ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
The photoionization and dissociation characteristics of (C2H2)n (n=2, 3, and 4) have been studied by the threshold photoelectron–photoion coincidence technique applied to a molecular beam. For photon energies between 10.20 and 11.27 eV, only dissociative ionization pathways were observed for the clusters; no direct ionization of (C2H2)2, (C2H2)3, and (C2H2)4 to the stable ions C4H4+, C6H6+, and C8H8+ was observed. The shape of the time of flight peaks is consistent with statistical dissociation dynamics, and statistical analysis of the fragment peak shapes suggests that the dimer, trimer, and tetramer produce the fragments C4H2+/C4H3+, C4H4+, and C6H6+, respectively. Consistent with these observations, ab initio calculations are presented which indicate that the neutral T-shaped dimer and triangular trimer geometries are unstable on the ionic potential energy surface, and indeed have vertical ionization energies well above the dissociation limits of many of their possible product channels. The statistical modeling of the C4H4+ peak width is consistent with a process in which the trimer neutral produces a C4H4+–C2H2 complex, which then dissociates via monomer evaporation without exploring any of the covalent C6H6+ potential energy surface, i.e., benzene ion is not formed. The heat of formation of the observed C4H4+ daughter is estimated to be 1240±15 kJ/mol, indicating that it is cyclobutadiene, butatriene, or vinylacetylene ion. The peak width of C6H6+ is also analyzed, and suggests a process whereby the tetramer produces a C6H6+–C2H2 complex, which dissociates via monomer evaporation without exploring any of the covalent C8H8+ potential energy surface. The heat of formation of the C6H6+ daughter fragment is estimated as 1160±15 kJ/mol, which is consistent with the formation of benzvalene ion.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.464653
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