On the energy dependence of the steric effect in atom–molecule reactive scattering. I. A quasiclassical approach

Rotationally correlated reactivity in the CH (v = 0, J, F(i)) + O2 → OH (A) + CO reaction

The rotational-state-selected CH (v = 0, J, F(i)) beam has been prepared by using an electric hexapole and applied to the crossed beam reaction of CH (v = 0, J, F(i)) + O(2) → OH (A) + CO at different O(2) beam conditions. The rotational state selected reactive cross sections of CH (RSSRCS-CH) turn out to depend remarkably on the rotational state distribution of O(2) molecules at a collision energy of ∼ 0.19 eV. The reactivity of CH molecules in the N = 1 rotational states (namely ∣J = 1∕2, F(2)> and ∣J = 3∕2, F(1)> states, N designates the angular momentum excluding spin) becomes strongly enhanced upon a lowering of the rotational temperature of the O(2) beam. The RSSRCS-CH in these two rotational states correlate linearly with the population of O(2) molecule in the specific K(O(2)) frame rotation number states: CH(|J = 1/2,F(2)>) with O(2)(|K(O(2)) = 1>);CH(|J = 3/2,F(1)>) with O(2)(|K(O(2)) = 3>). These linear correlations mean that the rotational-state-selected CH molecules are selectively reactive upon the incoming O(2) molecules in a specific rotational state; here, we use the term "rotationally correlated reactivity" to such specific reactivity depending on the combination of the rotational states between two molecular reactants. In addition, the steric asymmetry in the oriented CH (∣J = 1∕2, F(2), M = 1∕2>) + O(2) (|K(O(2)) = 1>) reaction turns out to be negligible (< ±1%). This observation supports the reaction mechanism as theoretically predicted by Huang et al. [J. Phys. Chem. A 106, 5490 (2002)] that the first step is an intermediate formation with no energy barrier in which C-atom of CH molecule attacks on one O-atom of O(2) molecule at a sideways configuration.

Steric effects in state-to-state scattering of OH (Π3∕22,J=3∕2,f) by HCl

In this paper we address stereo-dynamical issues in the inelastic encounters between OH (chi2pi) radicals and HCl (chi1sigma+). The experiments were performed in a crossed molecular-beam machine at the nominal collision energy of 920 cm(-1). Prior to the collisions, the OH molecules were selected using a hexapole in a well-defined rotational state v=0, omega=32, J=32, M(J)=32, f, and subsequently oriented in a homogeneous electrical field. We have measured rotationally resolved relative cross sections for collisions in which OH is oriented with either the O side or the H side towards HCl, from which we have calculated the corresponding steric asymmetry factors S. The results are presented in comparison with data previously obtained by our group for the inelastic scattering of OH by CO (E(coll)=985 cm(-1)) and N2 (E(coll)=985 cm(-1)) studied under similar experimental conditions. The dissimilarity in the behavior of the OH+HCl system revealed by this comparison is explained on the basis of the difference in the anisotropy of the interaction potential governing the collisions. The interpretation of the data takes into account the specific features of both nonreactive and reactive parts of the potential-energy surface. The results indicate that the scattering dynamics at this collision energy may be influenced by the HO-HCl van der Waals well and by reorientation effects determined by the long-range electrostatic forces and, furthermore, may involve reactive collisions.