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

Dissociative electron impact ionization of methane and ethane is studi ed by probing the nascent rovibrational distributions of the H-2.+ pro duct ions. The H-2.+(V, N) distribution is obtained by determining the kinetic energy release distribution of dissociating H-2(v',N') molec ules formed in neutralization of H-2.+ ions On cesium atoms. Methane y ields rotationally hot H-2.+ ions (T = 1 0000 K) with a flat vibration al population distribution up to v = 6. Ethane yields rotationally col der H2.+ ions (T = 1700 K) with a broad vibrational population distrib ution that peaks at v = 0. In the case of methane the high rotational temperature reflects an asymmetric transition state, i.e., an asynchro nous stretching of the two C-H bonds. For ethane a more symmetric tran sition state is involved.