Theoretical investigations in vibrational spectroscopy

Abstract

Radiationless transitions. Franck-Condon factors govern many important molecular processes, including electronic spectroscopy and radiationless transitions. Those processes that are classified as "vertical" transitions are well understood via the Franck-Condon principle and involve large overlaps. On the other hand, those that fall in the "nonvertical" transition category are more problematical. Not only are the overlaps small, the nuclei must tunnel from one configuration to another in order for the processes to occur, i.e. they are highly nonclassical. Consequently, our usual intuitions based on the Franck-Condon principle may fail. Thus, it is our goal to provide a framework whereby we may establish new intuitions to understand these processes. We adopt the concept of accepting modes and develop a technique which generalizes these to accepting zones in phase space.Helium clusters. Currently, much research concentrates on elucidating the properties of helium clusters by probing how they affect embedded impurities. A Diffusion Monte Carlo study was conducted to calculate frequency shifts for the asymmetric stretch, symmetric stretch, and bend fundamentals of a water monomer embedded in helium clusters. We included all degrees of freedom in the simulation and studied clusters ranging in size from one to fifteen helium atoms. The asymmetric stretch fundamental exhibits a red shift for most cluster sizes studied and is on the order of 1.5 $cm\sp{-1}$. To the contrary, the symmetric stretch and bend fundamentals exhibit shifts to the blue in the range from 4.0 to 6.0 $\sp{cm-1}$. The helium cluster is not isotropically distributed about the water monomer, which may account for the differences.