The internal state distributions of HCl products from the reactions of C1 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 Clz to produce the C1 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 C1 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 C1 + isobutane reaction was found to be mainly backward scattered with respect to the incoming C1 atom. These observations of the product intemal state distributions and angular distribution are consistent with a mechanism involving abstraction of hydrogen atoms from the hydrocarbon reagent through a collinear C1-H-R geometry.

Dynamics of the O+-H2 reaction. II. Reactive and nonreactive scattering of O+(4 S 3/2) at relative energies above 13 eVWe present a study of reactive and nonreactive collisions between 0and H 2 0 over the collision energy range from 0.67 to 1.07 eV. Kinetic energy analysis of the 0 -scattered nonreactive1y from H 2 0 shows two components: The first arises from direct scattering and is nearly quasielastic, while the second occurs at significantly lower barycentric energies and corresponds to 0 -ejected without reaction from electrostatically bound 0 -. H2 0 complexes formed by approaching reagents. This latter flux is significantly more intense than the reactive OH -flux. The kinetic energy distributions for the low energy 0 -nonreactive flux are in qualitative agreement with statistical phase space theory, although recoil distributions that model the exit channel by an r -4 potential underestimate the kinetic energy release. The reactive flux distributions show a strong energy dependence. At the lowest collision energy, the OH -is produced through two pathways, the first involving the participation of a complex living a fraction of a rotational period, the second producing OH -strongly backward scattered and with a much broader kinetic energy distribution. With increasing collision energy, the complex contribution to the scattering falls off rapidly, and product formation moves from the backward hemisphere to the forward direction. The angular distribution asymmetries at the lowest collision energies can be interpreted in terms of the osculating model for chemical reactions taking place in a fraction of a rotational period of the intermediate complex. This model suggests that the complex lifetime is ~ 250 fs at collision energies between 0.7 and 0.8 eV, a result in good agreement with Rice-Ramsperger-Kassel-Marcus (RRKM) calculations. The kinetic energy distributions at these energies are in good agreement with statistical phase space theory calculations. At the highest collision energies, still below the threshold for impUlsive stripping collisions, the OH -product is scattered sharply forward with a broad kinetic energy distribution peaking near 0.3 eV. We interpret the high energy dynamics as direct, but still involving significant interaction among all four atoms. The rapid variation in dynamics over a narrow collision energy range is attributed to the heavy-light-heavy mass combination of this system.

The reaction of Cl atoms with the selectively deuterated isobutane, 2-methylpropane-2-d 1 , has been investigated under single-collision conditions using state-selective detection of the products by resonance-enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The reaction was initiated in a crossed, pulsed flow of the reagents by 355 nm photolysis of Cl 2 precursor. The internal state distributions of HCl and DCl products, formed by abstraction of primary and tertiary hydrogen atoms, respectively, from the hydrocarbon reagent are reported. The degree of rotational excitation of both products was found to be very low. By comparison of the intensities of the HCl and DCl REMPI signals, the ratio of the yield of HCl to DCl product was found to be 3.3 ( 0.4. With the known speed and angular distribution of the Cl reagent, it was possible to obtain information on the product center-of-mass angular distributions from measurement of the masses 37 and 38 time of arrival profiles. The DCl product is found to be mainly backward scattered with respect to the incoming Cl atom, while the HCl product is sideways peaked. Inferences on the dynamics of the two possible abstraction pathways from these results are discussed.

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. Can. J. Chem. 72,828 (1994).We present the angular and kinetic energy distributions for the products of the proton transfer reaction 0-+ HF + OH + F a t center-of-mass collision energies of 45.0 and 55.8 kJ mol-' (0.47 and 0.58 eV, respectively). At both collision energies, the product angular distributions show forward-backward symmetry, characteristic of the decay of a transient complex living at least several rotational periods. The product kinetic energy distributions show structure that is clearly attributable to the formation of OH in v' = 0, 1, and 2. The kinetic energy distribution for a single vibrational state of OH is equivalent to the rotational state distribution for that state. At the higher collision energy, the product kinetic energy distribution shows a clear angular dependence, from which we infer a transition to more direct dynamics involving low impact parameter collisions that access the repulsive wall of the potential surface in bent geometries. The vibrational energy in the products decreases with increasing collision energy, with fv', the fraction of available energy appearing in vibration, decreasing from 0.28 to 0.22 over the reported collision energy range. We attribute this behavior to a transition from mixed energy release of a Heavy + Light-Heavy collision system dominated by the strong attractive well to induced repulsive energy release as the system reaches the low energy repulsive wall of the potential energy surface. [Traduit par la rkdaction]