REACTION DYNAMICS OF MG(3S3P P-1(1)) WITH CH4 - ELUCIDATION OF REACTION PATHWAYS FOR THE MGH PRODUCT BY THE MEASUREMENT OF TEMPERATURE-DEPENDENCE AND THE CALCULATION OF AB-INITIO POTENTIAL-ENERGY SURFACES

Using a pump-probe method, we have obtained the nascent bimodal rotati onal distribution of MgH (v ''=0 and 1) products formed in the reactio n of Mg(3s3p P-1(1)) with CH4. The low-N component of the distribution in the v ''=0 state is much larger than that in the v ''=1 state, whe reas the high-N component in the v ''=0 state is roughly equivalent to that in the v ''=1 state. The MgH (v ''=0) rotational distributions a t three temperatures, 770, 830, and 880 K, were measured. The bimodal distribution does not change with temperature within a small experimen tal error. The findings suggest that the bimodal nature results from t he same process, supporting a mechanism of Mg insertion into the C-H b ond, irrespective of the geometry of the entrance approach. The result is consistent with that of Kleiber et al. using the far-wing scatteri ng technique, and is supported by Chaquin et al.'s theoretical calcula tions. We also calculated two-dimensional potential energy surfaces fo r the excited and ground states of the reaction system. The calculatio n suggests that two possible trajectories are responsible for the prod uction of MgH following a nonadiabatic transition. One trajectory, wea kly dependent on the bending angle of H-Mg-CH3, is related to formatio n of the low-N component. The other trajectory evolves through a linea r geometry of the intermediate complex prior to dissociation, causing a strong anisotropy in the PES. This second trajectory corresponds to the population of rotationally and vibrationally hot states. An altern ative explanation of the low-N distribution is also discussed. (C) 199 6 American Institute of Physics.