NMR Study of Ortho-Para Conversion at High Pressure in Hydrogen
NMR Study of Ortho-Para Conversion at High Pressure in Hydrogen
复制标题
氢气高压下邻位-对位转化的核磁共振研究
DOI：
10.1103/physrevlett.81.4180
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发表时间：
1998
影响因子：
8.6
通讯作者：
I. Silvera
中科院分区：
文献类型：
--
作者：
M. Pravica;I. Silvera
分子氢具有显著的性质，即具有两种物质，邻位和帕拉，这是由于在自旋12质子交换下波函数反对称的要求。仲氢（p-H2）的单分子波函数具有核自旋0和量子数为J，M的偶转动态，而邻氢（o-H2）具有总自旋1和奇转动态[1]。由于在低温下，帕拉是一个球对称的分子状态的J <$0自由转子状态，而邻位是在取向不均匀的J <$1状态，这两种物质的固体有深刻的差异，无论是在他们的结构和激发光谱。近年来，人们对高压下氢的相态进行了广泛的研究，但对邻-对位（o-p）浓度的了解甚少，可能除了极限值。由于邻位浓度在0和1之间是连续变化的，因此存在一个连续的相图，该相图应该由它们的邻位浓度来标记。在这封信中，我们报告了核磁共振研究的o-p转换速率常数作为压力或密度的函数，以12.8 GPa（128千巴）。纯固体o-H2在零压力下以1.9%yh的速率向平衡转化[1-4]。低压研究（至0.6 GPa）表明，速率常数增加到几个单位小时[1-4]，然后降低[3-4]。测量o-p转换有两种基本方法：（1）我们使用的方法，NMR：具有I 1的邻位分子有助于核磁化，从而有助于NMR信号，而I 0分子则没有，以及（2）测量由于晶格释放和吸收的转换能量引起的加热。NRM方法应该在最高压力下仍然有效，因为预计核状态几乎不会受到压力的干扰，直到分子解离成原子状态，这在几兆巴压力下是预期的[5]。之前在金刚石对顶砧单元（DAC）中对氢的最高NMR研究为6.8 GPa [6]，但没有测量o-p转化率。我们开发了一种新型的脉冲NMR系统[7]，使我们能够研究DAC中的o-p转换。虽然速率常数在我们的最低压力4 GPa下仅为2%yh，但它迅速增加，在我们的最高压力下上升到58%yh。
Molecular hydrogen has the remarkable property of having two species, ortho and para, due to the requirement that the wave function be antisymmetric under exchange of the spin12 protons. The single-molecule wave functions for para-hydrogen (p-H2) have nuclear spin 0 and even rotational states with quantum numbers J, M, while ortho-hydrogen (o-H2) has total spin 1 and odd rotational states [1]. Since at low temperature, para is a spherically symmetric molecular state in the J 0 free rotor state, while ortho is in the orientationally nonuniform J 1 states, solids of the two species have profound differences, both in their structures and excitation spectra. In recent years there have been extensive studies of phases of hydrogen at high pressure with little knowledge of the ortho-para (o-p) concentration, except perhaps for limiting values. As there is a continuous variation of ortho concentration between 0 and 1, there is a continuum of phase diagrams which should be labeled by their ortho concentration. In this Letter we report an NMR study of the o-p conversion rate constant as a function of pressure or density to 12.8 GPa (128 kbar). Pure solid o-H2 converts towards equilibrium at a rate of 1.9%yh at zero pressure [1–4]. Low pressure studies (to ,0.6 GPa) have shown the rate constant to increase to several percentyh [1–4] and then decrease [3–4]. There are two fundamental methods of measuring o-p conversion: (1) the method we have used, NMR: the ortho molecules with I 1 contribute to the nuclear magnetization and thus the NMR signal, while the I 0 molecules do not, and (2) measurement of the heating due to conversion energy liberated and absorbed by the lattice. The NRM method should remain valid to the highest pressures as the nuclear states are expected to be little perturbed by pressure until the molecules dissociate into the atomic state, which is expected at multimegabar pressures [5]. The highest previous NMR study of hydrogen in a diamond anvil cell (DAC) was to 6.8 GPa [6], but o-p conversion rates were not measured. We have developed a novel pulsed NMR system [7] allowing us to study o-p conversion in a DAC. Although the rate constant is only 2%yh at our lowest pressure of 4 GPa, it rapidly increases, rising to 58%yh at our highest pressure.