Electronic and chemical changes in shocked liquid carbon disulfide inferred from time resolved reflection experiments and analysis

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Abstract
Time resolved, spectroscopic reflection measurements (300–500 nm) have been used to examine the electronic and chemical changes in liquid carbon disulfide shocked to peak pressures as high as 110 kbar. Multiple-shock loading and unloading and double-shock experiments were performed to examine the influence of temperature, and to compare the present data with previous absorption and continuum measurements. The reflectance increases markedly with pressure in multiple-shock loading experiments. At 300 nm, the reflectance increases from less than 0.33% at ambient conditions to 10% at 105 kbar. However, the reflectance changes exhibit wavelength dependence, being smaller for longer wavelengths, and are reversible upon pressure unloading. A phenomenological model was developed to calculate the complex refractive index for carbon disulfide. This model in conjunction with Fresnel’s reflection equations can be used to analyze the multiple-shock reflection and absorption data in a consistent manner. The experimental results can be understood in terms of the growth of absorption bands due to increasing overlap of the π electron wave functions of neighboring molecules due to compression; this overlap is likely a precursor to associative chemical reactions. The complete reversal in reflectance at pressures above 90 kbar is in contrast to the absorption data and arises because the present reflection measurements do not sample the bulk material, but, instead, are collected from a very thin, cooled, unreacted layer of carbon disulfide. The double-shock experiments show no evidence of a chemical reaction below 90 kbar and are in agreement with multiple-shock data. At higher pressures, the double-shock experiments show evidence of chemical reactions and display a complicated reflectance history that depends markedly on the wavelength. The wavelength dependence and associated complexities in the double-shock experiments are consequences of pressure dependent changes in absorption bands, cooling due to heat conduction to the sapphire optical windows, and the temperature dependence of the reaction threshold pressure. The present work has provided a link between the absorption measurements obtained under multiple-shock loading and continuum measurements obtained under double-shock loading.