Vibrational relaxation of an amino acid in aqueous solution

Time-resolved Raman spectroscopy is used to study vibrational energy redistribution after CH-stretch excitation, of glycine zwitterion in aqueous solution. Anti-Stokes Raman monitors the glycine vibrations and Stokes Raman monitors heating of the surrounding water, which acts as a molecular thermometer. A three-step mechanism is proposed for vibrational cooling (VC) that includes 0.8 ps decay of the parent CH-stretch, 1.0 ps relaxation of the midrange vibrations and 1.2 ps relaxation of the lower-energy vibrations. The overall VC process observed by the molecular thermometer is represented by a 1.8 ps time constant.