Time resolved optical Kerr effect analysis of urea-water system

The nuclear dynamics of urea aqueous solution was analyzed by time resolved optical Kerr effect (OKE). The data analysis was achieved in time and in f requency domains. Three relaxation times characterize the time decay of the OKE signal at high mole fractions of urea, while only two relaxation times characterize this decay for the low mole fractions. The observed slowest r elaxation time increases with increasing the mole fraction of urea. The com parison between this relaxation time and the ones determined by Raman and n uclear magnetic resonance spectroscopies suggests that the slow relaxation time is related to the reorientation of an axis lying in the plane of the u rea molecule. At high mole fractions, the power spectra derived from the Fo urier transform of the OKE signal are characterized by one broad peak at ar ound 70 cm(-1) and by a shoulder at around 160 cm(-1) in the high frequency part of the former peak. This shoulder is related to the hydrogen bond int eractions which involve urea molecules. Molecular dynamics simulation resul ts on urea/water system suggest that the power spectra derived from OKE dat a could be interpreted in terms of translational motions (caging effect) an d in terms of rotational motion (libration) of urea molecules. (C) 2001 Ame rican Institute of Physics.