We have succeeded in observing nuclear excitation by electron transition (NEET) in 197Au by a new method. Monochromatic x-rays of synchrotron radiation were used to ionize the K shell of gold atoms in a target foil. The internal-conversion electrons emitted from excited nuclei were detected with a silicon avalanche photodiode. At a photon energy of 80.989 keV, the NEET probability in 197Au was determined to be (5.0+/-0.6)x10(-8) from a comparison of the event number per photon between NEET and the nuclear resonance at 77.351 keV.

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...

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