First-principles calculations of K-shell x-ray absorption spectra for warm dense ammonia* *Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0403200), the National Natural Science Foundation of China (Grant Nos. 11775031, 11975058, 11625415, and 11675024), and the Science Challenge Project (Grant No. TZ2016001).

Abstract

The x-ray absorption spectroscopy is a powerful tool for the detection of thermodynamic conditions and atomic structures on warm dense matter. Here, we perform first-principles molecular dynamics and x-ray absorption spectrum calculations for warm dense ammonia, which is one of the major constituents of Uranus and Neptune. The nitrogen K-shell x-ray absorption spectrum (XAS) is determined along the Hugoniot curve, and it is found that the XAS is a good indicator of the prevailing thermodynamic conditions. The atomic structures at these conditions are ascertained. Results indicate that the ammonia could dissociate to NH x (x = 0, 1, or 2) fragments and form nitrogen clusters, and the ratios of these products change with varying conditions. The contributions to the XAS from these products show quite different characteristics, inducing the significant change of XAS along the Hugoniot curve. Further model simulations imply that the distribution of the peak position of atomic XAS is the dominant factor affecting the total XAS.