Akihide Wada

Adsorption of CO and NO on NiO(111)Ni(111) surface studied by infrared-visible sum frequency generation spectroscopy

Abstract Adsorption of CO and NO on NiO(111) thin films epitaxially grown on Ni(111) substrate has been studied by infrared-visible sum frequency generation (SFG) and the results were compared with those of infrared reflection absorption spectroscopy. From SFG measurements, the CO stretching band of adsorbed CO was observed at 2144 cm −1 for both p- and s-polarized visible (532 nm) light whereas the adsorbed NO gave the NO stretching band at 1800 cm −1 only for the p-polarized visible light. These observations suggested that the CO molecule was inclined to the surface whereas the tilt angle of NO from the surface normal was smaller than that of CO. The adsorption sites of CO and NO molecules are located on the slopes of trigonal microfacets formed by the reconstruction of the NiO(111) surface. CO adsorbed on Ni(111) instead of NiO(111) was also examined: the SFG signal corresponding to the CO stretching mode of linearly bonded CO was observed at 2076 cm −1 only for the p-polarized visible light, but that of the bridge-bonded one (at saturation coverage) was not detected by SFG.

In situ SFG spectroscopy of film growth. I. General formulation and the analysis of the signal observed during the deposition of formic acid on Pt(110)-(1×2) surface

The application of infrared-visible sum-frequency generation (SFG) spectroscopy to the in situ monitoring of film growth was examined by the observation of the growth of the physisorbed layer of formic acid on a reconstructed Pt(110)-(1×2) surface. The signal by the CH stretching band displayed a characteristic interference pattern as deposition proceeded at 158 K to prove that the signal originated from inside the deposited film and the CH bond and molecular plane of formic acid was angled to the surface. The expressions for the electric fields of the SF beams generated inside the film, at the top layer, and at the bottom layer were derived by incorporating the interference effects of the exciting and generated light beams. The formula was used to simulate the observation and to estimate the growth rate.

The effect of adsorbed noble gas atoms on vibrational relaxation of hydroxyl group in zeolite

By means of tunable infrared pump–probe experiment, vibrational relaxation rates (v=1→0) of OH stretching mode of Bro/nsted acidic hydroxyl groups in a mordenite zeolite, which were interacting with noble gases (He, Ar, Kr, or Xe), were measured. The population lifetime, T1, decreased with the increase of the interaction with noble gases, e.g., 170 ps and 58 ps for isolated hydroxyl groups and ones interacting with Xe, respectively. Using a simple linear chain model, the mechanism for the enhancement of the relaxation rate was studied.

TRANSIENT ABSORPTION SPECTRA OF VIBRATIONALLY EXCITED OH/OD GROUPS IN MORDENITE ZEOLITES : EFFECT OF XE ADSORPTION

Transient spectra following the v=1←0 excitation of the OH stretching mode of Bro/nsted acidic hydroxyl groups in normal and deuterated mordenite zeolites have been measured by a two‐color picosecond pump–probe technique. When the OH and OD stretching bands were excited by resonant IR pulses, transient bleaching of the fundamental band (v=1←0) and the transient hot band (v=2←1) absorption signal were observed. For the OH and OD groups free of adsorbates, the spectral widths of the pump‐induced signals were broadened by ∼4 cm−1 due to pure dephasing from the convoluted width of the pump and probe IR pulses. When the OH and OD groups were adsorbed by a Xe atom, the frequencies of the ν(OH) and ν(OD) bands shifted to lower frequency and at the same time the widths of both the transient bleaching and the transient hot band absorption were much broader than those of the isolated hydroxyl groups by the factor of more than three. Actually, the widths of the transient signals were about the same as those of the l...

A TPD and SFG study of propionic acid adsorbed on Ni(110) surface

Abstract Propionic acid adsorbed on Ni(110) surface was studied by temperature programmed desorption (TPD) and infrared-visible sum frequency generation (SFG) spectroscopy. The TPD results indicated that propionic acid molecularly adsorbed on the Ni(110) surface at 113 K and that the multilayers of propionic acid started to desorb at 213 K. The molecularly adsorbed propionic acid on the first layer decomposed on further heating to form propionate species which decomposed at 390 K to H 2 (g) and CO 2 (g), leaving carbon on the surface. The SFG spectra of the propionic acid adsorbed in various thickness at 113 K gave vibrational peaks at 2885 cm −1 , 2956 cm −1 , and 2990 cm −1 , assigned to the symmetric CH stretching mode of the CH 2 group, symmetric CH stretching mode of the CH 3 group, and antisymmetric CH stretching mode of the CH 2 group, respectively, of propionic acid. The multilayered films gave peaks under both the (pp) and (sp) polarization combinations of visible and infrared pulses but the signals disappeared for the (sp) polarization combination when the coverage was lower than one monolayer (ML). The absence of the SFG peaks for the (sp) polarization combination on the low coverage surface was ascribed to the difference in molecular orientation and molecular SFG tensor. The SFG spectrum of the surface which was dosed at 300 K gave vibrational peaks at 2885 cm −1 , 2948 cm −1 , and 2990 cm −1 only for the (pp) polarization combination. These peaks were assigned to the symmetric CH stretching mode of the CH 2 group, symmetric CH stretching mode of the CH 3 group, and the antisymmetric CH stretching mode of the CH 2 group of propionate, the product of dissociative adsorption.

