Photoexcitation and ionization in acetylene

Abstract

Theoretical studies are reported of photoexcitation and ionization processes in the acetylene molecule. The calculations performed employ Franck—Condon and static-exchange approximations, and implicitly invoke vibrational and rotational closure. Detailed comparisons with recent spectral assignments, electron-impact excitation studies, and total and partial-channel cross-sectional measurements indicate that the theoretical results provide a useful first approximation to the complete dipole excitation/ionization spectrum of C 2H 2. The calculated discrete spectra are seen to include contributions from virtual 4σ g( n/σ*) and 1π g(π*) orbitals, which appear an valence interlopers in the 1π u→ nsσ g, 2σ u → nsσ g, 1σ u → nsσ g and 2σ u → ndπ g, 1σ u → ndπ g Rydberg series, respectively, in good accordance with spectral measurements. By contrast, the 3σ u(σ*) virtual valence orbital gives rise to prominent diabatic resonance features above threshold in the 3σ g → kσ u, 2σ g → kσ u, and 1σ g → kσ u cross-sections, results that are in good quantitative accordance with corresponding experimental studies. Prominent features in the measured (1π u −1) X 2Π u partial cross-section, which is in good accordance with the present values, are attributed to autoionization 2σ u → 1π g(π*) and 4σ g( n/σ*) intravalence transitions.