Ortho-para conversion of hydrogen molecules at solid surfaces : elucidation of the mechanism and application to a new spin probe at surfaces

氢分子具有内部自由度、核自旋和旋转运动，它们在与固体表面的相互作用中起着重要的作用。本项目的目的是阐明邻氢和对氢与固体表面相互作用差异和邻位-对位转化的微观机理。为了实现这一点，我们发展了一种利用共振增强多光子电离的旋转/自旋态选择性热脱附谱的新实验技术，利用该技术，我们实验研究了正氢和对氢分子在介孔氧化铝和铜表面的脱附能差和邻位-对位转化率。结果表明：1.氢分子在氧化铝介孔上的脱附能高于在颗粒表面的脱附能；2.在介孔上的邻位-对位转化时间为32min；3.在颗粒表面的脱附时间小于7min；4.正位氢的脱附能比对位氢高约6 meV。邻位-对位转换被认为是由表面与杂质自旋的磁偶极子相互作用驱动的。结果表明，正氢和对氢的脱附能差异源于这两种表面上吸附势能的各向异性。

Hydrogen molecules have internal degrees of freedom, nuclear spin and rotational motion, which are known to play an important role in the interaction with solid surfaces. The purpose of the present project is to elucidate the microscopic mechanism of the interaction difference between the ortho-and para-hydrogen with solid surfaces and the ortho-para conversion. In order to achieve this, we have developed a new experimental technique of rotational/spin-state selective thermal desorption spectroscopy by using resonance-enhanced multiphoton ionization, with which we have experimentally investigated the difference of the desorption energy between ortho- and para-hydrogen molecules and the ortho-para conversion rate at mesoporous alumina and copper surfaces. We have found the following results : 1. The desorption energy of hydrogen molecules at meso-pores of alumina is higher than that at particle surfaces, 2. The ortho-para conversion time is 32 min at meso-pores, 3. The ortho-para conversion time at particle surfaces is shorter than 7 min, 4. The desorption energy of the ortho-hydrogen is higher than that of para-hydrogen by about 6 meV. The ortho-para conversion is considered to be driven by the magnetic dipole interaction with impurity spins at surfaces. The difference of the desorption energy between ortho- and para-hydrogen is concluded to originate from the anisotropy of the adsorption potential at these surfaces.