Kiyohiko Tabayashi

Anisotropic velocity distribution of desorbing product in carbon monoxide oxidation on palladium (110)

Significant anisotropy was found in the velocity distributions of desorbing product CO2 from a Pd(110) surface. The velocity distributions were determined by a cross‐correlation time‐of‐flight technique combined with angle‐resolved thermal desorption. Heating the coadlayer of CO and oxygen produces five peaks in the CO2 formation spectrum; P1– (around 420 K), P2– (∼370 K), P3– (∼300 K), P4– (∼230 K), and P5–CO2 (∼170 K). The translational temperature of each CO2 is much higher than the corresponding surface temperature, and increases in the sequence of P1– <P2– <P3– <P4– <P5–CO2. It decreases rapidly with an increase in the desorption angle perpendicular to the surface trough and more slowly parallel to it. This anisotropy is correlated to the reaction site symmetry.

Absorption spectrum of C60 in the gas phase: Autoionization via core‐excited Rydberg states

The absolute absorption cross section of C60 in the gas phase (830–870 K) was measured as a function of the photon energy (3.5–11.4 eV) (absorption spectrum). Absorption peaks at 7.87, 8.12, 8.29, 9.2 eV and a dip at 8.45 eV observed are assigned as Feshbach resonances in the photoexcitation involving superexcited states. The superexcited states responsible for the 7.87, 8.12, and 9.2 eV peaks are assigned to be core‐excited Rydberg states converging to the second, the third and the fourth ionization limits of C60 (8.89, 9.12, 10.82–11.59 eV), respectively. The 8.29 eV peak is considered to originate from vibrational excitation of a totally symmetric pentagonal pinch mode of the superexcited state responsible for the 8.12 eV peak. Further, a relative photoionization quantum yield was estimated from the absorption cross section measured and the relative photoionization cross section reported. The yield increases particularly in the vicinity of 8 eV in accordance with a high efficiency of autoionization of ...

CHARACTERIZATION OF SILICA GLASSES SINTERED UNDER CL2 AMBIENTS

Chlorine incorporated into silica glasses by sintering porous soot rods under Cl2/He atmosphere or by fabricating with plasma methods were found to be mostly as ≡SiCl, which gives rise to an optical absorption tail above ≂7.5 eV. The cross section of the optical absorption at ≂7.77 eV (160 nm) was found to be 1.14×1020 cm2. This value was almost the same as that of ≡SiCl in SiCl4 molecule in the gas phase. A minor fraction (10−3∼10−1) was found to be present as Cl2 molecule, which gives the absorption band at 3.8 eV.

Lack of a heavy-atom effect on fluorescence lifetimes of 9-cyanoanthracene—rare gas clusters in a supersonic free jet

Abstract Fluorescence lifetimes of 9-cyanoanthracene (9CNA) and its rare gas (Ar, Kr, Xe) clusters have been measured under supersonic free jet conditions. For the bare 9CNA molecule, the lifetime is 28.0±0.2 ns upon OO band excitation and is practically independent of excess energy up to 590 cm −1 above the electronic origin. The fluorescence lifetimes of the clusters in the vibrationless states are 26–32 ns. indicating absence of a heavy-atom effect on the energy dissipating path in these clusters. But in the vibrationally excited states, the lifetimes are greatly shortened. The results are discussed in terms of intramolecular energy relaxation and vibrational energy transfer in the 9CNA—rare gas clusters.

Hydrogen bonding in methanol clusters probed by inner-shell photoabsorption spectroscopy in the carbon and oxygen K-edge regions

Hydrogen bonding in methanol clusters has been investigated by using inner-shell photoabsorption spectroscopy and density functional theory (DFT) calculations in the carbon and oxygen K-edge regions. The partial-ion-yield (PIY) curves of H(CH(3)OH)(n)(+) were measured as the soft x-ray absorption spectra of methanol clusters. The first resonance peak in the PIY curves, which is assigned to the sigma*(O-H) resonance transition, exhibits a 1.20 eV blueshift relative to the total-ion-yield (TIY) curves of molecular methanol in the oxygen K-edge region, while it exhibits a shift of only 0.25 eV in the carbon K-edge region. Decreased intensities of the transitions to higher Rydberg orbitals were observed in the PIY curves of the clusters. The drastic change in the sigma*(O-H) resonance transition is interpreted by the change in the character of the sigma*(O-H) molecular orbital at the H-donating OH site due to the hydrogen-bonding interaction.

Dissociative excitation of water by metastable rare gas atoms: Rg(3P0,2)+H2O→Rg+OH(A 2Σ+)+H (Rg=Ar,Kr)

The title reactions were studied over the relative collision energy range 0.3–1.8 eV in crossed molecular beams. Vibrational and rotational state distributions of the nascent OH(A 2Σ+) product were determined by analysis of fluorescence from the OH(A 2Σ+–X 2Π) bands. The rotational distributions could be represented by simple Boltzmann distributions. With Ar*(3P0,2) excitation, both vibrational and rotational distributions were found to have no significant dependence on the collision energy and compare well with results previously obtained at near‐thermal energies. With Kr(3P0,2) excitation, however, the state distributions were found to be strongly collision‐energy dependent, the rotational temperature Tr (v=0) increasing from 850 to 1750 K and the vibrational population ratio Nv=1/Nv=0 from ≤0.09 to 0.14 as the collision energy was increased from 0.35 to 0.65 eV. Time‐of‐flight (TOF) energy selection was used to measure the integral cross sections for the formation of the OH(A). The collision energy dep...

