Comparative dynamics of Cl(P-2) and O(P-3) interactions with a hydrocarbonsurface

The dynamics of the interactions of atomic chlorine with the surface of a s aturated hydrocarbon liquid, squalane, were investigated and compared to th e results of an earlier study on analogous oxygen-atom interactions. Beams of continuous supersonic chlorine atoms were directed onto a squalane surfa ce, and the volatile products, Cl and HCl, were observed by mass spectromet ry as a function of incident angle, final angle, and incident Cl-atom energ y. Both the Cl and HCl time-of-flight (from the surface to the detector) di stributions revealed thermal and hyperthermal interaction channels, in anal ogy to the dynamical behavior of the O and OH signals observed in the previ ous study. The thermal HCl product may arise from two mechanisms: (i) desor ption of trapped HCl product and (ii) reaction of trapped Cl atoms to form thermal HCl, which subsequently desorbs. In contrast, the reaction of O ato ms with squalane led to a thermal OH signal, which could only come from des orption of trapped OH. The hyperthermal HCl signal is the result of an Eley -Rideal direct-abstraction mechanism. An analogous reaction gave rise to hy perthermal OH products in O-atom reactions with squalane. The HCl and OH hy perthermal products were scattered preferentially in the specular direction in the laboratory frame. However, a kinematic analysis showed a propensity for scattering sideways or even backwards (with respect to the direction o f the incident atomic beam) in the center-of-mass (c.m.) frame. The kinemat ic analysis also allowed the determination of the effective mass of the sur face collision partner, the c.m. collision energy, and the final c.m. trans lational energy. Similar c.m. scattering dynamics have been observed in gas -phase reactions of O and Cl atoms with saturated hydrocarbons. (C) 2000 Am erican Institute of Physics. [S0021-9606(00)70713-6].