Katsuyuki Fukutani

UV-laser-induced desorption of NO from Pt(111)

Abstract Ultraviolet-laser-induced desorption of NO molecules from a Pt(111) surface is investigated at 80 K by state-selective detection. We find two distinctive desorption channels are operative on an as-adsorbed surface and a surface annealed at 220 K. Rotational energy distributions are Boltzmann on as-adsorbed Pt(111) and non-Boltzmann on annealed Pt(111) with inverted population of spin-orbit states. Rotational, vibrational, and translational temperatures of NO from an as-adsorbed surface are increased with the incident photon energy. The effect of the adsorption state on desorption and the desorption mechanism are discussed.

Study of the adsorption structure of NO on Pt(111) by scanning tunneling microscopy and high-resolution electron energy-loss spectroscopy

The structure of nitric oxide (NO) on a Pt(111) surface was studied by scanning tunneling microscopy (STM ) and high-resolution electron energy-loss spectroscopy (HREELS ). The coexistence of two species is observed by STM after saturating the surface with NO at 70 K and annealing to 215 K. Two pairs of NMO and PtMN stretching vibrational frequencies are observed in HREELS spectra after annealing, which are assigned to the two species observed by STM. Comparison between the spectra of HREELS and infrared absorption spectroscopy (IRAS) indicates that IRAS is less sensitive to the lower-frequency mode. A new model for the adsorption structure of NO on the Pt(111) surface is proposed and discussed.

Photo-stimulated desorption of NO from a Pt(001) surface

Abstract Desorption of NO molecules chemisorbed on a Pt(001) surface at 80 K induced by ultraviolet laser radiation is investigated by a resonance-enhanced multiphoton ionization technique. The desorption yield of neutral NO is proportional to pump laser fluence, which is indicative of a single-photon process. The translational, rotational, and vibrational temperatures of desorbed molecules are approximately 650, 300, and 1200 K, respectively. The substantial difference of these values from sample temperature implies that desorption is not thermally driven, but induced by electronic excitation. Polarization and wavelength dependences of a pump laser are examined to characterize the electronic excitation relevant to desorption. No significant difference is observed for either s- and p-polarization and at λ = 193, 248, and 352 nm. The incidence angle dependence of the desorption yield is similar to that of the substrate ultraviolet light absorption. These results suggest that the initial step of photodesorption is substrate valence electron excitation.

Adsorption and desorption of NO and CO on a Pt(111)-Ge surface alloy

Abstract Adsorption of NO and CO on Pt(111) alloyed with a few per cent of Ge is investigated by reflection—absorption infrared spectroscopy and thermal desorption spectroscopy. Both molecules exclusively occupy the on-top site in contrast to bridge and on-top adsorption on clean Pt(111). The adsorption energy of NO is dramatically reduced compared with that on clean Pt(111). Photodesorption of CO observed on the clean Pt(111) is noticeably suppressed on the Pt(111)Ge surface alloy, while NO desorption is induced by photon irradiation. The rotational and translational temperatures of photodesorbed No are similar to those on clean Pt(111). The change in chemical properties of Pt(111) for molecular adsorption is discussed in terms of d-band filling of the substrate.

Influence of H2-annealing on the hydrogen distribution near SiO2/Si(100) interfaces revealed by in situ nuclear reaction analysis

Employing hydrogen depth-profiling via 1H(15N,αγ)12C nuclear reaction analysis (NRA), the “native” H concentration in thin (19–41.5 nm) SiO2 films grown on Si(100) under “wet” oxidation conditions (H2+O2) was determined to be (1–2)×1019 cm−3. Upon ion-beam irradiation during NRA this hydrogen is redistributed within the oxide and accumulates in a ∼8-nm-wide region centered ∼4 nm in front of the SiO2/Si(100) interface. Annealing in H2 near 400 °C introduces hydrogen preferentially into the near-interfacial oxide region, where apparently large numbers of hydrogen trap sites are available. The amount of incorporated H exceeds the quantity necessary to H-passivate dangling Si bonds at the direct SiO2/Si(100) interface by more than one order of magnitude. The H uptake is strongly dependent on the H2-annealing temperature and is suppressed above 430 °C. This temperature marks the onset of hydrogen desorption from the near-interfacial oxide trap sites, contrasting the thermal stability of the native H, which pre...

Photodesorption of CO and CO+ from Pt(111): Mechanism and site specificity

Ultraviolet photodesorption of CO and CO+ from Pt(111) at 80 K is investigated by (2+1) resonance‐enhanced multiphoton ionization and reflection absorption infrared spectroscopy. Desorption of CO and CO+ occurs at the on‐top site as single‐photon and three‐photon processes, respectively. The rotational, vibrational, and translational temperatures of desorbed CO are approximately 130, 3700, and 2000 K, which are considerably higher than the sample temperature. The threshold energy of neutral CO desorption lies between 2.3 and 3.5 eV suggesting that an unoccupied 2π state is responsible for the desorption.

