Two-fold Bridge Research Articles

Visibility of TiO2(110)(1×1) bridging oxygen in core level photoelectron spectroscopy

O 1$s$ core level and valence band photoelectron spectroscopy data have been acquired from adsorbate-free TiO${}_{2}$(110)(1 $\ifmmode\times\else\texttimes\fi{}$ 1) in ultrahigh vacuum. Careful analysis of the O 1$s$ spectra demonstrates that there is no distinct feature arising from twofold coordinate surface bridging oxygen atoms. This result resolves apparent ongoing confusion in the literature about this issue.

Adsorption of carbon monoxide on Pd (210) and (510) surfaces

Adsorption of carbon monoxide on Pd (210) and (510) stepped surfaces has been investigated by the extended London‐Eyring‐Polyani‐Sato method constructed using a five‐parameter Morse potential. Pd (210) and (510) stepped surfaces consist of terrace with (100) structure and step with (110) character. These results show that there exist common characteristics of CO adsorption on these two surfaces. At low coverage, CO adsorbs in twofold bridge site of the (100) terrace. The critical characteristics inherit that of CO molecule adsorbed in twofold bridge site of (100) original surface. When the coverage is increased, the top site of (110) step is occupied. The critical characteristics resemble that of CO molecule adsorbed in top site of (110) original surface. A number of new sites are exposed on the boundary regions, for example, the fivefold hollow site (H) of these two surfaces. There are stable adsorption sites at high coverage. Because of the different length of the (100) terrace, the (210) and (510) stepped surfaces have some different characteristics. First, CO is tilted adsorption on bridge site of terrace of (210), but perpendicular on terrace of (510) surface. Second, the bridge site (B1) where one Pd atom at the top of the step and the other at the bottom of the step is a stable adsorption site on (210), but the same type of site on Pd (510) surface is not. Copyright © 2012 John Wiley & Sons, Ltd.

A Density Functional Theory Study on the Effect of Zero‐Point Energy Corrections on the Methanation Profile on Fe(100)

The thermodynamics and kinetics of the surface hydrogenation of adsorbed atomic carbon to methane, following the reaction sequence C+4H(-->/<--)CH+3H(-->/<--)CH(2)+2H(-->/<--)CH(3)+H(-->/<--)CH(4), are studied on Fe(100) by means of density functional theory. An assessment is made on whether the adsorption energies and overall energy profile are affected when zero-point energy (ZPE) corrections are included. The C, CH and CH(2) species are most stable at the fourfold hollow site, while CH(3) prefers the twofold bridge site. Atomic hydrogen is adsorbed at both the twofold bridge and fourfold hollow sites. Methane is physisorbed on the surface and shows neither orientation nor site preference. It is easily desorbed to the gas phase once formed. The incorporation of ZPE corrections has a very slight, if any, effect on the adsorption energies and does not alter the trends with regards to the most stable adsorption sites. The successive addition of hydrogen to atomic carbon is endothermic up to the addition of the third hydrogen atom resulting in the methyl species, but exothermic in the final hydrogenation step, which leads to methane. The overall methanation reaction is endothermic when starting from atomic carbon and hydrogen on the surface. Zero-point energy corrections are rarely provided in the literature. Since they are derived from C-H bonds with characteristic vibrations on the order of 2500-3000 cm(-1), the equivalent ZPE of 1/2 hν is on the order of 0.2-0.3 eV and its effect on adsorption energy can in principle be significant. Particularly in reactions between CH(x) and H, the ZPE correction is expected to be significant, as additional C-H bonds are formed. In this instance, the methanation reaction energy of +0.77 eV increased to +1.45 eV with the inclusion of ZPE corrections, that is, less favourable. Therefore, it is crucial to include ZPE corrections when reporting reactions involving hydrogen-containing species.

A density functional theory study on the Ag n H (n = 1–10) clusters

Density functional GGA-PW91 method with DNP basis set is applied to optimize the geometries of Ag n H (n = 1–10) clusters. For the lowest energy geometries of Ag n H (n = 1–10) clusters, the hydrogen atom prefers to occupy the two-fold coordination bridge site except the occupation of single-fold coordination site in AgH cluster. After adsorption of hydrogen atom, most Ag n structures are slightly perturbed and only the Ag6 structure in Ag6H cluster is distorted obviously. The Ag–Ag bond is strengthened and the strength of Ag–H bond exhibits a clear odd–even oscillation like the strength of Au–H bond in Au n H clusters, indicating that the hydrogen atom is more favorable to be adsorbed by odd-numbered pure silver clusters. The adsorption strength of small silver cluster toward H atom is obviously weaker than that of small gold cluster toward H atom due to the strong scalar relativistic effect in small gold cluster. The pronounced odd–even alternation of the magnetic moments is observed in Ag n H systems, indicating that the Ag n H clusters possess tunable magnetic properties by adsorbing hydrogen atom onto odd-numbered or even-numbered small silver cluster.

