Nuclear Spin Conversion of Molecularly Chemisorbed H₂ on Pd(210)

It has been known that physisorbed hydrogen undergoes nuclear spin conversion (NSC) in the order of minutes on non-magnetic metal surfaces such as Ag(111)[1]. This has been described by the virtual surface-molecule electron transfer and the hyperfine Fermi contact interaction within second order perturbation theory. Recent experiments have shown that on Cu(510)[2] and Pd(210)[3] surfaces, H₂ is in the so-called molecularly chemisorbed state and behaves as a 2-dimensional quantum rotor, while the NSC rate is 1 order magnitude faster than on flat surfaces.

In this work, we investigate the NSC of the molecularly chemisorbed H₂ on Pd(210). The nuclear spin transition probability is evaluated by means of second order perturbation theory using the parameters derived from first principles calculations[4] . Our results show that the NSC time of H₂/Pd(210) is around 2 seconds, in agreement with experiment. Such a fast NSC rate can be attributed to a number of factors such as the substrate work function, the density of states at the Fermi level the electronic molecule-surface coupling potential. Both the work function and electronic density of states at the Fermi level of Pd(210) are larger compared with Ag(111), resulting in the increased NSC probability. In addition, the increased molecule-surface interaction in the molecularly chemisorbed state enhances the matrix element describing the virtual electron transfer. This effect further increases the NSC probability and rate.