Showing papers by "Keith A. Nelson published in 1991"
TL;DR: In this article, the authors used terahertz-rate trains of femtosecond pulses for repetitive impulsive excitation of individual phonon modes in an α-perylene molecular crystal.
Abstract: Optical control of elementary molecular motion through impulsive stimulated Raman scattering is enhanced by means of timed sequences of femtosecond pulses that are produced by pulse-shaping techniques. In particular, terahertz-rate trains of femtosecond pulses are used for repetitive impulsive excitation of individual phonon modes in an α-perylene molecular crystal. When the pulse repetition rate is matched to the desired phonon frequency, mode-selective vibrational amplification is achieved. A comparison of data acquired with the transient-grating and the pump–probe experimental geometries reveals the timing dynamics of the induced phonon oscillations with respect to the driving femtosecond pulse sequence.
165 citations
TL;DR: In this paper, the role of solvent in elementary chemical reactions using femtosecond transient absorption spectroscopy was studied using femto-condient transient absorption (FTE) spectrograms.
109 citations
TL;DR: In this paper, the authors used a wide range of scattering angles to characterize longitudinal acoustic waves in the 20 MHz-5 GHz frequency range and observed time-dependent thermal expansion on nanosecond-microsecond time scales.
Abstract: Picosecond time‐resolved impulsive stimulated light‐scattering experiments are conducted on polypropylene glycol at temperatures above the glass transition temperature Tg. Through the use of a wide range of scattering angles, longitudinal acoustic waves are characterized in the 20 MHz–5 GHz frequency range. In addition, time‐dependent thermal expansion is observed on nanosecond–microsecond time scales. The results are consistent with an empirical description of structural relaxation dynamics in terms of a stretched exponential relaxation function with exponent β=0.4 and with the average relaxation time given by the Vogel–Tamman–Fulcher form. Comparisons to Brillouin scattering and photon correlation spectroscopy results indicate that this description holds for a dynamic range of over 10 orders of magnitude. Comparison to dielectric relaxation measurements which probe mainly orientational motions of polymer segments indicates that density fluctuations involve different segmental motions with significantly ...
64 citations
TL;DR: In this paper, a femtosecond time-resolved impulsive stimulated scattering (ISS) data from carbon disulphide at three temperatures is presented and analyzed in terms of molecular orientational motions which show weakly oscillatory, librational dynamics at short times (<1 ps) and rotational diffusion at longer times.
Abstract: Femtosecond time‐resolved impulsive stimulated scattering (ISS) data from carbon disulphide at three temperatures are presented and analyzed in terms of molecular orientational motions which show weakly oscillatory, librational dynamics at short times (<1 ps) and rotational diffusion at longer times. The data are collected with a light polarization combination that eliminates signal due to electronic polarizabilities. Compared to ISS data reported earlier [S. Ruhman et al., Chem. Phys. Lett. 141, 16 (1987)], this permits improved examination of molecular motion at times shorter than 150 fs. The data also extend to longer times (several picoseconds) with good signal/noise ratios, permitting a more complete analysis of the molecular response. Finally, an improved analysis of the data is presented. Angular position correlation functions, which are determined from the data, are related to angular velocity correlation functions (AVCFs) through a Kubo line shape analysis. Approximate analytical models of liquid...
63 citations
TL;DR: In this article, a heavily damped soft optic phonon mode associated with the displacive character of the transition and relaxational modes associated with order-disorder character were examined.
Abstract: Femtosecond time-resolved impulsive stimulated scattering experiments were conducted on KNbO3 at temperatures spanning the orthorhombic phase. A heavily damped soft optic phonon mode associated with the displacive character of the transition and relaxational modes associated with order-disorder character were examined. Previous frequency-domain light scattering experiments had yielded conflicting results for the dynamics of the soft mode and the symmetry of the order-disorder motions. We find the relaxational motion to be of A1 symmetry and the soft mode to be the only low-frequency exictation of B2 symmetry. The soft mode is well described by a damped polariton model without any coupling to a relaxational mode. These results are consistent with a previously proposed eight-site model for the phase transitions.
25 citations
TL;DR: In this paper, the results of a study of the glass-forming liquid triphenylphosphite covering the normal liquid, supercooled liquid, and glass regimes by a picosecond time-domain light scattering technique are reported.
Abstract: The results of a study of the glass-forming liquid triphenylphosphite covering the normal liquid, supercooled liquid, and glass regimes by a picosecond time-domain light scattering technique are reported. The first time-resolved observations of propagating shear waves in a liquid are reported. Longitudinal and shear acoustic modes are observed and characterized over a total frequency range of 20 MHz to 5 GHz. Molecular orientational motion and time-dependent thermal expansion are also characterized. A Cole-Cole relaxation function is used to provide an approximate characterization of the frequency dependent acoustic behavior. At all temperatures examined, the distribution of structural relaxation processes that couple to shear waves appears to be narrower than the distribution that couples to longitudinal waves. The orientational relaxation and thermal expansion times are shown to be significantly slower than the average relaxation time of the distribution that couples to the longitudinal acoustic waves.
7 citations
TL;DR: The application of impulsive stimulated light scattering, an entirely time-domain form of light scattering spectroscopy, to the study of lattice dynamics in crystals near structural phase transitions is reviewed in this article.
Abstract: The application of impulsive stimulated light scattering, an entirely time-domain form of light scattering spectroscopy, to the study of lattice dynamics in crystals near structural phase transitions is reviewed. Advantages of the time-domain approach are demonstrated for study of low-frequency, heavily damped, and/or dispersive modes such as soft acoustic phonons. Impulsive stimulated Brillouin and Raman scattering data showing time-dependent oscillations and decay of acoustic and optic phonons, respectively, are presented and explained. Results of experiments on acoustic phonons in uniaxial and incommensurate ferroelectrics are summarized. Time-domain spectrosocopy of soft optic phonons and other overdamped modes is discussed.
5 citations
01 Jan 1991
TL;DR: In this article, the possibility of inducing mode-selective chemistry in ordered crystalline solids excitation and observing its occurrence in real time through impulsive stimulated Raman scattering is discussed.
Abstract: A discussion is presented on the possibility of inducing mode-selective chemistry in ordered crystalline solids excitation and observing its occurrence in real time through impulsive stimulated Raman scattering. Localized reactions of individual molecules or molecule pairs and collective chemical reactions associated which structural phase transitions are discussed.
4 citations
TL;DR: In this paper, the glass-forming aqueous electrolyte LiCl/H2O (13 mol%) was studied using impulsive stimulated light scattering, a time delayed four-wave mixing (transient grating) technique.
Abstract: The glass-forming aqueous electrolyte LiCl/H2O (13 mol%) was studied using impulsive stimulated light scattering, a time delayed four-wave mixing (‘transient grating’) technique. The excitation pulses deposit heat in the sample through weak optical absorption, and the time-dependent density response of the sample is monitored by variably delayed probe pulses. In most viscoelastic samples, longitudinal acoustic waves (the transient density response) and gradual thermal expansion (leading to a steady-state density response which eventually decays due to thermal diffusion) were observed. In LiCl/H2O an additional and unusual response was observed. Thermal expansion is followed by contraction on a timescale much faster than that of thermal diffusion. Possible microscopic origins of this response are suggested.
2 citations