Microscopic self-dynamics in liquid hydrogen and in its mixtures with deuterium
Microscopic self-dynamics in liquid hydrogen and in its mixtures with deuterium
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液氢及其与氘的混合物中的微观自动力学
DOI：
10.1103/physreve.70.061202
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发表时间：
2004
期刊：
影响因子：
--
通讯作者：
M. Zoppi
中科院分区：
文献类型：
--
作者：
D. Colognesi;M. Celli;M. Neumann;M. Zoppi
我们用非相干非弹性中子散射测量了不同热力学条件下纯液氢和掺氢液氢的动态结构因子。实验是在TOSCA-II上进行的，这是一种新型的飞行时间、逆几何、晶体分析光谱仪。经过精确的数据归约，利用修正的Young和Koppel模型研究了后向散射记录的中子谱的高能部分，并由此估算了氢分子的平均动能。此外，在高斯近似的框架下分析了前向散射中记录的中子谱的低能部分，并通过Levesque-Verlet模型对速度自相关函数进行了拟合。从而确定了各种物理量，并与精确的路径积分蒙特卡罗模拟进行了比较。尽管这些拟合的质量很好，但从前向散射数据导出的速度自相关函数似乎完全无法正确描述后向散射数据。这些发现证明了高斯近似在半量子液体中的无可置疑的崩溃。目前的结果似乎引起了极大的兴趣，并建议进一步研究广泛使用的高斯近似的局限性。
We have measured the dynamic structure factor of liquid parahydrogen, pure and mixed with deuterium, in various thermodynamic conditions using incoherent inelastic neutron scattering. The experiments were carried out on TOSCA-II, a new time-of-flight, inverse-geometry, crystal-analyzer spectrometer. After an accurate data reduction, the high-energy parts of the neutron spectra recorded in backward scattering were studied through the modified Young and Koppel model, from which the mean kinetic energy values for a hydrogen molecule were estimated. In addition the low-energy parts of the neutron spectra recorded in forward scattering were analyzed in the framework of the Gaussian approximation and fitted through a Levesque-Verlet model for the velocity autocorrelation function. Thus various physical quantities are determined and compared with accurate path integral Monte Carlo simulations. Despite the excellent quality of these fits, the velocity autocorrelation functions derived from the forward-scattering data appear totally unable to properly describe the backward-scattering ones. These findings prove an unquestionable breakdown of the Gaussian approximation in semiquantum liquids. The present results appear of great interest and suggest further investigation on the limits of the widely used Gaussian approximation.