Photoacoustic determination of the speed of sound in single crystal cyclotrimethylene trinitramine at acoustic frequencies from 0.5 to 15 GHz

Abstract

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.