Study of the reaction dynamics of Li+HF, HCl by the crossed molecular beams method

Abstract

The reactions of (I) Li+HF→LiF+H and (II) Li+HCl→LiCl+H have been studied by the crossed molecular beams method. Angular distributions [N(Θ)] of product molecules have been measured at 4 collision energies (E c ) ranging from about 2 to 9 kcal/mole and time‐of‐flight (TOF) measurements of product velocity distributions were made at approximately E c =3 and 9 kcal/mole for both reactions (I) and (II). The combined N(Θ) and TOF results were used to generate contour maps of lithium–halide product flux in angle and recoil velocity in the center‐of‐mass (c.m.) frame. For reaction (I) at E c =3 kcal/mole the c.m. angular distribution [T(ϑ)] shows evidence of complex formation with near forward–backward symmetry; slightly favored backward peaking is observed. The shape of this T(ϑ) indicates there is significant parallel or antiparallel spatial orientation of initial and final orbital angular momentum L and L′, even though with H departing L′ must be rather small and L?J′, where J′ is the final rotational angular momentum vector. It is deduced that coplanar reaction geometries are strongly favored. At E c =8.7 kcal/mole the T(ϑ) of reaction (I) becomes strongly forward peaked. The product translational energy distributions P(E T ′) at both these collision energies give an average E T ′of ∼55% of the total available energy; this appears consistent with a theoretically calculated late exit barrier to reaction. The T(ϑ) at E c =2.9 and 9.2 kcal/mole for reaction (II) are forward–sideways peaked. Most of the available energy (∼70%) goes into recoil velocity at both E c for LiCl formation. This suggests a late energy release for this 11 kcal/mole exoergic reaction. Both reactions (I) and (II) show evidence of no more than a minor partitioning of energy into product vibrational excitation. Integral reactive cross sections (σ R ) are evaluated by integrating the product distributions in the c.m. frame and using small angle nonreactive scattering of Li as an absolute calibrant. Values of σ R are: for LiF formation σ R ?0.8 Å2 and 0.94 Å2 at E c =3 and 8.7 kcal/mole, while for LiCl formation σ R = 27 Å2 and 42 Å2 at E c =2.9 and 9.2 kcal/mole, with estimated absolute and relative uncertainties of a factor of 2, and 30%, respectively. Average opacities for reaction have been estimated from the reaction cross sections and the extent of rotational excitation of products to be about 0.1 for reaction (I) and 1 for reaction (II), for L values allowed to react. These results are discussed in some detail with regard to the kinematic constraints, reaction dynamics, and potential energy surfaces for these two reactions, and related experimental and theoretical works are noted. In addition, angular distributions of nonreactive scattering of Li off HF and HCl are measured at 4 different E c each. Rainbow structure is observed at low E c and the angular distributions are fit by a spherically symmetric piecewise analytic potential. The resulting values of the potential’s well depth (ε) and minimum position (r m ) are: for Li+HF ε=0.46 kcal/mole and r m =4.34 Å and for Li+HCl ε=0.32 kcal/mole and r m =4.7 Å. These results differ significantly from some earlier estimates based on the measurements of integral scattering cross sections.