Collision energy dependence of the cross sections for the electronic excitation transfer reactions: Rg(3P0,2)+N2(X 1Σg)→Rg(1S0) +N2(C 3Πu) (Rg=Ar, Kr)

The collision energy dependence of the integral cross sections for the title reactions were determined using crossed molecular beams and time‐of‐flight energy selection techniques. Applying arc‐heated rare gas atom beams, the relative collision energies of metastable Ar(3P0,2) and Kr(3P0,2) atoms were selected between 0.4 and 2.5 eV for Ar(3P0,2)+N2, and 0.6 and 1.7 eV for Kr(3P0,2)+N2 systems. The negative energy dependence of the cross sections for the prototype reaction Ar(3P0,2)+N2 agrees well with the results of Parr and Martin in the overlapped energy range (0.4–0.8 eV). The absolute cross sections were determined by normalizing our cross sections to the ones of Parr and Martin. As to the endoergic Kr(3P0,2)+N2 reaction, the product fluorescence from N2(C 3Πu–B 3Πg) was also observed. The total cross section for this reaction exhibits a steep increase near the threshold for each component state [ΔH(3P0)=0.47 and ΔH(3P2)=1.12 eV] and then tends to level off. Assuming the component ratio Kr(3P2)/Kr(3P...

Theoretical study of ion desorption from poly-(methyl methacrylate) and poly-(isopropenyl acetate) thin films through core excitation

Site-specific chemical reactions following core excitation of poly-(methyl methacrylate) (PMMA) and poly-(isopropenyl acetate) (PiPAc) thin films were investigated. New x-ray absorption spectra of PMMA and PiPAc at the C and O K edges and theoretical spectra within the framework of density functional theory using model molecules were reported, and some new peak assignments were proposed for these spectra. Core-hole excited state molecular dynamics simulations were performed to discuss dissociation dynamics for the target systems, and some specific reaction mechanisms were discussed and explained theoretically; for example, the amount of CH3 ion fragments for PMMA was enhanced at the C and O K edges through the existence of the repulsive sigma*(O-CH3) excited state.

Hydrogen bonding in acetone clusters probed by near-edge x-ray absorption fine structure spectroscopy in the carbon and oxygen K-edge regions

Hydrogen bonding in acetone clusters was investigated using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory calculations in the carbon and oxygen K-edge regions. The partial-ion-yield (PIY) curves of the cluster ions were measured as the NEXAFS spectra of acetone clusters. In the carbon K-edge region, the first resonance peak, which was assigned to the C(CO) 1s-->pi( *)(C=O) resonance transition, showed no substantial change in the PIY curves of the acetone clusters, while the C(CH3) 1s-->3ppi(CH(3)) excitation feature was found to be strongly suppressed. The selective suppression of the C(CH3) 1s-->3ppi(CH(3)) resonance transition can be explained by the change in the character of the 3ppi(CH(3)) orbital due to the C=O...H-C type of hydrogen-bonding interaction. On the other hand, the NEXAFS spectra of the acetone molecule and clusters were almost identical in the oxygen K-edge region, except for a small shift in the pi( *)(C=O) resonance of 0.13 eV, because the character of the pi( *)(C=O) orbital remained, regardless of the C=O...H-C hydrogen bonding interaction.

Inner-shell excitation spectroscopy and fragmentation of small hydrogen-bonded clusters of formic acid after core excitations at the oxygen K edge

Inner-shell excitation spectra and fragmentation of small clusters of formic acid have been studied in the oxygen K-edge region by time-of-flight fragment mass spectroscopy. In addition to several fragment cations smaller than the parent molecule, we have identified the production of HCOOH.H+ and H3O+ cations characteristic of proton transfer reactions within the clusters. Cluster-specific excitation spectra have been generated by monitoring the partial ion yields of the product cations. Resonance transitions of O1s(C[double bond]O/OH) electrons into pi(CO)* orbital in the preedge region were found to shift in energy upon clusterization. A blueshift of the O1s(C[double bond]O)-->pi(CO)* transition by approximately 0.2 eV and a redshift of the O1s(OH)-->pi(CO)* by approximately 0.6 eV were observed, indicative of strong hydrogen-bond formation within the clusters. The results have been compared with a recent theoretical calculation, which supports the conclusion that the formic-acid clusters consist of the most stable cyclic dimer andor trimer units. Specifically labeled formic acid-d, HCOOD, was also used to examine the core-excited fragmentation mechanisms. These deuterium-labeled experiments showed that HDO+ was formed via site-specific migration of a formyl hydrogen within an individual molecule, and that HD2O+ was produced via the subsequent transfer of a deuterium atom from the hydroxyl group of a nearest-neighbor molecule within a cationic cluster. Deuteron (proton) transfer from the hydroxyl site of a hydrogen-bond partner was also found to take place, producing deuteronated HCOOD.D+ (protonated HCOOH.H+) cations within the clusters.