Time-Resolved Reflection Spectroscopy on Shock - Liquid Carbon Disulfide.

An experimental method has been developed for making time resolved reflection measurements on shock compressed liquid CS_2 in the 300-500 nm spectral range. CS_2 was contained between transparent windows and shock loaded by impact of a third material on the front window of the target. To compliment existing absorption measurements, the reflectance of CS _2 was studied to peak pressures ranging from 40-110 Kbar, using multiple shock, double shock, and ramp wave loading. An analytic method was developed for analyzing the transmission and reflection experiments in terms of sample geometries, the optical properties of CS_2, the optical properties of the cell materials, and the Fresnel reflection equations. This analysis was augmented with methods for calculating the optical constants of CS_2 from an assumed electronic structure. In multiple shock experiments the reflectance increases continuously with pressure and, inside spectral regions of absorption, is found to change at times coincident with shock wave arrival at the rear CS_2 /sapphire boundary. While the reflectance of the CS_2/sapphire interfaces is less than.5% at ambient pressure, at 105 Kbar it has increased to 10% at 300 nm and 2% at 500 nm. These results are consistent with existing absorption measurements. Unlike the absorption band edge shift, the reflectance change is reversible on pressure release at pressures above the reported 90 Kbar threshold for chemical reactions in multiple shock experiments. Potential mechanisms to explain this difference between the absorption and reflection measurements are discussed. A reflection experiment on a thin (~ .9mum) cell loaded to a peak pressure of 40 Kbar using ramp wave loading showed the evolution of a multiple peaked spectrum. The peaks in this spectrum were determined to be due to interference effects. Double shock experiments at peak pressures up to 89 Kbar showed no evidence of chemical reactions. At peak pressures of 96 and 99 Kbar the reflectance at wavelengths less than 400 nm decrease with time. The nature of this decrease was in agreement with the pressure history, at the rear CS_2/sapphire interface, expected from a chemical reaction in the CS_2 . At longer wavelengths large oscillations were also observed. These features were taken as evidence of a chemical reaction. Analysis using the Fresnel reflection equations and a complex refractive index for CS_2 indicates that the observed spectral changes are due to an increase in the oscillator strength and bandwidth of an absorption band centered near 320 nm.