基于Morse势的可重建有限温度下太赫兹吸收谱的半经验模型

Semi-Empirical Model to Retrieve Finite Temperature Terahertz Absorption Spectra using Morse Potential

太赫兹吸收谱起源于对材料低频振动模或低频声子模的激发,强烈依赖于体系的化学组成和微观结构,它是材料的一种指纹特性.太赫兹振动激发能较低(0.414~414 meV),导致振动模对应的势能面较浅,且振动激发态可通过热涨落占据.较浅的势能面通常导致振动模非谐性较强,使得激发态吸收发生红移;结合较为明显的振动激发态占据,在温度升高时,太赫兹特征信号通常发生不同程度的红移.本文基于第一性原理振动分析,结合已有的低温太赫兹实验谱和Morse势函数,发展了一个半经验模型,用于描述低频振动模的非谐性效应.通过对嘌呤分子晶体的测试,模型可以重现不同温度下的太赫兹吸收谱;对实验结果的准确预测表明,该模型可用于分子晶体的太赫兹光谱表征.

Terahertz(THz)absorption is a ngerprint property of materials,due to the underlying low-frequency vibration/phonon modes being strongly dependent on the chemical constitutions and microscopic struc-tures.The low excitation energies(0.414−41.4 meV)are related to two intrinsic properties of THz vibrations:the potential energy surfaces(PESs)are shallow,and the vibrationally excited states are usually populated via thermal uctuations.The shallow PESs make the vibrations usually anharmonic,leading to redshifted vibrational excited state absorption;combined with considerable vibrational excited states population,charac-teristic THz signals are usually redshifted and congested with varying degrees at di erent temperatures.Combining existing experimental THz spectra at low temperatures,rst principles vibration analysis,and the Morse potential,we developed a semi-empirical model to evaluate the anharmonicity of the low-frequency modes.The model was benchmarked with purine molecular crystal to generate THz spectra at di erent temperatures,the results were consistent with experiments.The good agreement suggests this model would facilitate the application of THz spectroscopy in molecular crystal characterization.