Substituent effects in molecular electronic relaxation dynamics via time-resolved photoelectron spectroscopy: \u3c0\u3c0* states in benzenes

We study the applicability of femtosecond time-resolved photoelectron spectroscopy to the study of substituent effects in molecular electronic relaxation dynamics using a series of monosubstituted benzenes as model compounds. Three basic types of electronic substituents were used: C=C (styrene), C=O (benzaldehyde), and CC (phenylacetylene). In addition, the effects of the rigidity and vibrational density of states of the substituent were investigated via both methyl (-methylstyrene, acetophenone) and alkyl ring (indene) substitution. Femtosecond excitation to the second * state leads, upon time-delayed ionization, to two distinct photoelectron bands having different decay constants. Variation of the ionization laser frequency had no effect on the photoelectron band shapes or lifetimes, indicating that autoionization from super-excited states played no discernible role. From assignment of the energy-resolved photoelectron spectra, a fast decaying component was attributed to electronic relaxation of the second * state, a slower decaying component to the first * state. Very fast electronic relaxation constants (