In situ SFG spectroscopy of film growth. II. Deposition of formic acid on Ni(110) surface

Infrared-visible sum-frequency generation (SFG) spectroscopy was used to monitor in situ the growth of multiple layers of formic acid on a Ni(110) surface. The signal by the CH stretching band displayed a characteristic interference pattern as the deposition proceeded and the feature was analyzed by the formula presented in the preceding article. The effect of substrate structure was examined to reveal that the deposition of a SFG active layer on alien surfaces requires the substrate temperature higher than 138 K with the optimum at 158 K, but the same growth as under the optimum condition was sustained at 143 K once the active layer had been formed beforehand. No SFG-wise change was observed when the sample temperature was changed in vacuum either from the SFG undetectable 143–153 K or to the opposite direction to indicate that the layers formed at the two temperatures are not related with phase transition. However, the deposition of the SFG-active layer took place at 143 K, when the sample was precovere...

Picosecond infrared pump–probe spectrum of D2O adsorbed at acid OD group of zeolite

Spectroscopic and dynamic features of the vibrationally excited D2O/zeolite system have been investigated by two-color infrared–infrared pump–probe experiment. The frequency- and delay-scanned probe intensities were measured by tuning the pump laser to the OD stretching bands of the D2O molecule hydrogen bonded to the acidic OD group of mordenite zeolite. Two types of pump-induced signals were observed: the ones which have the population lifetime of 43±5 ps and display frequency shift by the pumping frequency, and the others, which have the lifetime of about 15 ps and exhibit no such frequency shift. Possible origins of the signals are discussed.

Vibrational dynamics of adsorbed D2O on Brønsted hydroxyl group in a zeolite

Abstract The spectroscopic and dynamic features of vibrationally excited water molecules (D2O) adsorbed on deuterated mordenite zeolite (DM-20) have been observed by infrared-infrared pump-probe experiments. The vibrational lifetimes of D2O molecules adsorbed on zeolite hydroxyl groups have been determined to be 43 ± 5 ps for the OD group which is rather free from interaction, and ≈ 15ps for the OD group under strong hydrogen bonding to the surface. By infrared pumping, a newly appearing absorption band (≈15ps) was observed at around 2730 cm−1. In basic accordance with the report by Graener and Seifert on a solution of H2O and D2O in solvents, a possible assignment of the absorption band is discussed.

Relaxation process of CO stretching mode of metal carbonyls adsorbed on Y zeolites studied by pump–probe experiment

The vibrational relaxation lifetimes of Mo(CO)6 and Cr(CO)6, adsorbed in a cage of HY, DY, and NaY zeolites, were measured at various temperatures by the pump–probe method using picosecond infrared laser pulses. The lifetimes at 110 K of the v=1 level of the CO stretching mode were 70±4, 53±5, and 150±9 ps for Mo(CO)6 on HY, DY, and NaY zeolite, respectively, and 87±6, 74±7, and 140±12 ps for Cr(CO)6, respectively. It was shown by comparison of the lifetimes at 110 K, as well as the temperature dependence, that the rate of vibrational relaxation is determined by the coupling of the CO stretching mode with four quanta of the accepting modes comprised of: (i) modes with displacement of central metals Cr or Mo in all of the three zeolite systems, (ii) modes associated with the OH and OD groups on zeolite surface in the HY and DY zeolite systems, and probably (iii) the frame vibrational modes of zeolite with energies at 300–800 cm−1.

VIBRATIONAL RELAXATION PROCESS OF POLYATOMIC MOLECULES ADSORBED IN ZEOLITES

The vibrational relaxation lifetimes of the CO stretching mode of Cr(CO)6 and Mo(CO)6 adsorbed in the cage of the HY, DY and NaY-type zeolites were measured at various temperature by pump-probe method using picosecond infrared laser pulses. It was shown by comparing the lifetimes that the accepting modes include both the vibrational modes of the metal carbonyl and those associated with the cations on the zeolite surface. The analysis of the temperature dependence of the lifetimes revealed that the number of the excited accepting modes are four with their energy lying around 500 cm-1.