Conformational barrier crossing in the water dimer cluster of coumarin 151 studied by infrared–optical double-resonance spectroscopy

Vibronically induced conformational relaxation is reported between two distinct structures of the jet-cooled cluster coumarin 151/(H2O)2. The process involves transfer of the water dimer in the electronically excited state over a distance of ≈8 Å between sites on the coumarin molecule, governed by an activation energy of 60 cm−1.