Investigations of molecular clusters: Excited state photochemistry, solvation effects and high energy processes.

摘要

Cluster science is an area of chemistry that has rightfully attracted more attention over the last half century. This matter of finite dimension is critical for understanding fundamental processes in a wide range of disciplines from atmospheric science to surface chemistry and beyond. The studies in this thesis focus on three main subjects. The first subject discusses a phenomenon termed Coulomb explosion and presents a new technique for determining the kinetic energy release from these events. The second subject presents photochemically induced reactions within mixed water and methanol clusters. Finally, the last topic presents photochemically initiated dynamical data concerning the atmospherically significant molecule sulfur dioxide.; Cluster species are studied in the laboratory with a femtosecond laser system for ionization followed by detection with a time-of-flight mass spectrometer. Two main areas of research include detailing the kinetic energy release following the Coulomb explosion of cluster species and probing the dynamics of molecular species using the established pump-probe spectroscopy method. Coulomb explosion experiments are performed on hydrogen-bonded systems using a novel technique to individualize specific ranges of kinetic energy released following the event. Molecular dynamics clocking experiments are performed on methanol cluster systems, with a focus on solvation effects, as well as on the sulfur dioxide monomer.; A new method is presented for the analysis of high energy multicharged cations produced by Coulomb explosion in a time-of-flight mass spectrometer. The technique employs energy gating through a manipulation of electric potentials in the Wiley McLaren lens assembly, and the use of a reflecting electric field. Water clusters of sizes greater than 20 molecules were irradiated with femtosecond laser pulses to generate protons and multicharged oxygen atoms. The kinetic energy plots for the multicharged oxygen atom indicate the possible existence of solvation shells that give access to higher ranges of kinetic energy. This method is also applied to methanol clusters.; The photochemistry of neat methanol and mixed methanol/water clusters is investigated. The electronic states probed in these experiments are the 3s and 3p Rydberg states. The photochemistry of methanol in the 3p Rydberg state is complex and leads to dissociation products that widely vary, but are dominated by the cleaving of the O-H bond. Neat methanol clusters yield dynamics associated with the time necessary for the internal conversion of methanol from the 3p to the 3s Rydberg state. Mixed clusters of methanol/water exhibit unique characteristics for specific sizes. Excitation of the neutral (MeOH)_nH₂O (n = 2,4) cluster results in dynamics attributed to the formation of a neutral H ₃O molecule that facilely ionizes for detection. (Abstract shortened by UMI.)