Ultrafast deactivation mechanisms of protonated aromatic amino acids following UV excitation.

Deactivation pathways of electronically excited states have been investigated in three protonated aromatic amino acids: tryptophan (Trp), tyrosine (Tyr) and phenylalanine (Phe). The protonated amino acids were generated by electrospray and excited with a 266 nm femtosecond laser, the subsequent decay of the excited states being monitored through fragmentation of the ions induced and/or enhanced by another femtosecond pulse at 800 nm. The excited state of TrpH+ decays in 380 fs and gives rise to two channels: hydrogen atom dissociation or internal conversion (IC). In TyrH, the decay is slowed down to 22.3 ps and the fragmentation efficiency of PheH+ is so low that the decay cannot be measured with the available laser. The variation of the excited state lifetime between TrpH+ and TyrH+ can be ascribed to energy differences between the dissociative pi sigma* state and the initially excited pi pi* state.