PRODUCT STATE DISTRIBUTIONS AND ANGULAR DIFFERENTIAL CROSS-SECTIONS FROM PHOTOINITIATED REACTIONS OF CHLORINE ATOMS WITH SMALL HYDROCARBONS

The internal state distributions of HCl products from the reactions of Cl atoms with methane, propane, and isobutane are reported. These measurements were carried out using state-selective detection of the HCl(v,J) vibration-rotation states by resonance-enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The reactions were initiated in a crossed, pulsed-flow of the reagents by 355 nm photolysis of Cl-2 to produce the Cl atom reagents. The rotational state distributions in the observed HCl(v = 0 and 1) vibrational levels were found to be quite cold. The relative intensities of REMPI signals for the detection of v = 0 and 1 products were compared in order to gain a measure of the degree of vibrational excitation. Because of the known speed and angular distribution of the Cl reagents, it was possible to obtain information on the product center-of-mass angular distribution from measurement of the product laboratory velocity distribution; the latter could be derived from the observed mass 36 time-of-arrival profile. The HCl(V = 0) product from the Cl + isobutane reaction was found to be mainly backward scattered with respect to the incoming Cl atom. These observations of the product internal state distributions and angular distribution are consistent with a mechanism involving abstraction of hydrogen atoms from the hydrocarbon reagent through a collinear Cl-H-R geometry.