2007 Fiscal Year Final Research Report Summary
Effects of magnetism and magnetic-field on the ortho-para conversion of hydrogen on solid surfaces
| Project/Area Number |
17360017
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Thin film/Surface and interfacial physical properties
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| Research Institution | The University of Tokyo |
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Principal Investigator |
FUKUTANI Katsuyuki The University of Tokyo, Institute of Industrial Science, Professor (10228900)
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| Co-Investigator(Kenkyū-buntansha) |
OKANO Tatsuo The University of Tokyo, Institute of Industrial Science, Professor (60011219)
WILDE Markus The University of Tokyo, Institute of Industrial Science, Research Associate (10301136)
KASAI Hideaki Osaka University, Graduate School of Engineering, Professor (00177354)
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| Project Period (FY) |
2005 – 2007
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| Keywords | Hydrogen / Nuclear spin / Ortho-para conversion / Magnetic-field effect / Magnetic surfaces / Rotational energy / Oxygen |
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| Research Abstract |
Molecular hydrogen has a remarkable feature that it has two nuclear-spin modifications of ortho and para species. The purpose of the present project is to investigate the magnetic effect on the ortho-para conversion of hydrogen on transition-metal oxide surfaces. To achieve this, we have first developed an apparatus that allows us to experimentally study the ortho-para conversion on single-crystal surfaces under a strong magnetic field. As a sensitive probe of ortho and para hydrogen, we have also developed laser induced fluorescence via the E, F state, where the infra-red emission between the E, F and B states were measured with a InGaAs photomultiplier.
With this technique, we measured the ortho-para conversion time of H2 and D2 adsorbed on a Cr203(0001) surface grown on a Cr(110) surface at 8 K. The conversion time was found to be 210 and 540 s for H2 and D2, respectively. These values are considerably longer than those at the 02-adsorbed surface, which were also investigated in the
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present project. In the Cr203 bulk, each Cr ion has a 3/2 spin, which is antiferromagnetically ordered. On the (0001) surface, only a half of the Cr ion is present on the surface to alleviate the electro-static energy. As a result, the surface looks ferromagnetically ordered thereby yielding a weak inhomogeneous magnetic field. As a next step, we measured the H2 ortho-para conversion under magnetic fields of 0, 1, and 3 T at a sample temperature of 44.5 K the conversion time was found to be 108, 108, and 116 min at 0, 1, and 3 T, respectively. A theory suggests that there appears a magnetic-field effect in the case that the rotational energy of ortho-H2 is dissipated into the electronic system of the substrate. The present experimental results indicate that the rotational energy should be accommodated by either magnetic excitation or phonon excitation of the Cr2O3 surface. Furthermore, it was found that the conversion time is strongly dependent on the sample temperature: while no definite conversion was observed at 300 K, the conversion was accelerated at 40 K These results imply that a dynamic conversion mechanism is operative in addition to the static ortho-para conversion in the physisorption well.
To further get insight into the effect of surface magnetism on the ortho-para conversion, the conversion time was measured on a Ag surface with magnetic impurities of O2 molecules. With increasing the O2 coverage, the conversion was found to be dramatically accelerated. With the aid of Monte Carlo simulations, the conversion time in the vicinity of adsorbed 02 was concluded to be 8.2 s. This value compares well with an existing theory. Less
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