Picosecond nonlinear optical study of acoustic transients and excited C(60) in benzene.

摘要

I study the properties of laser-induced acoustic waves in benzene solutions and the nonlinear optical properties of excited state C{dollar}sb{lcub}60{rcub}{dollar} molecules dissolved in benzene using time-resolved degenerate four-wave mixing technique.; In particular, I study the laser-induced acoustic waves originating from both heating and electrostriction. In order to characterize completely the coupling between light and density and temperature variations in benzene, I determine the change of refractive index with respect to temperature change at constant density {dollar}left({lcub}partial noverpartial T{rcub}right)sbrho.{dollar} I find {dollar}rm 0.95times 10sp{lcub}-4{rcub}Ksp{lcub}-1{rcub}geleft({lcub}partial noverpartial T{rcub}right)sbrhoge -0.79 times 10sp{lcub}-4{rcub}Ksp{lcub}-1{rcub},{dollar} the first measurement of this quantity in a liquid to my knowledge. Using the laser-induced acoustic wave technique, I study the dispersion of sound attenuation and velocity in benzene, in 0.5-9 GHz frequency range. This dispersion is due to the energy exchange between the center-of-mass and internal degrees of freedom of the molecules. I find that this exchange occurs in 312ps {dollar}pm{dollar} 39ps, in agreement with results obtained by Brillouin Scattering.; I also measure the optical polarizability of excited state C{dollar}sb{lcub}60{rcub}{dollar} molecules dissolved in benzene using a transient grating technique. I develop a model that incorporates the contribution to the signals from the population grating of excited state C{dollar}sb{lcub}60{rcub}{dollar} molecules superposed on the contributions from a stationary thermal grating and from an acoustic wave grating. I find that the (in-phase) polarizability of the lowest excited electronic levels (both singlet and triplet) of C{dollar}sb{lcub}60{rcub}{dollar} at the wavelength of 532 nm is less than 3% different from that of the ground state. Using a pump-probe technique, I find that C{dollar}sb{lcub}60{rcub}{dollar} molecules in their lowest-energy excited states have an absorption cross-section nearly 3 times larger than in their ground states. I study the effects of large excited-state absorption of the dissolved C{dollar}sb{lcub}60{rcub}{dollar} on the laser-induced acoustic wave in solution, and find a fourth-order dependence of the laser-induced acoustic wave signal on the laser pulse energy due to this effect.