Ultrafast dynamics in transparent fluids investigated via time-resolved third-order spectroscopy.

摘要

The ultrafast dynamics of near critical fluids and other transparent materials are investigated with time-resolved optical heterodyne detected (OHD) third-order spectroscopy. Among the main results of this dissertation are: supercritical fluctuations on a picosecond timescale are revealed by H2 rotational coherence decay; a giant acoustic response due to enhanced near critical compressibility is optically generated and characterized; a new pulse reconstruction algorithm is developed.;The decay of J-specific Raman coherence birefringence of H2 rotors in supercritical CO2 (0.8 rhoc) are measured in order to understand the ∼picosecond timescale solvent fluctuations around the critical point. The H2 anisotropic Raman time correlation functions possess long-time exponential tails attributed to motional narrowing. Nonexponential early-time behavior indicates the inhomogeneities in anisotropic local potential interactions. Mixed classical nonadiabatic quantum molecular dynamics simulations are performed. The excellent agreement between experiment and the theoretical treatment supports the use of this methodology. Multiregional studies are conducted for mixtures ranging from gas-like to liquid-like densities. A non-monotonic dependence of the Raman coherence decay times and transition frequencies are observed.;A novel third-order polarization due to accumulated acoustic grating is characterized in near critical CHF3 and CO2. The electrostrictively generated acoustic response is pi out of phase with the normal OKE birefringence. Repetition rate dependence identifies the accumulated origin of the phenomenon with an acoustic relaxation timescale of ∼100 ns. The combined effect of efficient coherence coupling excitation and accumulation results in a birefringent signal strength inversely dependent on the sound velocity to the fifth power.;In contrast a single-pulse coherence coupling effect routinely observed in transparent media can be shown to originate from nuclear responses. Its impact on frequency selected dichroic Raman spectral density recovery is demonstrated for CCl4. For linearly-chirped pulses, the antisymmetric coherent coupling trace dominates the frequency integrated dichroism. Its absolute sign indicates the sign of the chirp. An assumption-free amplitude-phase reconstruction algorithm combining the coherent coupling dichroism, the intensity autocorrelation and the power spectrum is developed.