Photodissociation of acetylene and bromoacetylene in the vacuum ultraviolet: Production of electronically excited C2H and C2

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Abstract
The photodissociation of C2H2 and BrC2H in the vacuum ultraviolet produces nearly identical quasicontinuous emissions in the region 4000 to above 5500 Å. It is concluded that the continua are most likely associated with the electronically excited C2H (C2H*) in both cases. Based on this conclusion the following spectroscopic and thermochemical data are derived from the thresholds of incident wavelength to produce the continua (1305±3 Å and 1540±3 Å, respectively, for C2H2 and BrC2H): The electronic energy of C2H, E0(C2H) ? 4.11±0.05 eV, D0(Br–C2H) = 91±1 kcal mol−1 (381±4 kJ mol−1), and ΔHmDfo(BrC2H) = 64.2±1.5 kcal mol−1 (269±6 kJ mol−1). The photolysis of BrC2H yields the C2 Swan bands in addition to the C2H* by the spin forbidden process. The fluorescence yield has been measured as a function of incident wavelength for C2H2 and BrC2H. The fluorescence yield curves follow the absorption spectra of C2H2 and BrC2H, indicating that the C2H* is predissociated from the initially formed electronic states. The C2H* yield from C2H2 increases rapidly below the incident wavelength, 1200 Å. Bond dissociation energies and the heats of formation of haloacetylenes are estimated in comparison with the corresponding cyanogen halides. Primary photochemical processes in C2H2 are discussed.

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