Dissociative Absorption Research Articles

Anomalous Difference between Replacements of D–H and H–D in Oxide Ceramics by Exposure to Air Containing H2O and D2O Vapors

We have investigated the exchange of deuterium (or protium) implanted into an oxide ceramic, SrCe0.95Yb0.05O3-δ, for protium (or deuterium) due to exposure to H2O (or D2O) vapor at room temperature by means of the elastic recoil detection (ERD) technique. It is found that D is completely exchanged for H by exposure to H2O vapor, while H is hardly exchanged for D by exposure to D2O vapor, namely there exists a great isotope difference between the exchange of D for H and that of H for D. This result suggests a new model of replacement reactions; dissociative absorption of H at the surface, diffusion of H and release of D due to H–D mix-molecular bulk recombination and the subsequent trapping of H in the vacant trapping sites. In order to clarify the reaction leading to the great isotope difference, the experiments on the retention of H and D by simultaneous H+, D+ implantation and the release of 5 keV D2+ implants by 0.5 keV H2+ irradiation and that of 5 keV H2+ implants by 0.5 keV D2+ irradiation have been done. The retention experiment shows that the D/H ratio of the saturation implantation concentration is 1.3. Competition among H–H, H–D and D–D bulk recombination prefers to enrich D more than H, which is opposite to the isotope difference observed. The release experiment shows that the slow and continuous decay of 5 keV D2+ implants is induced by long-term 0.5 keV H2+ irradiation, while that of 5 keV H2+ implants is hardly induced by long-term 0.5 keV D2+ irradiation. The latter result suggests that the isotope difference in the hydrogen diffusion is one of the origins of the major isotope difference.

SiC growth on Si(111) from a C60 precursor: A new experimental approach based on a hyperthermal supersonic beam

Abstract Silicon carbide (SiC) films are grown on Si(111) using as a precursor fullerene seeded in helium supersonic beams. The regime of energy and flux distributions, achievable by changing the beam parameters, is well suited to synthesizing SiC films under well ordered conditions, because of better control of the synthesis process and of the C60 dissociative absorption. This is confirmed by the first experimental results and by morphological and structural characterization of the films produced. Further developments are briefly discussed.

SiC growth on Si(111) from a C 60 precursor: a new experimental approach based on a hyperthermal

Abstract Silicon carbide (SiC) films are grown on Si(111) using as a precursor fullerene seeded in helium supersonic beams. The regime of energy and flux distributions, achievable by changing the beam parameters, is well suited to synthesizing SiC films under well ordered conditions, because of better control of the synthesis process and of the C60 dissociative absorption. This is confirmed by the first experimental results and by morphological and structural characterization of the films produced. Further developments are briefly discussed.

Dissociative water vapour absorption in BaZr0.85Y0.15O2.925/H2O: pressure–composition isotherms in terms of Fermi–Dirac statistics

Thin films of proton-conducting BaZr0.85Y0.15O2.925 perovskite-type ceramics, prepared in a sol–gel process by multiple dip-coating on silicon single crystal wafers, and powder samples, prepared by the conventional carbonate route, were charged with hydrogen by dissociative water absorption at definite values of water vapour pressures and temperatures. The absolute content of the absorbed hydrogen was determined ex situ at room temperature using the 15N nuclear resonance reaction analysis. From the resulting water vapour pressure–hydrogen composition isotherms the absorption enthalpies and entropy were calculated, using our two site model, based on Fermi–Dirac statistics.

Photoinduced Surface Reactions on TiO2 and SrTiO3 Films: Photocatalytic Oxidation and Photoinduced Hydrophilicity

We have evaluated photocatalytic oxidation activities and photoinduced hydrophilicities for TiO2 and SrTiO3 films. Although both types of films had almost the same photocatalytic oxidation activity, the reaction of photoinduced hydrophilicity was proceeding only on TiO2. The photoinduced hydrophilicity peculiar to TiO2 is not caused by the photocatalytic oxidation of organic compounds adsorbed on the surface, but is based on a surface structural change which is favorable for dissociative water absorption.

Substituent and solvent effects on the short-time A-band photodissociation dynamics of CF 2I 2

Resonance Raman spectra of CF 2I 2 in cyclohexane and in methanol solutions were taken with excitation within the directly dissociative A-band absorption. The CF 2I 2 in cyclohexane solution resonance Raman spectrum is substantially different than that in methanol solution, which suggests that solvent–solute interactions noticeably affect the short-time photodissociation dynamics of CF 2I 2. We compare the qualitative short-time photodissociation dynamics of CF 2I 2 and CH 2I 2 to examine substituent and solvent effects.

