REACTION OF CL WITH VIBRATIONALLY EXCITED CH4 AND CHD3 - STATE-TO-STATE DIFFERENTIAL CROSS-SECTIONS AND STERIC EFFECTS FOR THE HCL PRODUCT

The mechanism for the reaction of atomic chlorine with vibrationally e xcited methane is investigated by measurement of correlated state and scattering distributions using the method of core extraction (see prec eding paper). Laser photolysis of molecular chlorine creates monoenerg etic chlorine atoms (>98% (ClP3/2)-P-2) that react with vibrationally excited methane molecules prepared by linearly polarized infrared lase r excitation. The resulting HCl product population distributions are d etermined by (2+1) resonance-enhanced multiphoton ionization (REMPI), and the differential cross section for each product rovibrational stat e is measured by core extraction. Approximately 30% of the product is formed in HCl(upsilon=1,J) with a cold rotational distribution; the re maining population is formed in HCl(upsilon=0,J) and is more rotationa lly excited. We observe a rich variation of the scattered flux that is dependent on the internal-energy state of the product. The HCl(upsilo n=1) product is sharply forward scattered for low J and becomes nearly equally forward-backward scattered for high J; the HCl(upsilon=0,J) p roduct is back and side scattered. The reactions of Cl with C-H stretc h-excited methane (CH4) and C-H stretch-excited CHD3 are found to have similar angular and internal-state distributions. Observation of the spatial anisotropy of the HCl(upsilon=0,J=3) product shows that signif icant vibrational excitation of the methyl fragment does not occur. Th e measured spatial anisotropy is most consistent with a model in which backscattered HCl(upsilon=0,J=3) is formed in coincidence with slight methyl vibrational excitation and the forward-scattered HCl(upsilon=0 ,J=3) is formed in coincidence with no methyl excitation. The approach of the attacking chlorine atom with respect to the C-H stretch direct ion can be varied by rotating the plane of polarization of the infrare d excitation. A marked steric effect is observed in which Cl atoms app roaching perpendicular to the C-H stretch preferentially yield forward -scattered HCl(upsilon=1) product. On the other hand, the reaction is weakly dependent on the rotational quantum state of CH4(upsilon(3)=1,J ), and on the rotational polarization. The data are consistent with a model that has a widely open ''cone of acceptance'' in which the impac t parameter controls the internal-state and scattering distributions o f the HCl product. (C) 1995 American Institute of Physics.