De Santis Alberto, Ercoli Alessandro, Rocca Dario
Facolta di Agraria, DISA and INFM-Viterbo, Universita della Tuscia, Via San Camillo De Lellis, 01100 Viterbo, Italy.
J Chem Phys. 2004 Jun 1;120(21):10194-9. doi: 10.1063/1.1736630.
The translational dynamics of supercooled and normal liquid water is investigated via a specific correlation function DeltaB with the aim of explaining the behavior of the centers of mass velocity correlation function (VCF). DeltaB is divided into diffusive and nondiffusive parts that yield separated contributions to the VCF, namely an Enskog-type diffusive one, modeled by an exponential function, and a nondiffusive one, made up by damped oscillations of a vanishing time integral. In the translational density of states (DOS), the oscillations yield the bands at omega(1) congruent with 50 cm(-1), omega(3) congruent with 240 cm(-1) (the two well-known experimental bands of the Raman spectra) and omega(2) congruent with 160 cm(-1) (the Einstein frequency of the liquid). It is shown that the chief negative lobe of the VCF is mainly due to the DOS component at the lowest frequency omega(1). The study of the relative pair dynamics shows that this lobe is due to the transverse dynamics, while the longitudinal one determines the fast DOS component at omega(3). The presence of a negative tail is highlighted. Its contribution extends beyond the region of the fast dynamics (t<0.7 ps) up to about 1.5 ps and is due to a low-frequency oscillating mode that produces a low-frequency DOS band centered at about omega(0)=20 cm(-1).
通过一个特定的关联函数ΔB研究过冷液态水和正常液态水的平移动力学，目的是解释质心速度关联函数（VCF）的行为。ΔB分为扩散部分和非扩散部分，它们对VCF产生单独的贡献，即由指数函数建模的恩斯科格型扩散部分，以及由消失的时间积分的阻尼振荡组成的非扩散部分。在平动状态密度（DOS）中，振荡产生了与50 cm⁻¹ 一致的ω(1)、与240 cm⁻¹ 一致的ω(3)（拉曼光谱的两个著名实验波段）以及与160 cm⁻¹ 一致的ω(2)（液体的爱因斯坦频率）处的能带。结果表明，VCF的主要负瓣主要归因于最低频率ω(1)处的DOS分量。相对对动力学的研究表明，这个瓣是由于横向动力学，而纵向动力学决定了ω(3)处的快速DOS分量。突出了负尾的存在。它的贡献延伸到快速动力学区域（t < 0.7 ps）之外，直至约1.5 ps，并且归因于一种低频振荡模式，该模式产生了一个以约ω(0)=20 cm⁻¹ 为中心的低频DOS带。
