An experimental and theoretical study of the S1←S0 transition of p-ethynyltoluene.

The one photon and the two photon S1←S0 spectra of jet-cooled p-ethynyltoluene have been measured for the first time, and a detailed vibronic analysis for both spectra has been attained. Mass analyzed resonance enhanced multiphoton ionization spectroscopy is the employed technique. In the one photon spectrum, the allowed component (origin and Franck-Condon bands) is much weaker than the forbidden component, and the same mechanisms as in the one photon spectrum of phenylacetylene are observed. The methyl torsional transitions are active. The 000 band is at 35 483 cm-1. The two photon spectrum is very strong and bears a resemblance to the two photon spectrum of phenylacetylene. The potential barrier of the methyl rotor in the S1 state has been determined as V6=-12 cm-1 with BCH3=5.55 cm-1. Ab initio calculations, MP2(full)/cc-pVTZ and CAS/cc-pVTZ, have been implemented for the geometry optimization and the normal mode vibration computation in the S0 and S1 states. [ABSTRACT FROM AUTHOR]