Ortho-paraH2conversion on a cold Ag(111) metal surface

Abstract

Electron-energy-loss experiments of H2 physisorbed on noble metals at low temperature indicate that the ortho-para conversion rate is 2 orders of magnitude faster on a Ag(l 11) surface than on a Cu(lOO) one. We suggest a process in which a metal electron is virtually transferred back and forth from a surface band to the molecule antibonding orbital, the ortho-para energy being dissipated by metal (electron-hole) triplet pairs. The ortho-para rates are about 1 min on Ag(lll) and 1 h on Cu(lOO), in agreement with experimental data. This large difference does not characterize Ag and Cu, but is inherent to different surface orientations.

Photostimulated Desorption and Ortho-Para Conversion ofH2on Ag Surfaces

“…To achieve a high cross section for rotational excitation in the EELS measurements [2], a primary electron energy of 3 eV was used, which lies in the shape resonance of H 2 . As pointed out in theoretical studies [10,17] and also indirectly supported by the photostimulated o-p conversion rate observed in the present paper, such a resonance may efficiently promote o-p conversion.…”

“…Later, Ilisca proposed a theoretical model for the fast o-p conversion on nonmagnetic metal surfaces, which involves a virtual electron transfer from the metal substrates to the adsorbed H 2 , followed by spin conversion through hyperfine-contact interaction. This hyperfine-Coulomb excitation model predicted an o-p conversion time on the order of 1-10 min [10,11].…”

“…This conversion time corresponds to the natural conversion process arising from the interaction of H 2 with the Ag substrate. According to theoretical studies [9,10], conversion via direct interaction with the substrate electrons takes 10 h, while indirect conversion mechanisms involving virtual electron transfer between the substrate and physisorbed H 2 followed by hyperfine-contact interaction occur on a scale of 100 s. Two types of intermediate states are considered for the electron exchange, neutral and charge transfer, and the conversion time on Ag(111) is calculated to be 420 and 120 s, respectively [11]. The experimentally measured value in the present work is apparently in good agreement with the indirect mechanism via the neutral intermediate.…”

“…According to the exchange-contact model, the conversion probability (W) is expressed as follows [10,17]:…”

Surface Temperature Dependence of Hydrogen Ortho-Para Conversion on Amorphous Solid Water

“…On magnetic surfaces and/or with magnetic impurities, such as O 2 , the OP conversion is induced via magnetic dipole interactions [4,5]. Furthermore, conversion on diamagnetic surfaces such as Ag [4][5][6][7][8][9], Cu [10,11], and graphite [12,13] have been reported. The proposed conversion mechanism for those surfaces is the Coulomb contact model [3].…”