Ultrafast Nonradiative Relaxation Channels of Tryptophan

The nonradiative relaxation channels of gas-phase tryptophan excited along the S1-S4 excited states (287-217 nm) have been tracked by femtosecond time-resolved ionization. In the low-energy region, λ ≥ 240 nm, the measured transient signals reflect nonadiabatic interactions between the two bright La and Lb states of ππ* character and the dark dissociative πσ* state of the indole NH. The observed dynamical behavior is interpreted in terms of the ultrafast conversion of the prepared La state, which simultaneously populates the fluorescent Lb> and the dissociative πσ* states. At higher energies, after excitation of the S4 state, the tryptophan dynamics diverges from that observed in indole, pointing to the opening of a relaxation channel that could involve states of the amino acid part. The work provides a detailed picture of the processes and electronic states involved in the relaxation of the molecule, after photoexcitation in the near part of its UV absorption spectrum.