ZEKE photoelectron spectra of trans-stilbene in the S₁ state

Measurements of zero kinetic energy (ZEKE) photoelectron spectra were carried out with nanosecond lasers for trans-stilbene in a free jet expansion, by means of two-color (1 + 1′) REMPI via several S₁ vibrational levels. A series of observed ZEKE photoelectron spectra of trans-stilbene show well-resolved low-frequency vibrational structure due to the cation in the energy region up to 1500 cm^-1. The adiabatic ionization potential of trans-stilbene has been determined as 61 748 ± 4 cm^-1. Vibrational frequencies of 290, 208, 49, and 39 cm^-1 have been assigned to the vibrational modes 24⁺, 25⁺, 36⁺, and 37⁺ of the cation, respectively. It has also been found that the observed congested vibrational bands originate not only from the initially prepared S₁ levels but also from relaxed S₁ vibrational levels produced by intramolecular vibrational redistribution (IVR). The present IVR results are consistent with an available picosecond laser dispersed fluorescence study. The present work also clearly demonstrates that (1 + 1′) ZEKE photoelectron spectroscopy has sufficient capability for studying the dynamics of electronically excited neutral molecules.