Photostimulated Desorption and Ortho-Para Conversion ofH2on Ag Surfaces

Abstract

Photostimulated desorption and change of the ortho-para ratio of H2 physisorbed on Ag at 7 K are reported. After pump laser excitation at 193 nm neutral H2 desorbing in a nonthermal process is state selectively detected by resonance-enhanced multiphoton ionization. At weak pump laser fluence the natural ortho-para conversion of H2 on Ag was monitored to proceed with a conversion time of approximately 780 s, which is in good agreement with predictions based on an electron-exchange-hyperfine-contact model. With increasing pump laser fluence, the ortho-para ratio decreases faster, suggesting photoassisted ortho-para H2 conversion.

Photostimulated desorption of Xe from Au(001) surfaces via transient Xe−formation

“…[1][2][3] Laserinduced thermal desorption (LITD) was investigated in detail for the systems of CO/Fe(110) (Ref. 4) and Xe/Cu, 5 and has been successfully applied to the studies of surface diffusion combined with low-energy electron microscopy 6 or with scanning tunneling microscopy.…”

Mechanism of theortho-paraconversion of hydrogen on Ag surfaces

“…After a fit of these data points to an exponential function ͓C exp͑−t / ͔͒, where C and are fit parameters, the conversion time was estimated to be 610Ϯ 90 s. This value agrees roughly with the previous experimental value. 10 Figures 1͑b͒ and 1͑c͒ demonstrate the photoexcitation effect on the o-p conversion. Figure 1͑b͒ denotes the evolution of the o-p ratio under the irradiation of photons of 193 nm with a fluence of 120 J / cm 2 / pulse and a repetition rate of 10 Hz.…”

“…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.…”

“…Although the actual coverage is not known, the H 2 ͑D 2 ͒ layer is considered to be in the monolayer regime as discussed in Ref. 10.…”

“…The ortho-para ratio of adsorbed H 2 ͑D 2 ͒ was monitored by a sort of pump-probe technique: 10 Pulse laser light at a wavelength of 193 nm ͑2.2 mm in diameter͒ irradiated the sample at an incidence angle of 81°, and subsequently the desorbed H 2 ͑D 2 ͒ molecules from the surface was J-state selectively detected by ͑2+1͒ resonance-enhanced multiphoton ionization ͑REMPI͒ via the E,F 1 ⌺ g + states. 13 The REMPI laser pulses with a wavelength of 201 nm and an energy of 120-360 J / pulse were focused at a distance of about 10 mm in front of the sample at a repetition rate of 10 Hz.…”