Predissociation of Excited Acetylene in the ÃA_u State around the Adiabatic Dissociation Threshold as Studied by LIF and H-Atom Action Spectroscopy

Abstract

The LIF and H-atom action spectra of the Ã−X̃ transition of acetylene in a supersonic jet were observed in the excitation-energy range of 47 000−50 600 cm -1, where 60 vibrational states were identified in the à state through rotational analyses of respective bands. Most of these states originate from anharmonic couplings of the Franck−Condon-allowed 2 m 3 n states with the 4 i 6 j states where i + j = even. This fact is consistent with the determined rotational constants of respective states whose values are mostly between those of the 2 m 3 n ( m ≤ 1, n ≤ 4) and the 4 1 or 6 1 state. The line intensities of the H-atom action spectra increase at the higher excitation energies, indicating more efficient predissociation. The lifetimes of respective excited states were evaluated from the observed spectral line widths. In a higher excitation-energy region, the lifetime is shortened from 100 ps at 47 000 cm -1 to 25 ps at 50 600 cm -1, which exceeds the adiabatic dissociation threshold of the à state by 933 cm -1 and is the wavelength limit (197 nm) of the third harmonic of the dye laser. To excite acetylene in the higher excitation energies, vibrationally excited acetylene, which was prepared by irradiation with an IR laser, was excited further by a UV laser. Utilizing this IR−UV double-resonance method, acetylene could be excited up to 53 500 cm -1, which corresponds to 187 nm. Beyond 50 600 cm -1, the lifetime of excited acetylene becomes longer, around 100 ps at 51 100 cm -1 . This sudden increase in the lifetime is attributed to the predissociation mechanism changes from the triplet state-mediated dissociation to the direct dissociation on the à state surface. This fact is also supported by the observed average kinetic energies of H atoms estimated from the Doppler line widths of the REMPI spectral lines of H atoms; below 50 600 cm -1, the kinetic energy is in proportion to the excitation energy, whereas beyond 50 600 cm -1, the observed average kinetic energies are around 1000−2000 cm -1, which are a little smaller than the excess energy for the production of C 2 H(Ã) + H. It was found that the predissociation mechanism or efficiency is not dependent on the vibrational parity, gerade or ungerade, of the initial excited state. This is attributed to the anharmonic coupling between the ν 3 ‘ and ν 4 ‘/ν 6 ‘ vibrational modes.