High rydberg states of jet-cooled toluene observed by ultraviolet two-photon absorption spectroscopy: Ultrafast radiationless decay and pseudo-Jahn-Teller effects

Three new Rydberg series converging to the first ionization potential of toluene are identified by the method of ultraviolet two-photon absorption in a supersonic jet. No Rydberg series converging to the near-degenerate second ionization potential are found, a result analogous to that known for the valence systems of benzenoid molecules. The vibronic spectrum of the toluene cation is discussed in light of this result, and the failure to observe a distinct splitting of the lowest two states is attributed to a complete pseudo-Jahn-Teller mixing of the vibronic states. In addition, the possibility of systematic perturbations between Rydberg series converging to vibrationally excited states of the ion and purely electronic series is examined in light of quantum defect variations. A striking feature of toluene's Rydberg spectrum is the dramatic sharpening of individual bands with increasing principal quantum number. This sharpening is associated with a decrease in radiationless transition rate, a proposition which is supported by agreement between present bandwidth measurements and the direct femtosecond decay measurement of Wiesenfeld and Greene. Evidence favoring a spiraling decay mechanism is summarized.