Mark A. Berg

Dynamics in low temperature glasses: Theory and experiments on optical dephasing, spectral diffusion, and hydrogen tunneling

Temperature dependent photon echo (PE) and nonphotochemical hole burning (NPHB) measurements are reported on resorufin in three organic glasses: ethanol (1.5–11 K), glycerol (1.1–25 K), and d‐ethanol (1.5–11 K). In all cases, the NPHB results are broadened considerably from the PE results at low temperatures, but the two measurements coalesce at high temperatures. The temperature dependences are found to deviate from the power law dependence expected for two‐level system dephasing, and the deviation is attributed to dephasing by a pseudolocal mode. The appropriate correlation functions for PE and hole burning experiments are shown to be different from each other. They also differ from the correlation function for the optical absorption (OA) experiment, which has been the basis for most calculations of optical dephasing in glasses. The broadening of hole widths beyond the PE result is shown to be a measure of the slow spectral diffusion processes in the glass. Other types of dephasing measurement are also ...

Studies of chemical reactivity in the condensed phase. I. The dynamics of iodine photodissociation and recombination on a picosecond time scale and comparison to theories for chemical reactions in solution

Picosecond transient absorption measurements from 1000–295 nm are used to monitor the recombination dynamics of iodine after photodissociation in a variety of inert solvents. The high time resolution and signal‐to‐noise ratio of these measurements permits the development of a detailed model of this reaction, which should resolve disagreements over the time scales of geminate recombination and vibrational relaxation and over the role of excited electronic state trapping. Most of the atoms which undergo geminate recombination do so in ≤15 ps, in agreement with the predictions of existing molecular dynamics simulations. The subsequent vibrational and electronic energy relaxation of the recombined molecule is relatively slow and accounts for most of the transient absorption dynamics. The relaxing X‐state vibrational population distribution is extracted with an approximate method using calculated spectra of the excited vibrational levels and is compared to recent models. Vibrational relaxation times vary from ...

Broadening of vibrational lines by attractive forces: Ultrafast Raman echo experiments in a CH3I:CDCl3 mixture

A definitive demonstration of inhomogeneous vibrational line broadening in a liquid is made from Raman echo measurements of the sym‐methyl stretching vibration of CH3I in a 50% mixture with CDCl3. The lifetime of the inhomogeneity is found to be 4–7 ps. The source of the inhomogeneity is identified as concentration fluctuations within the first solvation shell. The range and time scale of the interaction are consistent with the predictions of Schweizer and Chandler [J. Chem. Phys. 76, 2296 (1982)] for an attractive force interaction.

A picosecond photon echo study of a chromophore in an organic glass: Temperature dependence and comparison to nonphotochemical hole burning

The first two‐pulse photon echo experiments on a chromophore in an organic glass are reported. The homogeneous electronic dephasing of resorufin in ethanol glass is measured from 1.5–11.4 K. The temperature dependence of the dephasing time does not fit the power law frequently predicted by theory for the dephasing characteristic of glasses. However, the temperature dependence can be accounted for by including dephasing from librations or acoustic phonons, mechanisms known to be important in crystals. The dephasing decay is found to be a single exponential for over six factors of e. The dephasing is also shown to be uncorrelated with the extent of nonphotochemical hole burning (NPHB). However, the homogeneous linewidth deduced from the photon echo is four times narrower than the linewidth obtained from NPHB, demonstrating that the hole is broadened by additional processes.

Dynamics of Water and Ions Near DNA: Comparison of Simulation to Time-Resolved Stokes-Shift Experiments

Time-resolved Stokes-shift experiments measure the dynamics of biomolecules and of the perturbed solvent near them on subnanosecond time scales, but molecular dynamics simulations are needed to provide a clear interpretation of the results. Here we show that simulations using standard methods quantitatively reproduce the main features of TRSS experiments in DNA and provide a molecular assignment for the dynamics. The simulations reproduce the magnitude and unusual power-law dynamics of the Stokes shift seen in recent experiments [ Andreatta, D., et al. J. Am. Chem. Soc. 2005, 127, 7270 ]. A polarization model is introduced to eliminate cross-correlations between the different components contributing to the signal. Using this model, well-defined contributions of the DNA, water, and counterion to the experimental signal are extracted. Water is found to have the largest contribution and to be responsible for the power-law dynamics. The counterions have a smaller, but non-negligible, contribution with a time constant of 220 ps. The contribution to the signal of the DNA itself is minor and fits a 30 ps stretched exponential. Both time-averaged and dynamic distributions are calculated. They show a small subset of ions with a different coupling but no other evidence of substates or rate heterogeneity.

Some Comparisons of LIBS Measurements using Nanosecond and Picosecond Laser Pulses

Laser-induced breakdown spectra were measured by using a 1.3 ps laser pulse on glass, steel, and copper. Material ablation with the use of picosecond excitation is very precise with well-formed sharp-edged craters. The spectra obtained with 570 nm, 1.3 ps excitation decay more quickly and show significantly lower background emission than those that use 1064 nm, ∼ 7 ns excitation. The background was low enough that excellent laser-induced spectroscopy (LIBS) spectra were obtained on the three samples by using a single 1.3 ps laser pulse and a nongated detector. Similar results were obtained by using nanosecond excitation but with higher relative background signals. The radiance was similar with the use of pico- or nanosecond excitation; however, the radiant intensity was larger with nanosecond excitation because of the larger plasma.

Temperature‐dependent ultrafast solvation dynamics in a completely nonpolar system

Transient hole burning measurements on dimethyl‐s‐tetrazine in n‐butylbenzene are reported from the low‐viscosity room‐temperature liquid down to the low‐temperature glass. The results give a detailed picture of the solvation of a nonpolar solute in a nonpolar solvent. The dynamics separate into a phonon modulated and a structural component, as was found previously for polar solvents. The structural component is frozen in the glass, but its relaxation rate increases into the subpicosecond range with increasing temperature. The time decay of the structural relaxation is highly nonexponential. The coupling of the solute electronic state to the structural coordinates is close to linear, but the coupling to the phonon coordinates cannot be accounted for by simple linear or quadratic coupling models. Effects are also found that are attributed to changes in coupling constants with changing density.

Transient hole burning of s‐tetrazine in propylene carbonate: A comparison of mechanical and dielectric theories of solvation

The solvation dynamics of s‐tetrazine, a nonpolar solute, in propylene carbonate, a polar solvent, have been measured in the temperature range of 190–300 K and the time range of 1.5–300 ps by transient hole burning. A detailed model of the gas‐phase spectrum of s‐tetrazine is used to extract purely solvent‐induced effects from steady‐state and ultrafast spectra. Absolute measurements of the solvation response function are extracted from these spectra and are compared to dielectric and mechanical theories of solvation. Although the theories postulate very different solute–solvent interactions mechanisms, either theory can account for the available data.

Time‐resolved nonpolar solvation dynamics in supercooled and low viscosity n‐butylbenzene

We have measured the time‐resolved dynamics of the structural solvation of dimethyl‐s‐tetrazine in n‐butylbenzene, a completely nonpolar system. A combination of transient hole burning and time‐resolved fluorescence has been used to measure Stokes shift dynamics from 155–250 K, spanning a viscosity range of 6.6×105–2.5 cP. The decays have a nonexponential shape that is well described by a stretched exponential with β∼0.5. The time constants for solvation are equal to the shear relaxation times derived from viscosity and ultrasound measurements, suggesting that mechanical relaxation of the solvent is of prime importance in the structural solvation of nonpolar electronic states. A potential correlation with solute rotational dynamics is argued to be less plausible, based on the small size of the implied hydrodynamic volume.

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.