Deduction of a three-phase model for the (√3 × √3)R30°-Cu 2Si/Cu(1 1 1) surface alloy

The (√3 × √3)R30°-Cu 2Si/Cu(1 1 1) surface alloy that forms during high temperature dosing of silane (SiH 4) on Cu(1 1 1) has been investigated using LCAO–DFT. Simulated STM images have shown that experimental images may be interpreted as a mixed phase system consisting of Si ion cores bound in HCP, FCC and twofold bridge sites with a ratio of 25:25:50 rather than previously proposed models where the Si ion cores were bound in only FCC and HCP sites. The new model is shown to be consistent with previously published NIXSW studies.

Molecular adsorption and metal-support interaction for transition-metal clusters in zeolites: NO adsorption on Pdn (n=1–6) clusters in mordenite

The adsorption of NO molecules on Pd(n) clusters of varying size (n=1-6) located in the main channel of mordenite and the interaction of the metallic clusters with the zeolitic framework were investigated using ab initio density-functional calculations under periodic boundary conditions. The supported clusters are created by binding Pd(n) (2+) cations to the inner cavity of a deprotonated Al-exchanged zeolite with an Al/Si ratio of 1/11, such that a charge-neutral system is created. Compared to the highly symmetric structures of the gas-phase clusters, the clusters bound to the zeolitic framework undergo appreciable geometric distortions lowering their symmetry. The distortions are induced by strong interactions with "activated" framework oxygens located close to the charge-compensating Al/Si substitution sites, but the cluster forms also weaker bonds to "nonactivated" oxygen atoms. The interaction with the framework also affects the electronic and magnetic properties of the clusters. While in the gas phase all clusters (except the isolated Pd atom with a closed d(10) ground state) have a paramagnetic moment of 2mu(B), in the zeolite clusters with two to four atoms have zero magnetic moment, while the Pd(5) cluster has a magnetic moment of 2mu(B) and for the Pd(6) cluster, it is even enhanced to 4 mu(B) (but the magnetic energy differences relative to low-spin configurations are modest). Analysis of the magnetization densities shows that in all clusters with zero total moment (singlet ground state), there are sites with excess spin densities of opposite sign. The influence of the cluster-support interaction on the chemical properties of the clusters has been tested by the adsorption of NO molecules. The results demonstrate the interplay between the molecule-cluster and cluster-framework interactions, which can lead to an increase or decrease in the adsorption energy compared to NO on a gas-phase cluster. While on the gas-phase cluster adsorption in low-coordination sites (vertex or bridge) is preferred, for the cluster in the zeolite adsorption in threefold coordinated hollow or twofold bridge sites is preferred. The magnetic properties of the clusters and of the paramagnetic NO molecule play an important role. For the supported clusters with zero magnetic moment, upon adsorption the spin of the molecule is transferred to the cluster (and induces also a modest polarization of the framework). For magnetic clusters, spin pairing induces a reduced magnetic moment of the NO-Pd(n) complex. The redshift of the NO stretching frequencies is reduced compared to the free clusters by the cluster-support interaction for the smaller clusters, while it remains essentially unchanged for the larger clusters. A detailed electronic analysis of the cluster-support interactions and of the adsorption properties is presented.

Adsorption and dissociation of molecular hydrogen on the (0001) surface of double hexagonal close packed americium

Hydrogen molecule adsorption on the (0001) surface of double hexagonal packed americium has been studied in detail within the framework of density functional theory using a full-potential all-electron linearized augmented plane wave plus local orbitals method (FP-L/APW+lo). Weak molecular hydrogen adsorptions were observed. Adsorption energies were optimized with respect to the distance of the adsorbates from the surface for three approach positions at three adsorption sites, namely t1 (one-fold top), b2 (two-fold bridge), and h3 (three-fold hollow) sites. Adsorption energies were computed at the scalar-relativistic level (no spin-orbit coupling NSOC) and at the fully relativistic level (with spin-orbit coupling SOC). The most stable configuration corresponds to a horizontal adsorption with the molecular approach being perpendicular to a lattice vector. The surface coverage is equivalent to one-fourth of a monolayer (ML), with the adsorption energies at the NSOC and SOC theoretical levels being 0.0997 eV and 0.1022 eV, respectively. The respective distance of the hydrogen molecule from the surface and hydrogen-hydrogen distance was found to be 2.645 A and 0.789 A, respectively. The work functions decreased and the net magnetic moments remained almost unchanged in all cases compared with the corresponding quantities of bare dhcp Am (0001) surface. The adsorbate-substrate interactions have been analyzed in detail using the partial charges inside the muffin-tin spheres, difference charge density distributions, and the local density of states. The effects of adsorption on the Am 5f electron localization-delocalization characteristics have been discussed. Reaction barrier for the dissociation of hydrogen molecule has been presented.