Proton conducting Ba3Ca1.18Nb1.82O8.73–H2O: pressure–compositions isotherms in terms of Fermi-Dirac statistics, concentration and fuel-cell measurements, and impedance spectroscopy

Thin films of Ba3Ca1.18Nb1.82O8.73, prepared in a sol-gel process by multiple dip-coating on silicon wafers, and powder samples, prepared by the conventional carbonate route, were charged with hydrogen by dissociative water absorption at definite values of water vapour pressure and temperature. The hydrogen content was determined ex situ at room temperature using nuclear resonance reaction analysis. From the resulting water vapour pressure–composition isotherms the absorption enthalpies Δ∅HO, Δ∅Ht and the absorption entropy Δ∅S were calculated, using a two-site model, based on Fermi-Dirac statistics. On Ba3Ca1.18Nb1.82O8.73 bulk material also impedance spectroscopy, concentration and fuel-cell measurements as well as neutron diffraction were performed. From these data, taken together, the proton diffusion coefficient DH could be evaluated and compared to quasielastic neutron scattering results.

The influence of molecular rotation on the direct subsurface absorption of H2 on Pd(111)

Within the generalized gradient approximation (GGA) of density functional theory (DFT) we have calculated a three-dimensional (3D) potential energy surface (PES) including an angular degree of freedom for a H2 molecule interacting with a Pd(111) surface. There is an entrance channel barrier (≈0.09 eV) to both dissociative chemisorption and direct subsurface absorption, but after this barrier is crossed direct subsurface absorption can proceed almost without a barrier. 3D quantum mechanical wave packet calculations incorporating the rotation of H2 in a plane perpendicular to the surface show a large part of the hydrogen going directly subsurface even at low incident kinetic energies. The wave packet calculations also show that in the low energy regime rotation inhibits direct subsurface absorption at low j0 and promotes it at high j0.

Proton conducting Ba 3Ca 1.18Nb 1.82O 8.73/H 2O: Sol–gel preparation and pressure/composition isotherms

Thin films of Ba 3Ca 1.18Nb 1.82O 8.73, prepared in a sol–gel process by multiple dip-coating on silicon wafers, were charged with hydrogen by dissociative water absorption at definite values of water vapour pressures and temperatures. The hydrogen content was determined using nuclear resonance reaction analysis. From the resulting water vapour pressure/hydrogen composition isotherms the absorption enthalpies and the absorption entropy were calculated in the framework of a two site model, based on Fermi–Dirac statistics. On Ba 3Ca 1.18Nb 1.82O 8.73 also impedance spectroscopy was performed yielding the bulk conductivity. From these data in combination with proton transport numbers and the thermodynamic results as noted above the proton diffusion coefficient could be evaluated.

Quasi-Coherent Molecular Vibrations with Energies above the Dissociation Threshold in the Ground Electronic State

The purpose of this paper is to present a new type of molecular vibration that may be prepared and monitored by IR and UV laser pulses in the femtosecond time domain: quasicoherent vibrations in the ground electronic state but with energies well above the dissociation threshold and even above any potential barriers. (The word “quasi-coherent” indicates certain deviations from perfect coherent vibrations, which exist only in ideal model systems, e.g., harmonic oscillators. 1 ) More specifically, we show that molecules may absorb vibrational energy in both dissociative and nondissocative “spectator” bonds such that the dissociative bond oscillates in a quasi-periodic fashion and with rather long lifetimes (.10 ps), that is, without any abrupt cleavage, which is blocked, even though the total amount of vibrational energy is sufficient to break the dissociative bond immediately. We also suggest how to excite these quasi-coherent vibrations above the dissociation threshold, that is, by means of a series of selective femtosecond IR laser pulses (the pumping process) and how to discover them, namely, by femtosecond UV laser pulses (the probing process). The new type of quasi-coherent vibrations in the continuum of the ground electronic state should be distinguished from other well-known quasi-coherent states or resonances with energies above the dissociation threshold. First, the molecules may be prepared in electronically excited states where they may vibrate quasicoherently before they eventually dissociate, that is, by tunneling through a potential barrier (predissociation) or by electronic transitions to lower, dissociative electronic states (dissociative internal conversion, IC). These types of quasi-coherent molecular vibrations in electronically excited states have been prepared and monitored since the early days of femtosecond chemistry (see ref 2 and the books and reviews 3-5 ). In contrast, we consider quasi-coherent molecular vibrations in the ground electronic state that do not decay by tunneling through any potential barriers or by electronic transitions. Second, molecules may possess vibrational resonances with energies Er above the dissociation thresholds and with finite lifetimes Ur, both in the electronic ground and excited states. In ideal cases of isolated, that is, nonoverlapping, resonances, they give rise to corresponding peaks in the dissociative absorption spectra, with specific energies Er and widths ir such that

A time-dependent study of the dynamics of IBr photodissociation

Time-dependent calculations of the photodissociation of IBr are presented. We have calculated the dissociative absorption spectrum in the range of frequencies from v = 14705 cm −1, the v = 0 threshold, to v = 22321cm −1, establishing the yields of Br( 2P 3 2 ) in the ground state and Br ∗( 2P 1 2 ) in the excited spin-orbit state. The effect of the vibrational excitation of the IBr molecule on the photodissociation yields was also examined. Two independent photodissociation paths arising from vibrational levels of the ground X( 1∑ o +) state and of the excited A( 3Π 1) state which are only a few cm −1 apart are considered. We show that the percentage yields of Br ∗ and Br are strongly dependent on the photodissociation path.

