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In a modification of a picosecond ultrasonic technique, a short acoustic pulse is launched into a liquid sample by a laser pulse absorbed in a semitransparent transducer film and is detected via coherent Brillouin scattering of a time-delayed probe pulse. With both excitation and probing performed from the transducer side, the arrangement is suitable for in vivo study of biological tissues. The signal is collected from a micrometer-thick layer next to the transducer and is not affected by the diffuse scattering of probe light deeper in the sample. The setup, utilizing a 33 nm thick single crystal SrRuO(3) transducer film, is tested on a full fat milk sample, with 11 GHz acoustic frequency recorded.

The detection of ultrashort laser-generated acoustic pulses at a metal surface and the reconstruction of the acoustic strain profile are investigated. A 2 ps-long acoustic pulse generated in an SrRuO 3 layer propagates through an adjacent gold layer and is detected at its surface by a reflected probe pulse. We show that the intricate shape of the transient reflectivity waveform and the ability to resolve acoustic pulses shorter than the optical skin depth are controlled by a single parameter, which is determined by the ratio of the real and imaginary parts of the photoelastic constant of the material. We also demonstrate a Fourier transform-based algorithm that can be used to extract acoustic strain profiles from transient reflectivity measurements.

We have recorded laser excitation spectra of transitions from the ground X(1)Sigma(+) state of gaseous gold fluoride (AuF) into three excited electronic states in the visible region. We prepared the sample in a dc electric discharge by flowing a dilute mixture of SF(6) in argon through a hollow gold cathode. Two of these electronic states give rise to the previously reported yellow bands of the molecule, for which a rotational analysis is given here for the first time. We have analyzed the (0,0), (1,1), (0,1), and (1,2) bands of these two transitions, which we identify as [17.8]0(+)-X(1)Sigma(+) and [17.7]1-X(1)Sigma(+); their red-degraded (0,0) band heads lie at 563.0 and 566.2 nm, respectively. The (0,0) band of a new, red-degraded [14.0]1-X(1)Sigma(+) transition at 715.1 nm has also been recorded and analyzed. An accurate set of molecular constants of the three excited states as well as the ground state has been determined by least-squares fitting all of the optical data together with measurements made by other workers of the pure rotational spectrum of AuF in its ground state. These constants include the electronic term energies, vibrational frequencies, rotational constants, and Omega-doubling constants. We discuss the nature of these three excited electronic states in terms of the ionic Au(+)F(-) electronic configurations from which they are derived.

We report photoacoustic measurements of the quasi-longitudinal speed of sound along different crystallographic directions in the energetic molecular crystal cyclotrimethylene trinitramine (RDX). Measurements in (100)-oriented RDX were made using two complimentary techniques to probe acoustic frequencies from 0.5 to 15 GHz to resolve large discrepancies in reported sound speed values measured using different techniques and frequency ranges. In impulsive stimulated light scattering (ISS), two laser beams were crossed at various angles in a sample to generate coherent acoustic waves with well-defined wavevectors. Picosecond acoustic interferometry (PAI) measurements were conducted in which a laser pulse heated a thin metal transducer layer coated on the sample surface to generate a broadband acoustic wave-packet that propagated into the sample. Time-dependent coherent Brillouin scattering of probe light from the acoustic waves revealed frequencies in the 0.5–3.5 GHz range in ISS measurements and at ∼15 GHz in the PAI measurements, yielding the speed of sound in each case. Our ISS results are in agreement with previous ultrasonic and ISS measurements at kilo- and megahertz frequencies. Our PAI results yielded a 15 GHz sound speed essentially equal to those at megahertz frequencies in contrast to an earlier report based on Brillouin light scattering measurements. The lack of acoustic dispersion over six orders of magnitude in frequency indicates that there is no relaxation process that significantly couples to acoustic waves in RDX at acoustic frequencies up to 15 GHz.