The (Lb)S1 <- S0 transition of phenylpropyne and phenylacetylene: an experimental and ab initio study

This work describes the first excited singlet electronic state, Lb, of phenylpropyne (PPR) and phenylacetylene (PA). Ab initio calculations have been performed for the geometry and normal modes in the S0 and S1 states. One-photon and two-photon S1 <- S0 spectra of jet cooled samples have been examined (REMPI detection) and a detailed vibronic analysis has been carried out for PPR and PA. The origin band of PPR (λ = 279.64 nm) is one order of magnitude weaker than the origin band (λ = 278.63 nm) of PA and the acetylenic modes are greatly reduced by the H → CH3 substitution. These changes and the parallel PPR-PA comparison provided a way for secure vibronic assignment in both molecules, which was also aided by theoretical predictions