Insights into the reaction dynamics of molecules and clusters using femtosecond spectroscopy.

摘要

In this dissertation, studies of the ultrafast dynamics of HBr, HBr-water clusters, HBr clusters, acetone-water clusters and SO₂ clusters are reported.; Interrogation of the state formed by the avoided crossing of the Rydberg E(1Σ+) state and valence state V( 1Σ+) of HBr and DBr revealed dynamic behavior that was different for the two isotopomers. Pump-probe experiments on HBr showed no change in the lifetime over the range of pump wavelengths of 256.7 to 254.7 nm. However, the lifetime of DBr decreased as the pump wavelength was tuned bluer. Tunneling is involved in the scheme.; Dissolution of acids is one of the most fundamental of solvation processes, where experimentally there is still a paucity of information regarding the dynamics of the phenomenon and the nature of the hydration complex leading to ion-pair formation. Herein we address these issues using femtosecond pump-probe spectroscopy. Evidence is presented showing that five water molecules are necessary for complete dissolution of an HBr molecule to form the contact ion-pair H+·Br−(H₂O) _n in the electronic ground state. Studies have revealed that in small mixed clusters (n 5), the ion-pair formation can be photoinduced by electronic excitation.; The ultrafast dynamics of HBr-water clusters have been investigated using the pump-probe technique coupled with reflectron time-of-flight mass spectrometry. HBr clusters, mixed HBr-water clusters and protonated water clusters are observed in the mass spectra. Dynamic studies reveal that when an HBr chromophore of a cluster is excited electronically, solvent reorganization occurs to form the solvent separated ion-pair. The influence of excitation wavelength and clustering on the dynamics are discussed. In addition, further evidence is presented confirming that complete dissolution of HBr requires five solvent molecules in the isolated complex.; The ultrafast dynamics of mixed clusters of acetone and water, H +(CH₃COH₃)_n(H₂O) _m, have been investigated using the pump-probe technique employing a femtosecond laser system coupled to a reflectron time-of-flight mass spectrometer. Studies using an excitation wavelength of 407 nm have revealed that the degree of solvation by both acetone and water molecules has a pronounced influence on the dynamics of acetone. The decay of the excited acetone moiety was found to increase with increasing cluster size in both pure acetone and mixed acetone-water clusters. (Abstract shortened by UMI.)