Picosecond Electronic Spectroscopy to Determine the Transformation Mechanism for the Pressure-Induced Phase Transition in Shocked CdS

摘要

Plate impact, shock wave experiments provide a unique method to investigate the time-dependent mechanisms and the kinetics associated with pressure-induced phenomena, such as chemical reactions and phase transformations. The very rapid and well defined loading conditions associated with plate-impact experiments permit real-time examination of the shock-induced changes. Further, the ability to propagate the shock wave along various crystallographic directions provides the means to perform careful analysis of the stress and orientational dependence. Recently, an experimental method has been developed to observe real-time changes in the absorption transmission of materials, with 100 or 200 ps resolution, in single-event, plate impact shock experiments (1-4). These data can provide useful information regarding the material under investigation. In particular, the dependence of the absorption edge on photon energy can distinguish between direct and indirect electronic transitions, and can provide an estimate of the band-gap energy of the material (5). Along with ab-initio techniques to calculate the electronic structure of a crystalline system, this electronic information can be used to gain insight regarding the crystal structure. As described in Ref. (1,2,4) the wurtzite-to-rocksalt phase transition in cadmium sulfide (CdS) is well suited to investigation through the use of fast electronic spectroscopy; the wurtzite and rocksalt phases exhibit a direct and indirect band gap with band gap energies of 2.5 and 1.5-1.7 eV, respectively (6-8). The intent of this work was to use picosecond electronic spectroscopy and ab-initio methods to examine the real-time structural changes that occur in the initial stages of the shock-induced wurtzite-to-rocksalt phase transition in single crystal CdS.