Childhood Abuse and Three Measures of Altered States of Consciousness (Dissociation, Absorption and Mysticism) in a Female Outpatient Sample

This study examines the relationships between childhood and adolescent trauma and neglect and three types of altered states of consciousness (dissociative episodes, absorption, and mysticism) in an outpatient sample of women. Thirty-eight female outpatients receiving counseling at a community clinic completed three scales measuring the tendency to experience altered states of consciousness (Dissociative Experiences Scale (DES), Absorption Scale of the MPQ, and the Mysticism Scale), two measures of negative childhood experiences (Child Abuse and Trauma Scale (CAT) and the Self-Injury and Abuse Scale), and the Beck Depression Inventory. Twenty-one of 38 (55%) women reported childhood sexual abuse on the CAT. There was close agreement regarding childhood sexual abuse between the two abuse questionnaires (Kappa = .783, p >.001). Childhood sexual abuse was associated with more suicide attempts, but not with other self-injurious behaviors. Higher DES scores were associated with more negative overall home environment ratings and with histories of sexual abuse. The tendency to have negative or pathological (dissociative) altered states of consciousness was not correlated with the tendency to have positive (mystical) types of altered states. To the extent that there is an experiential or learned component underlying pathological dissociation, it does not appear to generalize to mystical-type experiences or even to the trait of absorption.

The influence of surface structure on the reaction of dimethylzinc on GaAs(100)

The reactions of dimethylzinc (DMZ) on the (4 × 2), (2 × 4) and c(4 × 4) reconstructions of GaAs(100) were studied using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). The results of the TPD experiments demonstrate that the strength of the Zn-surface interaction is a function of the surface As Ga ratio. Zn atoms produced via dissociative absorption of DMZ desorb at 565, 600 and 620 K from the (4 × 2), (2 × 4) and c(4 × 4) reconstructions, respectively. In contrast, methyl groups were found to desorb from all three reconstructions at 610 K. HREELS data for the (4 × 2) and c(4 × 4) reconstructions show that the energy of the surface-C stretching mode for absorbed methyl groups is the same on these surfaces. These results indicate that the absorption sites for methyl groups are similar on the three reconstructions. Based on these experimental results, models of the absorption sites for Zn atoms and methyl groups on the (2 × 4) and c(4 × 4) reconstructions of GaAs(100) are proposed.

Diffusion of hydrogen and oxygen in the high temperature proton conductor srceo3investigated by dynamic conductivity measurements

Abstract In SrCeO3 the oxygen vacancy concentration at elevated temperatures depends on the oxygen partial pressure in the surrounding atmosphere. Changes of the oxygen vacancy concentration are accompanied by change in the hole concentration and therefore can be measured by electrical conductivity measurements. We apply a dynamic method for studying the diffusion of oxygen vacancies by measurements of the time dependence of the electrical conduction after a sudden change in the oxygen partial pressure has taken place. In doped samples and under wet conditions the protonic charge carriers are mainly incorporated by a dissociative absorption of water and lead to a noticeable protonic conduction. The proton diffusion in SrCe0.95 Y0.05O3 is determined by relaxation measurements at a polarised sample in wet and hydrogen containing atmosphere. The electrochemical cell discussed here is characterised by a combination of one reversible and one (hydrogen) ion blocking electrode. From the time dependence of the de...

Short-time photodissociation dynamics of chloroiodomethane in solution from resonance Raman spectroscopy

Resonance Raman and preresonance Raman spectra of chloroiodomethane in cyclohexane solution have been obtained at excitation wavelengths of 282, 309, 320, 342, 355, and 532 nm. The 282 nm spectrum is in resonance with the directly dissociative A-state absorption band of chloroiodomethane. We observe a large degree of Raman intensity in the C-1 stretch overtone progression as well as combination bands of the C-Cl stretch mode with the C-1 stretch overtones indicating that most of the initial photodissociation dynamics in the Franck-Condon region involves the C-1 stretch motion as well as some C-Cl stretch motion.

