H-2(.+) LOSS FROM METHANE AND ETHANE UNDER ELECTRON-IMPACT - AN EXPERIMENTAL-STUDY OF TRANSITION-STATE GEOMETRIES

Dissociative electron impact ionization of methane and ethane is studied by probing the nascent rovibrational distributions of the H-2.+ product ions. The H-2.+(V, N) distribution is obtained by determining the kinetic energy release distribution of dissociating H-2*(v',N') molecules formed in neutralization of H-2.+ ions On cesium atoms. Methane yields rotationally hot H-2.+ ions (T = 1 0000 K) with a flat vibrational population distribution up to v = 6. Ethane yields rotationally colder H2.+ ions (T = 1700 K) with a broad vibrational population distribution that peaks at v = 0. In the case of methane the high rotational temperature reflects an asymmetric transition state, i.e., an asynchronous stretching of the two C-H bonds. For ethane a more symmetric transition state is involved.