Structural Melting of an Amino Acid Dimer upon Intersystem Crossing

We present a spectroscopic investigation of the excited-state dynamics of the phenylalanine (Phe)/serine (Ser) protonated dimer in the gas phase. Using an ultraviolet (UV) laser pulse, we promote individual isomers to the S-1 state and probe their fate with an infrared (IR) pulse. We find that the S-1 state has a lifetime of similar to 70 ns and undergoes intersystem crossing (ISC) to the T-1 state. Time-resolved IR spectra allow us to follow the structural evolution of the dimer. In the S-1 state, the different isomers retain the hydrogen-bonding pattern of the ground state. Intersystem crossing triggers a sudden increase of the vibrational energy, so that the dimers can overcome isomerization barriers and explore large parts of the potential energy surface (PES). Their broad IR spectra largely resemble one another and indicate that the dimers adopt a molten structure.