Solvation effects and short-time photodissociation dynamics of CH 2I 2 in solution from resonance Raman spectroscopy

Resonance Raman spectra of CH 2I 2 in cyclohexane solution have been obtained within the directly dissociative A-state absorption band of CH 2I 2 at excitation wavelengths of 355 and 341.5 nm. We find several intense vibrational bands in the solution phase spectra that are not observed in a previously reported 355 nm gas phase spectrum indicating that solvation effects are important in the Franck-Condon region of the first electronic excited state of CH 2I 2. The solution phase short-time photodissociation dynamics appears to have more motion in the ICI bending coordinate than the gas phase photodissociation.

Photodynamics in chlorine-doped xenon under high pressures: a diamond anvil cell study

High-pressure studies of molecular dissociation, atomic mobility, diffusion, and recombination photodynamics are reported for chlorine-doped solid xenon contained in a diamond anvil cell. The photogeneration of atoms is monitored by following emission from Xe2CI, the emission spectra and relaxation dynamics of which are characterized as a function of pressure and temperature. At 308 nm, the direct dissociation of CI2 via its - X dissociative absorption is prohibited at all studied pressures (2-1 0 GPa) and temperatures (30-300 K). Instead, dissociation via the two-photon-induced harpoon process: Xe + CI, + 2hv - [Xe+CI2-] - CI + Xe'CI-, is observed. The cross section of the latter process, at 2 GPa, is large and shows a strong temperature dependence-IO4 cm4 s at 300 K, cm4 s at 40 K. At room temperature and pressures above 5 GPa, atomic CI is stable with respect to recombination for periods of many weeks, implying a diffusion constant less than cmz 8. Recombination at 2 GPa proceeds over the period of several hours. The radiative dissociation of Xe2CI leads to atomic mobility and subsequent diffusion-controlled recombination. The extent of dissociation is controlled by the competition between rates of photogeneration and photomobility driven recombination.

Theoretical determination of bound–free absorption cross sections in Ar+2

A b initio calculations have been carried out for the potential energy curves and transition moments of the 2Σ+u, 2Πg, 2Πu, and 2Σ+g states of Ar+2 which arise from the 2P+1S ion–atom asymptote. These data have been used in a theoretical calculation of the dissociative absorption cross sections from the bound 2Σ+u state to the repulsive 2Πg and 2Σ+g states. The 2Σ+u→2Πg transition, which is dominated by spin–orbit effects, has a maximum absorption cross section of 2.6×10−19 cm2 centered at 716 nm with a full width at half-maximum of 185 nm at room temperature. The 2Σ+u→2Σ+g transition is found to be much stronger with a maximum cross section of 0.5×10−16 cm2 centered at 300 nm with a full width at half-maximum of 75 nm at room temperature.

Molecular alignment and photofragment spectroscopy

By illumination with polarized light, molecules with a dissociative absorption can be aligned by the selective photodissociation of different M states at different rates, as was first shown by Dehmelt and co-workers for H+2. We have extended this technique to neutral molecules, demonstrating it for a diatomic, IBr pumped by the second harmonic of a neodymium laser at 18 780 cm−1, by observing the recoiling Br fragments with photofragment spectroscopy. Classical and quantum calculations of the selective photodissociation effect give equivalent predictions except in the limit in which only a few M states remain populated. Several methods are discussed for experimentally monitoring the degree of alignment as a function of light flux: (i) the change in fragment flux, (ii) the change in relative strength of two transitions of different symmetry as reflected in their joint angular distributions, and (iii) the change in the shape of the angular distribution of the recoiling atomic fragments for a single transition. The first two methods are demonstrated for IBr molecules. In addition, experiments using two sequential light pulses are suggested, the second pulse monitoring the alignment produced by the first either through the photofragment angular distribution or through absorption. Our results show that at achievable light fluxes, molecules with dissociative transitions of moderate strength can be strongly aligned. Molecular populations can be prepared which contain only a few M states for each J, or even only M=±J. This technique can potentially provide a wide variety of aligned molecules for molecular beam or bulb studies of relaxation as well as elastic, energy transfer, and reactive scattering, and can provide further directionality for photon and fragment angular recoil studies.

The interaction of chlorine with the ZnO(0001)-Zn surface: A LEED, Auger and workfunction study

Saturation of the ZnO(0001)-Zn surface with chlorine at 300K takes place according to Auger spectroscopy at approximately 25% of a monolayer: sufficient to satisfy the charge stabilization criterion. Random dissociative absorption into two states of differing energy is observed until the temperature of the substrate is increased, when LEED indicates that ordering into complex superstructures takes place. A very small amount of chlorine, approximately 0.01 monolayer, remains after thermal treatment and this is incorporated into a stable reconstructed surface. Ion-bombardment is necessary for the complete removal of this chlorine. Large electron-beam desorption effects were observed throughout the work and these were characterized by cross sections for the two surface sites of (2+or-1)*10-19 and (7+or-4)*10-20 cm2.