The surface restricted transient grating is demonstrated as a sensitive probe of ultrafast surface reaction dynamics. Studies of doped single crystal n-Ti0 2 (00 1) surfaces in air demonstrate linear trapping processes, assigned to crystal defects within the surface deformation layer, that limit carrier lifetimes to 5 ns. Direct in situ grating studies at photochemically active n-Ti0 2 / H 0 interfaces demonstrate that the dominant mechanism of interfacial electron transfer in this system involves thermalized hole carriers at the atomic surface. The d~namics are.consistent with adsorbed OH-as the initial hole acceptor. In addition, optical generatIOn of coherent surface acoustic modes is demonstrated. A detailed theory is presented for the grating excitation ofthe surface acoustics. Acoustic propagation in the H 2 0 half-space of the Ti0 2 / H 2 0 liquid interface gives evidence for a phase change of the water layer at the polar Ti0 2 (001) surface to a solid phase.

A number of different approaches have been developed to estimate and image the elastic properties of tissue. The biomechanical properties of tissues are vitally linked to function and pathology, but cannot be directly assessed by conventional ultrasound, MRI, CT, or nuclear imaging. Research developments have introduced new approaches, using either MRI or ultrasound to image the tissue response to some stimulus. A wide range of stimuli has been evaluated, including heat, water jets, vibration shear waves, compression, and quasistatic compression, using single or multiple steps or low-frequency (<10 Hz) cyclic excitation. These may seem to be greatly dissimilar, and appear to produce distinctly different types of information and images. However, our purpose in this tutorial is to review the major classes of excitation stimuli, and then to demonstrate that they produce responses that fall within a common spectrum of elastic behavior. Within this spectrum, the major classes of excitation include step compression, cyclic quasistatic compression, harmonic shear wave excitation, and transient shear wave excitation. The information they reveal about the unknown elastic distribution within an imaging region of interest are shown to be fundamentally related because the tissue responses are governed by the same equation. Examples use simple geometry to emphasize the common nature of the approaches.

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Two-dimensional simulations of gas cavity responses to both weak shocks (p ≤ 30 MPa) and strong shocks (p ranging from 500 to 2000 MPa) are performed using a finite volume method. An artificial viscosity to capture the shock and a simple, stable, and adaptive mesh generation technique have been developed for the computations. The details of the shock propagation, rarefaction, transmission and bubble wall motions are obtained from the numerical computations. A weak shock is defined in the present context as one that does not cause liquid jet formation upon impact with the bubble. For this case, a large pressure is created within the gas upon collapse due to rapid compression of the gas, ultimately causing the re-expansion of the bubble. The bubble collapse and re-expansion time predicted by this model agree well with spherically symmetric computations. When impacted by strong shock waves, the bubble will collapse and a liquid jet is formed that propagates through the bubble to the opposite bubble wall. Jet speeds as high as 2000 m s−1 are predicted by this model.