Torsional vibrational structure of the propene radical cation studied by high-resolution photoelectron spectroscopy

The pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the (X) over tilde (+2)A ''<- (X) over tilde (1)A' transition of CH3CHCH2 (propene), CD3CDCD2, and several partially deuterated isotopomers have been recorded in the region of their adiabatic ionization thresholds and up to 2000 cm(-1) of internal energy of the cations. The vibrational structure has been assigned on the basis of the frequency shifts resulting from deuteration of selected sites of the propene molecule. Two highly anharmonic progressions have been identified and assigned to the two torsional modes of the propene cation, the methyl and methylene torsions. The positions of the torsional levels could be approximately reproduced using one-dimensional models, allowing a semi-quantitative description of the potential energy surface along each torsional coordinate. The observation of forbidden vibrational bands and the analysis of their partially resolved rotational contours reveal the importance of the vibronic coupling between the (X) over tilde (+2)A '' and the (A) over tilde (+2)A' states mediated by the methylene (nu(20)) and methyl (nu(21)) torsional modes. (C) 2011 American Institute of Physics. [doi:10.1063/1.3638182]