Mechanism of theortho-paraconversion of hydrogen on Ag surfaces

Abstract

Ortho-para conversion of H 2 and para-ortho conversion of D 2 on an Ag surface were investigated by resonance-enhanced multiphoton ionization combined with photostimulated desorption. The conversion processes were found to be accelerated by laser irradiation at 193 nm, while photo-irradiation at 532 nm had little effect on the conversion time. The natural conversion time of H 2 and D 2 was estimated to be 610 and 1030 s, respectively. The roles of electron transfer and rotational-energy dissipation in the conversion are discussed.The simplest molecule of hydrogen has a remarkable feature due to the quantum statistics. According to the symmetry of the total nuclear spin ͑I͒ function, H 2 and D 2 are classified into ortho and para species: Ortho ͑para͒ H 2 has I =1 ͑0͒, whereas ortho ͑para͒ D 2 has I = 2 or 0 ͑1͒. Since protons and deuterons are fermions and bosons, respectively, the total wave function of H 2 and D 2 must be antisymmetric and symmetric with respect to the exchange of the two nuclei, respectively. This requires that ortho and para H 2 are in the rotational states with odd and even rotational quantum numbers ͑J͒, while para and ortho D 2 have odd and even J, respectively. 1,2One particular feature is that the transition between the ortho and para states is so slow in the gas phase with a conversion time of the order of 10 10 s because of the small coupling terms related to the nuclear spins. 1 Nevertheless, the ortho-para ͑o-p͒ conversion is promoted in the presence of magnetic impurities and on magnetic surfaces. The mechanism of the o-p conversion has been discussed on the basis of the Wigner model where magnetic dipole interaction is responsible for the nuclear-spin flip. 3,4 Interestingly, however, the o-p conversion has also been observed on Ag, 5,6 Cu, 7 and graphite 8,9 surfaces, though these surfaces are diamagnetic. High-resolution electron-energy-loss spectroscopy ͑HREELS͒ studies showed that the H 2 o-p conversion time is 10 2 -10 3 s on Ag and Cu, 5-7 while a laser spectroscopic experiment combined with photostimulated desorption ͑PSD͒ revealed the conversion time of ϳ800 s. 10 Furthermore, the o-p conversion is promoted at the step site of Cu͑510͒ with a conversion time as short as 1 s, 11 while the o-p H 2 conversion is accelerated under photon irradiation. 10 A theoretical study showed that direct interaction of the substrate electrons with the adsorbed H 2 leads to a conversion time in the order of hours. 12 On the other hand, a Coulomb-contact model predicted an o-p conversion time on the order of 1 -10 min. This is a second-order perturbation theory where electrons are virtually transferred between metal substrates and the adsorbed H 2 followed by nuclearspin flip through the hyperfine contact interaction. 4 Although the conversion time obtained by this model is in agreement with the experimental values, the role of the electron transfer and rotational-energy dissipation have not been experimentally confirmed to date.In an attempt to clarify the mechanism of the o-p conversion on m...

Effects of coadsorbedO2on hydrogen ortho-para conversion on Ag surfaces

“…On bare Ag, the conversion mechanism has been discussed to be the Coulomb-contact model in which electron transfer between the substrate and hydrogen is involved. In our previous work, 14 we discussed the isotope dependence on bare Ag in terms of electron transfer and/or rotational-energy dissipation. We suggested that the probability of electron transfer or rotational-energy release is larger for D 2 than H 2 , resulting in deviation of the conversion time ratio from 7.1.…”

“…13,14 Briefly, a Ag film deposited on a Cu block was used as the sample surface. Ag of ϳ5 nm ͑purity Ն 99.99%͒ was deposited in an ultrahigh vacuum ͑UHV͒ chamber ͑2 ϫ 10 −8 Pa͒ with a W filament at a substrate temperature of 6.8 K followed by annealing of the film at 300 K. Ag forms islands with an average size of 50 nm consisting of an average terrace width of 6 nm as characterized by scanning tunnel microscope ͑STM͒ in a separate chamber.…”

“…The ion intensity was normalized to the REMPI laser intensity following the relation between the laser fluence and the ion yield. 13,14 After each experiment, both hydrogen and oxygen molecules were desorbed from the surface by heating the sample up to room temperature. After the heat treatment, the o-p conversion time without O 2 exposure was confirmed to be identical to the value obtained on a freshly prepared Ag surface.…”

“…12 Photoacceleration of the o-p conversion furthermore suggested that the electron transfer is important for the o-p ͑p-o͒ conversion of H 2 ͑D 2 ͒. 13,14 O 2 molecules have two unpaired electrons in the antibonding orbitals of 2p ‫ء‬ revealing a diradical nature. When molecular O 2 is adsorbed on a metal surface, two possible adsorption states exist: 15 ͑1͒ physisorbed state, which is neutral O 2 , and ͑2͒ chemisorbed state, which is either superoxolike O 2 − or peroxolike O 2 2− .…”

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].…”