Ultraviolet Photodissociation Dynamics of the Propargyl Radical

Abstract

Ultraviolet (UV) photodissociation dynamics of jet-cooled propargyl (C(3)H(3)) radical is studied in the photolysis wavelength region of 230 to 250 nm with high-n Rydberg atom time-of-flight (HRTOF) and resonance enhanced multiphoton ionization (REMPI) techniques. In this wavelength region, the photofragment yield (PFY) spectra of the H + C(3)H(2) product channel are obtained by using propargyl chloride, allene, and propyne as precursors of the C(3)H(3) radicals, and they have a broad peak centered near 240 nm and are in good agreement with the previous UV absorption spectrum of C(3)H(3) by Fahr et al. The H + C(3)H(2) product translational energy distributions, P(E(T))'s, are obtained from all three precursors and are essentially the same. The P(E(T)) distributions peak at approximately 5 kcal/mol, and the fraction of average translational energy in the total excess energy, f(T), is approximately 0.3. The H-atom product angular distribution is isotropic, with the anisotropy parameter beta approximately 0. The dissociation mechanism is consistent with internal conversion of the electronically excited propargyl followed by unimolecular decomposition on the ground state. Our study supports the previously observed UV absorption spectrum of propargyl near 240 nm by Fahr et al. and is in general agreement with the results in the UV photodissociation of propargyl by the groups of Chen and Neumark, but disagrees with the recent theoretical calculations by Eisfeld.