Photoelectron Angular Distribution in Acetylene/Vinylidene

Timur Osipov, Lewis Cocke
(Department of Physics, Kansas State University, Manhattan, KS 66506)

Mike Prior
(Lawrence Berkeley National Laboratory, Berkeley, CA 94720)

Reinhard Doerner, Thorsten Weber, Schmidt Lothar, Horst Schmidt-Böcking
(Institut fur Kernphysik, Universitat Frankfurt, Germany)

Allen Landers
(Department of Physics, Western Michigan University, Kalamazoo, MI 49008)

Amine Cassimi
(CIRIL/CEA/CNRS, Caen, France)

The molecular dissociation of acetylene (C2H2) following the photoionization of one carbon K-shell, and subsequent Auger decay, has been analyzed using a momentum imaging technique. Momenta of the photoelectron and all charged molecular fragments measured in coincidence reveal two important break up channels: CH+ + CH+ and C+ + CH2+. The latter is the result of the dissociation of the dication of vinylidene (C-CH2)2+, the isomer of the acetylene dication. The photoelectron angular distribution, measured in the body-fixed frame, appears to be different for the two channels. The difference can be explained by the change of the vinylidene molecular orientation with respect to the initial photoelectron direction due to the C=C bond rotation before the dissociation process occurs. The data indicates that this bond rotates by approximately 30 ° prior to the dissociation. This result can be interpreted as due primarily to the rearrangement of the hydrogen atoms during isomerization and places an upper limit on the isomerization time.

This work was supported by the Chemical Sciences, Geosciences and Biosciences Division,
Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy.

Submitted to DAMOP 2002, May 2002 in Williamsburg, VA.

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