Low-temperature catalysis of para-deuterium to ortho-deuterium conversion on Grafoil

Yucel, S.; Alexander, Neil B.; Hönig, A.

doi:10.1103/physrevb.42.820

Cited by 12 publications

(8 citation statements)

References 14 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…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.…”

mentioning

confidence: 85%

Mechanism of theortho-paraconversion of hydrogen on Ag surfaces

et al. 2008

View full text Add to dashboard Cite

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...

show abstract

mentioning

confidence: 85%

Mechanism of theortho-paraconversion of hydrogen on Ag surfaces

et al. 2008

View full text Add to dashboard Cite

show abstract

“…Despite the absence of a magnetic field, rapid conversions have also been reported on diamagnetic surfaces such as Ag, − Cu, , and graphite, , with a conversion time constant of typically τ = 1–1000 s. Recent experiments observed that H 2 and D 2 molecules desorbed from ASW have low spin temperatures because of NSC on the ASW surface at low temperatures. ,,, For weakly physisorbed H 2 on the surface, such as Ag or ASW, it can rotate and a significant decrease of Δ E op is not expected. Yucel first estimated the conversion times due to magnetic dipole–dipole, Fermi hyperfine contact, and spin–orbit interactions between metal electrons and nuclear spins of H 2 , where the Δ E op is dissipated by emitting phonons to balance the energy.…”

Section: Nuclear-spin Modifications Of Moleculesmentioning

confidence: 99%

Surface Processes on Interstellar Amorphous Solid Water: Adsorption, Diffusion, Tunneling Reactions, and Nuclear-Spin Conversion

2013

View full text Add to dashboard Cite

Formation through H-Atom Recombination 8794 3.3.2. Energy Partitioning in H 2 Formation through Recombination 8796 4. Quantum-Tunneling Reactions on Surfaces 8797 4.1. Water Formation 8798 4.1.1. O 2 + H (Sequential H-Atom Addition to O 2 ) 8800 4.1.2. OH + H 2 8800 4.1.3. Other Reactions 8805 4.2. Formaldehyde and Methanol Formation and Deuteration 8805 4.2.1. Hydrogen and Deuterium Addition to CO 8805 4.2.2. Deuterium Enrichment of CH 3 OH and H 2 CO 8807 4.3. Carbon Dioxide Formation 8811 4.3.1. CO + O 8812 4.3.2. CO + OH 8813 5. Nuclear-Spin Modifications of Molecules 8814 5.1. orthoand para-H 2 Molecules and Their Roles in Interstellar Chemistry 8814 5.1.1. Nascent Ortho-to-Para Ratio of H 2 Molecules Formed on Ice Mantles 8815 5.1.2. Nuclear-Spin Conversion of H 2 Molecules 8816 5.2. Ortho and Para States of Other Abundant Molecules (H 2 O, etc.) 8819 5.2.1. Nuclear-Spin Conversion of Polyatomic Molecules by Collisions 8820 5.2.2.

show abstract

“…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].…”

mentioning

confidence: 98%

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

et al. 2016

View full text Add to dashboard Cite

The surface temperature dependence of the ortho-to-para conversion of H-2 on amorphous solid water is first reported. A combination of photostimulated desorption and resonance-enhanced multiphoton ionization techniques allowed us to sensitively probe the conversion on the surface of amorphous solid water at temperatures of 9.2-16 K. Within a narrow temperature window of 8 K, the conversion time steeply varied from similar to 4.1 x 10(3) to similar to 6.4 x 10(2) s. The observed temperature dependence is discussed in the context of previously suggested models and the energy dissipation process. The two-phonon process most likely dominates the conversion rate at low temperatures

show abstract

Low-temperature catalysis of para-deuterium to ortho-deuterium conversion on Grafoil

Cited by 12 publications

References 14 publications

Mechanism of theortho-paraconversion of hydrogen on Ag surfaces

Mechanism of theortho-paraconversion of hydrogen on Ag surfaces

Surface Processes on Interstellar Amorphous Solid Water: Adsorption, Diffusion, Tunneling Reactions, and Nuclear-Spin Conversion

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

Contact Info

Product

Resources

About