The photoionization and dissociation dynamics of energy-selected acetylene dimers, trimers, and tetramers

Journal Article · Thu Dec 31 23:00:00 EST 1992 · Journal of Chemical Physics; (United States)

DOI:https://doi.org/10.1063/1.464653· OSTI ID:7186617

Booze, J A; Baer, T ^[1]

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1. Department of Chemistry, University of North Carolina-Chapel Hill, Chapel Hill, North Carolina 27599-3290 (United States)

The photoionization and dissociation characteristics of (C[sub 2]H[sub 2])[sub [ital n]] ([ital 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 (C[sub 2]H[sub 2])[sub 2], (C[sub 2]H[sub 2])[sub 3], and (C[sub 2]H[sub 2])[sub 4] to the stable ions C[sub 4]H[sub 4][sup +], C[sub 6]H[sub 6][sup +], and C[sub 8]H[sub 8][sup +] 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 C[sub 4]H[sub 2][sup +]/C[sub 4]H[sub 3][sup +], C[sub 4]H[sub 4][sup +], and C[sub 6]H[sub 6][sup +], respectively. Consistent with these observations, [ital ab] [ital 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 C[sub 4]H[sub 4][sup +] peak width is consistent with a process in which the trimer neutral produces a C[sub 4]H[sub 4][sup +]--C[sub 2]H[sub 2] complex, which then dissociates via monomer evaporation without exploring any of the covalent C[sub 6]H[sub 6][sup +] potential energy surface, i.e., benzene ion is not formed. The heat of formation of the observed C[sub 4]H[sub 4][sup +] daughter is estimated to be 1240[plus minus]15 kJ/mol, indicating that it is cyclobutadiene, butatriene, or vinylacetylene ion.

• DOE Contract Number: FG05-88ER13950
• OSTI ID: 7186617
• Journal Information: Journal of Chemical Physics; (United States), Journal Name: Journal of Chemical Physics; (United States) Vol. 98:1; ISSN JCPSA6; ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400500* -- Photochemistry
ACETYLENE
ALKYNES
BEAMS
COINCIDENCE METHODS
COUNTING TECHNIQUES
DIMERS
DISSOCIATION
ENERGY
ENTHALPY
FORMATION HEAT
HYDROCARBONS
IONIZATION
MATHEMATICAL MODELS
MOLECULAR BEAMS
MOLECULES
ORGANIC COMPOUNDS
PHOTOIONIZATION
PHYSICAL PROPERTIES
POLYATOMIC MOLECULES
POTENTIAL ENERGY
REACTION HEAT
STATISTICAL MODELS
THERMODYNAMIC PROPERTIES
THRESHOLD ENERGY