Jongwan Yu

Limitations on measuring solvent motion with ultrafast transient hole burning

A number of limitations to hole burning in the liquid phase are identified. As spectral diffusion becomes more rapid, a point is reached where the narrowest hole width no longer measures the homogeneous linewidth. Spectral congestion of the solute is also shown to limit the fastest detectable spectral diffusion rate. An optimal pulse length is found which allows observation of the most rapid spectral diffusion. In addition, the coherence effect observed when the pulses overlap is shown to have resonances at both the excited and ground state vibrational spacings. The coherence effect greatly resembles the hole burning spectrum, but contains no dynamical information. Because of these limitations, hole burning may not be observed even when the homogeneous spectrum is well resolved and spectral diffusion is slow. Experimental confirmation is found in the hole burning spectrum of iodine in hexane, which shows no hole burning despite having a narrow homogeneous linewidth.

Solvent-electronic state interactions measured from the glassy to the liquid state. I. Ultrafast transient and permanent hole burning in glycerol

Picosecond transient and permanent hole burning spectroscopies are used to study the interaction between glycerol and the electronic states of the nonpolar solute dimethyl‐s‐tetrazine. Variable temperature measurements from the low temperature glass into the fluid region have identified a phonon‐modulated interaction in agreement with a previous hypothesis [J. Chem. Phys. 94, 5787 (1991)]. However, this mechanism alone cannot account for the magnitude of the subpicosecond line broadening seen at room temperature. A second relaxation mechanism is identified, which is connected to the structural coordinates involved in the glass transition and reaches subpicosecond relaxation times at room temperature.

SUBPICOSECOND RELAXATION OF SOLVENT PERTURBATIONS OF NONPOLAR ELECTRONIC STATES MEASURED BY TRANSIENT HOLE BURNING

The results of transient hole burning of the nonpolar solute dimethyl‐s‐tetrazine (DMST) in glycerol, glycerol triacetate, sorbitol hexaacetate, squalane, and hexane solutions are reported. Despite the high viscosity and diversity of chemical types of the solvents, the solvent‐induced perturbations of the electronic states of DMST relax in less than 1 ps in all cases. This contrasts strongly with the relaxation of polar electronic states, which are typically slow in viscous solvents. A new type of solvent‐interaction mechanism appears to be important. We suggest that short‐range repulsions modulated by phonon‐like solvent modes are responsible.

Solvent‐electronic state interactions measured from the glassy to the liquid state. II. Fluorescence line narrowing spectroscopy in glycerol

Fluorescence line narrowing spectroscopy is extended to the study of liquid state dynamics. Measurements are made on solutions of dimethyl‐s‐tetrazine in glycerol from room temperature to below the glass transition. The structural broadening identified in the preceding paper is related to a relaxation function by the use of a simple configuration coordinate model. This relaxation function is ∼150 times faster than the structural relaxation function measured by other experiments, showing that electronic state solvation is sensitive to a unique aspect of the structural dynamics of the liquid.

Resorufin as a probe for the dynamics of solvation by hydrogen bonding

Abstract Resorufin is shown to be good probe solute for the direct measurement of the rates of formation and breaking of solvent—solute hydrogen bonds. In ethanol, a hydrogen-bond equilibration time of 40 ps is found for both the ground and excited states. Deuteration of the solvent has no effect on the equilibration time. The same is also found in more viscous ehtylene glycol solutions, indicating that hydrogen-bond breaking requires only local motion of the bonding moiety.

Rapid solvation of a nonpolar solute measured by ultrafast transient holeburning

Abstract Holeburning spectra from 1 ps to 1.2 ns have been measured for dimethyl- s -tetrazine dissolved in glycerol triacetate. Clean excitation of the well-separated 0–0 vibronic transition insures that only solvent dynamics contribute to the hole broadening. The majority of the interaction relaxes in less than a picosecond, with a second process relaxing in ≈ 2 ps. This solvation process is much faster than the 700 ps dielectric solvation of a highly polar solute in the same solvent, as measured by time-resolved Stokes shifts.

Structural relaxation in liquids and glasses by transient hole burning

Ultrafast transient and permanent hole burning and fluorescence line narrowing are used to study the dynamic interaction between the electronic states of a nonpolar solute (dimethyl-s-tetrazine) and its solvent (glycerol) from low temperature glass to room temperature liquid. A model for phonon-modulated interactions not only describes the glass phase results, but also extrapolates to describe the short time dynamics in the liquid phase. However, this mechanism does not account for all of the ultrafast interaction seen at room temperature. A second mechanism connected to the structural coordinates of the liquid is identified. It has a strongly temperature dependent relaxation rate, which becomes subpicosecond at room temperature. This mechanisms rate is significantly faster than rates measured by other structural relaxation experiments.

Solvent Dynamics by Transient Holeburning: Nonpolar Versus Polar Solutes

Solvent interactions with the electronic states of a non-polar solute are measured with transient holeburning. Large interactions much more rapid than dielectric relaxation are observed.