On the Excited State Dynamics of Vibronic Transitions. High-Resolution Electronic Spectra of Acenaphthene and Its Argon van der Waals Complex in the Gas Phase

Rotationally resolved fluorescence excitation spectroscopy has been used to study the dynamics, electronic distribution, and the relative orientation of the transition moment vector in several vibronic transitions of acenaphthene (ACN) and in its Ar van der Waals (vdW) complex. The 0 0 band of the S-1 <- S-0 transition of ACN exhibits a transition moment orientation parallel to its a-inertial axis. However, some of the vibronic bands exhibit a transition moment orientation parallel to the b-inertial axis, suggesting a Herzberg-Teller coupling with the S-2. state. Additionally, some other vibronic bands exhibit anomalous intensity patterns in several of their rotational transitions. A Fermi resonance involving two near degenerate vibrations has been proposed to explain this behavior. The high-resolution electronic spectrum of the ACN-ArvdW complex has also been obtained and fully analyzed. The results indicate that the wealdy attached argon atom is located on top of the plane of the bare molecule at similar to 3.48 angstrom away from its center of mass in the So electronic state.