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

点击翻译按钮获取中文摘要

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.