochemistry of the solvated species. In this thesis, the photochemistry of two cluster-solvated atmospheric species is discussed. The first system is that of SO2 clusters and SO2 solvated in water clusters. The second is the excited-state ion-pair formation of HI in water clusters.; The SO2 studies presented here comprise an investigation of the influence of cluster solvation on the excited-state dynamics of the SO 2, molecule. Studying the dynamics of SO2 solvated in SO 2 and H2O clusters sheds light on the effect that interactions with species such as cloud droplets and aerosol particles may have on the potential energy surface and thus the reactivity of atmospheric SO2. The excited-state dynamics of (SO2)m and SO2(H 2O)n clusters following excitation by ultrafast laser pulses in the range of 4.5 eV (coupled 1A2, 1B 1 states) are investigated and the results are found to be in good agreement with published computational work. Possible sources of the observed trends in the measured lifetimes associated with these states are considered. A mechanism involving charge transfer to solvent (CTTS) is proposed as the source of the excited-state dynamics that follow the 9 eV excitation of the SO2 F band within (SO2)m clusters. The CTTS mechanism is supported by calculations of the energetics of the process and the observed trends in the excited-state lifetimes which correlate very well with the calculated energies. CTTS is not expected to be feasible in SO2(H2O) n clusters due to the low electron affinity of water, a factor that is believed to play a key role in the process for pure SO2 clusters. However, reactions within the cluster leading to ion-pair formation are energetically feasible and may account for the observed dynamics.; The HIm(H2O)n cluster dynamics studies presented here elucidate details of the ion-pair formation process that occur during acid solvation. An important requirement for the reactions of halide ions to take place on the surface of atmospheric particulates is that a sufficient number of mobile water molecules must be present for the dissolution of the halogen-containing acid to take place.