Transient Grating Signal Research Articles

Consequences of initial condition analysis for the interpretation of transient grating observations

On the basis of a recent theory, we present a detailed analysis of initial state consequences on transient grating (and Ronchi ruling) signals. We show explicitly that the required changes in the interpretation of data, while substantial in principle, are of no practical importance in experiments reported so far. For instance, the interpreted mean-free-path of Frenkel excitons in molecular crystals remains unaffected for all experiments except one at 1.8 K in anthracene, for which the uncertainty factor is 2 under the worst of conditions. We also present a revised table of coherence lengths and mean-free-paths updated from our previous work.

Gas phase dynamics and spectroscopy probed with picosecond transient grating experiments

Picosecond transient grating experiments in sodium and iodine vapors, involving the 3S→3P and X→B transitions, respectively, are discussed in detail. Population gratings in sodium demonstrate that the technique can be used to measure velocity distributions in the gas phase. It is shown that the time dependent transient grating signal is related to the Fourier transform of the velocity distribution. Similar experiments on iodine illustrate the effect of state changing collisions on the grating signal. Theoretical calculations for a model in which the change of state is caused by a single collision are given. Close agreement with the data is observed for the situation in which the collision takes the initial velocity into a random velocity distribution. From this model a collision cross section is determined. The results demonstrate that information on collision dynamics can be obtained from grating experiments. In addition, the sodium experiments are used to illustrate a new type of time domain high resolution spectroscopy. When the grating excitation pulses have perpendicular polarizations, a polarization grating, rather than the usual population grating, is formed. Diffraction from the sodium polarization grating shows larger time dependent oscillations in the diffraction efficiency. These oscillations yield the ground state and excited state hyperfine frequencies (1.77 GHz and 189 MHz, respectively). A detailed theoretical description of the origin of the oscillations is presented. The results suggest that polarization grating spectroscopy can have applications in other areas, such as molecular rotational dynamics.

Surface quenching of optically generated carriers in thin-film hydrogenated amorphous silicon: Picosecond transient-grating experiments.

Subnanosecond-timescale transient-grating experiments are used to investigate the dynamics of optically generated charge carriers in thin-film hydrogenated amorphous silicon (a-Si:H) at room temperature. By examining the time dependence of the transient-grating signal as a function of fringe spacing for small fringe spacing, the diffusion constant in the plane of the film (${D}_{?}$) is determined. At large fringe spacing, the grating decay becomes independent of fringe spacing and is highly nonexponential. The functional form and rate of the large-fringe-spacing decay change with sample thickness. A model for the effect of surface quenching on the transient-grating decay is presented. The calculations are in good agreement with the data and yield a diffusion constant (${D}_{\ensuremath{\perp}}$) for motion perpendicular to the thin-film plane. ${D}_{\ensuremath{\perp}}$=0.4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}2}$ ${\mathrm{cm}}^{2}$/sec, while ${D}_{?}$=1.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}2}$ ${\mathrm{cm}}^{2}$/sec.

Simple connection between signals in transient grating experiments and memories in generalized master equations for excitons

An exact, simple, and useful connection is shown to exist between transient grating signals and generalized master equations used in the analysis of exciton transport in molecular crystals. It is model independent and akin to the connection between the van Hove correlation function and scattering observables.