Chemical dynamics in time and energy space

Abstract

The development of a versatile picosecond ultraviolet/vacuum ultraviolet temporal spectrometer and its potential use for measuring internal energy redistribution in isolated molecules are described in detail. A detailed description of the double-pass Nd:YAG amplifier and the dye amplifiers is given with the pulse energies achieved in the visible, ultraviolet, and vacuum ultraviolet. The amplified visible pulses are shown to be of sub-picosecond duration and near transform limited. The instrument`s temporal response (≤10 ps) is derived from an instrument limited measurement of the dissociation lifetime of methyl iodide at 266 nm. The methyl iodide experiment is used to discuss the various sources of noise and background signals that are intrinsic to this type of experiment. Non-time-resolved experiments measuring the branching ratio and kinetic energy distributions of products from the 193 nm photodissociation of cyclopentadiene and thiophene are presented. These studies were done using the molecular beam Photofragment Translational Spectroscopy (PTS) technique. The results from the cyclopentadiene experiment confirm that H atom elimination to yield the cyclopentadienyl radical is the dominant dissociation channel. A barrier of ≥5 kcal/mol can be understood in terms of the delocalization of the radical electron of the cyclopentadienyl fragment. A concerted elimination yielding cyclopropene and acetylene was also observed and is proposed to occur via a bicyclo-[2.1.0]pent-2-ene intermediate. Two other channels, yielding acetylene plus the CH2CHCH triplet carbene, and CH2 plus 1-buten-3-yne, are postulated to occur via ring opening. The implications of the experimental results for bulk thermal oxidation and pyrolysis models are discussed. The thiophene experiment shows six competing dissociation channels. The postulated intermediates for the various thiophene dissociation channels include bicyclo, ring opened, and possibly ring contracted forms.