First-principles calculations of K-shell x-ray absorption spectra for warm dense ammonia  

作　　者：Zi Li Wei-Jie Li Cong Wang Dafang Li Wei Kang Xian-Tu He Ping Zhang 李孜;李伟节;王聪;李大芳;康炜;贺贤土;张平(Institute of Applied Physics and Computational Mathematics,Beijing 100088,China;Center for Applied Physics and Technology,Peking University,Beijing 100871,China)

机构地区：[1]Institute of Applied Physics and Computational Mathematics,Beijing 100088,China [2]Center for Applied Physics and Technology,Peking University,Beijing 100871,China

出　　处：《Chinese Physics B》2021年第5期576-582,共7页中国物理B（英文版）

基　　金：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);the Science Challenge Project(Grant No.TZ2016001).

摘　　要：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.

关 键 词：FIRST-PRINCIPLES warm dense x-ray absorption spectrum AMMONIA 

分 类 号：O657.3[理学—分析化学] P172.2[理学—化学]