Novel insight into the hydrogen absorption mechanism at the Pd(110) surface.

The microscopic mechanism of low-temperature (80 K < T < 160 K) hydrogen (H) ingress into the H2 (<2.66 × 10(-3) Pa) exposed Pd(110) surface is explored by H depth profiling with (15)N nuclear reaction analysis (NRA) and thermal desorption spectroscopy (TDS) with isotope (H, D) labeled surface hydrogen. NRA and TDS reveal two types of absorbed hydrogen states of distinctly different depth distributions. Between 80 K and ∼145 K a near-surface hydride phase evolving as the TDS α1 feature at 160 K forms, which initially extends only several nanometers into depth. On the other hand, a bulk-absorbed hydrogen state develops between 80 K and ∼160 K which gives rise to a characteristic α3 TDS feature above 190 K. These two absorbed states are populated at spatially separated surface entrance channels. The near-surface hydride is populated through rapid penetration at minority sites (presumably defects) while the bulk-absorbed state forms at regular terraces with much lower probability per site. In both cases, absorption of gas phase hydrogen transfers pre-adsorbed hydrogen atoms below the surface and replaces them at the chemisorption sites by post-dosed hydrogen in a process that requires much less activation energy (<100 meV) than monatomic diffusion of chemisorbed H atoms into subsurface sites. This small energy barrier suggests that the rate-determining step of the absorption process is either H2 dissociation on the H-saturated Pd surface or a concerted penetration mechanism, where excess H atoms weakly bound to energetically less favorable adsorption sites stabilize themselves in the chemisorption wells while pre-chemisorbed H atoms simultaneously transit into the subsurface. The peculiarity of absorption at regular Pd(110) terraces in comparison to Pd(111) and Pd(100) is discussed.

Theoretical study on the photostimulated desorption of CO from a Pt surface

Photostimulated desorptions (PSD’s) of CO, CO+, and CO− from a Pt surface are studied theoretically using Pt2–CO model cluster including image force correction. Calculations are performed by the single excitation configuration interaction and the symmetry adapted cluster (SAC)/SAC‐CI methods. The PSD’s of the ground state CO occur as the Menzel–Gomer–Redhead (MGR) process and those of CO+ (n cation) and excited (n→π*) CO* through the modified MGR process in which the upper repulsive potential curves are nonadiabatic; the process proceeds through a sequence of nonadiabatic transitions between the similar pertinent states embedded in the metal excited bands. The excited states as the desorption channels are characterized by the excitations from the Pt–CO bonding orbitals to the antibonding MO’s: metal‐adsorbate chemical bond cleavage by photons which leads to a repulsive potential is essential for the PSD. The electrostatic image force interaction plays only a minor role and the present result does not support the Antoniewicz model. The calculated excitation‐energy thresholds for the CO, CO+, and CO* desorptions are 1.6∼2.6, 11.3, and 11.3–12.7 eV, respectively, which explains the energy thresholds and the fluence dependencies of the incident laser in the PSD experiments. On the other hand, the PSD giving CO− would occur with the energy range of 6.2–8.2 eV, one to two photon energy of the 193 nm (6.4 eV) laser. Since the upper nonadiabatic potential curves have shallow minima, in this case, the lifetime of the CO− species would be larger than those of the CO+ and CO* species. The present study clarifies the electronic structures of the desorbed CO+, CO−, and CO* species, which have not been identified experimentally.

Below-surface behavior of hydrogen studied by nuclear reaction analysis

Abstract The behavior of hydrogen in the near-surface region can be investigated by a nuclear reaction analysis (NRA) technique. The H atom adsorbed on a surface might penetrate the surface and in-diffuse toward the bulk. During metal–semiconductor interface formation, H might be trapped at the interface or out-diffuse toward the surface of the overlayer. The NRA method combined with surface science techniques provides insight into the ‘below-surface’ behavior of H on a nanometer scale.

Adsorption structures of NO/Pt(111) investigated by scanning tunneling microscopy

The adsorption structure of nitric oxide (NO) on Pt(111) was studied at 10 and 70 K by scanning tunneling microscopy (STM). The island growth modes at both temperatures are similar except for the domain size of the 2×2 structure. In these low temperature region, two phases can coexist at medium coverages. These phases are assigned to the two NO species occurring at different stretching-vibrational frequencies observed in the previous vibrational spectroscopic studies. The relative location of two different species observed by STM and its stretching-vibrational frequencies suggests that the adsorption sites of NO on the Pt(111) surface at low and high coverages correspond to the hollow and the on-top sites, respectively.