Photochemical Recoil Spectroscopy: Chlorine Spectrum and Scattering Analysis

Abstract

The measurement and analysis of the velocity distribution of the recoiling chlorine atoms from the laser induced photodissociation of Cl2 at 3471 Å using a time-of-flight technique is described. The expected velocity distribution is numerically calculated from the experimental geometry by appropriate center-of-mass (c.m.)-laboratory (LAB) coordinate transformations. By making the measurements at moderate densities in the interaction zone, the elastic scattering of Cl atoms by Cl2 molecules is observed. Since scattering angles in c.m. correspond to velocity changes in LAB coordinates, the direct recoil signal must be corrected for the effects of multiple elastic scattering. Careful comparison of expected and observed velocity profiles enables one to deduce directly the energy state of the products, the bond dissociation energy D0 ○, and the rainbow scattering angle. The analysis on Cl2 yields D0 ○ (Cl–Cl)=57.05±0.2 kcal/mole, which is in excellent agreement with the JANAF value of 57.04±0.06 kcal/mole and the precise value of 57.1743 kcal/mole [R. J. LeRoy and R. B. Bernstein, J. Chem. Phys. 52, 3869 (1970)]. Using a Lennard-Jones (12–6) potential, the rainbow scattering angle for Cl–Cl2 collisions corresponds to a Cl3 well depth (ε) of 2±0.5 kcal/mole.