A quantum dynamical examination of the vibronic structure of singlet and triplet spectra of acetylene

Abstract

The intensity distributions for the optical transitions S₀ → S₁ and S₁ → S₀ and the electron impact induced transitions S₀ → T_n (n = 1–3) were calculated using quantum dynamical methods. The problem was analyzed in three degrees of freedom, the CC-bond length and the two CCH-bond angles. The three-dimensional potential energy surfaces needed for such a treatment were calculated at the CASPT2 level employing an ANO-type basis set and an active space that consisted of 10 electrons in 10 orbitals. The complex intensity distribution of the S₀ ↔ S₁ transition in absorption and emission is reproduced in great detail. Spectral quantization allows an analysis of the modes that dominate vibronic coupling. Most of these modes bear no resemblance to the normal modes of the trans and cis form of S₁ . An emission from a higher excited state starting at 6.23 eV could tentatively be assigned to T₄ . T₂ and T₃ most likely do not contribute to the first band observed in the electron energy loss (EEL) spectrum. The same is true for the band that starts at 6.6 eV in the EEL spectrum. It results mostly from S₁ and possibly in part from T₄ .