Photoexcitation and ionization in acetylene

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 → kσu 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