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.

X 射线吸收光谱是检测热致密物质上的热力学条件和原子结构的有力工具。在这里，我们对作为天王星和海王星的主要成分之一的暖致密氨进行第一性原理分子动力学和 X 射线吸收光谱计算。沿 Hugoniot 曲线测定氮 K 壳层 X 射线吸收光谱 (XAS)，发现 XAS 是主要热力学条件的良好指标。确定了这些条件下的原子结构。结果表明氨可以解离成NH _x ( x = 0、1 或 2) 断裂并形成氮簇，这些产物的比例随条件的变化而变化。这些产品对 XAS 的贡献表现出截然不同的特征，导致 XAS 沿 Hugoniot 曲线发生显着变化。进一步的模型模拟表明，原子 XAS 峰位置的分布是影响总 XAS 的主要因素。