Vibrational state dependence of beta and D asymmetry parameters: The case of the highest occupied molecular orbital photoelectron spectrum of methyl-oxirane

The beta angular asymmetry and D dichroic asymmetry parameters have been experimentally investigated using synchrotron radiation for the vibrationally resolved photoionization of the highest occupied orbital (HOMO) of methyl-oxirane. A theoretical calculation, in the Born-Oppenheimer harmonic approximation, of the Franck-Condon factors between vibrational ground state and different ionic vibronic states has been performed in order to gain information on the vibrational states mainly involved in the HOMO photoelectron band. The general good agreement between theoretical and experimental results allows a reliable assignment of the major features of the HOMO band. Experimental results show a dependence of beta and D asymmetry parameters from different final vibrational states. This paper reports the first evidence of a dependence of the dichroic D parameter as a function of the vibrational modes of the ion in the ionization of a chiral molecule.