Adsorption of carbon monoxide on Pd(3 1 1) and (2 1 1) surfaces

Adsorption of carbon monoxide on Pd(3 1 1) and (2 1 1) stepped surfaces has been investigated by the extended London-Eyring-Polyani-Sato (LEPS) method constructed using a 5-parameter Morse potential. The calculated results show that there exist common characteristics of CO adsorption on the two surfaces. At low coverage, CO occupies threefold hollow site of the (1 1 1) terrace and is tilted with respect to the surface normal. Among the threefold hollow sites on the (1 1 1) terrace, the nearer the site is to the step, the greater is the influence of the step. The twofold bridge site on the (1 0 0) step is also a stable adsorption site at high coverage. Because of the different lengths of the (1 1 1) terraces, the (3 1 1) and (2 1 1) stepped surfaces have different characteristics. A number of new sites are exposed on the boundary regions, including the fourfold hollow site (H 4) of the (3 1 1) surface and the fivefold hollow site (H 5) of the (2 1 1) surface. At high coverage, CO resides in the H 5 site of the (2 1 1) surface, but the H 4 site of the (3 1 1) surface is not a stable adsorption site. This study further shows that the on-top site on the (1 0 0) step of Pd(3 1 1) is a stable adsorption site, but the same type of site on Pd(2 1 1) is not.

A density functional study of atomic hydrogen and oxygen chemisorption on the relaxed (0001) surface of double hexagonal close packed americium

Ab initio total energy calculations within the framework of density functional theory have been performed for atomic hydrogen and oxygen chemisorption on the (0001) surface of double hexagonal packed americium using a full-potential all-electron linearized augmented plane wave plus local orbitals method. Chemisorption energies were optimized with respect to the distance of the adatom from the relaxed surface for three adsorption sites, namely top, bridge, and hollow hcp sites, the adlayer structure corresponding to coverage of a 0.25 monolayer in all cases. Chemisorption energies were computed at the scalar-relativistic level (no spin-orbit coupling NSOC) and at the fully relativistic level (with spin-orbit coupling SOC). The two-fold bridge adsorption site was found to be the most stable site for O at both the NSOC and SOC theoretical levels with chemisorption energies of 8.204 eV and 8.368 eV respectively, while the three-fold hollow hcp adsorption site was found to be the most stable site for H with chemisorption energies of 3.136 eV at the NSOC level and 3.217 eV at the SOC level. The respective distances of the H and O adatoms from the surface were found to be 1.196 Ang. and 1.164 Ang. Overall our calculations indicate that chemisorption energies in cases with SOC are slightly more stable than the cases with NSOC in the 0.049-0.238 eV range. The work functions and net magnetic moments respectively increased and decreased in all cases compared with the corresponding quantities of bare dhcp Am (0001) surface. The partial charges inside the muffin-tins, difference charge density distributions, and the local density of states have been used to analyze the Am-adatom bond interactions in detail. The implications of chemisorption on Am 5f electron localization-delocalization are also discussed.

Control of electrostatic interactions between F-actin and genetically modified lysozyme in aqueous media

The aim for deterministic control of the interactions between macroions in aqueous media has motivated widespread experimental and theoretical work. Although it has been well established that like-charged macromolecules can aggregate under the influence of oppositely charged condensing agents, the specific conditions for the stability of such aggregates can only be determined empirically. We examine these conditions, which involve an interplay of electrostatic and osmotic effects, by using a well defined model system composed of F-actin, an anionic rod-like polyelectrolyte, and lysozyme, a cationic globular protein with a charge that can be genetically modified. The structure and stability of actin-lysozyme complexes for different lysozyme charge mutants and salt concentrations are examined by using synchrotron x-ray scattering and molecular dynamics simulations. We provide evidence that supports a structural transition from columnar arrangements of F-actin held together by arrays of lysozyme at the threefold interstitial sites of the actin sublattice to marginally stable complexes in which lysozyme resides at twofold bridging sites between actin. The reduced stability arises from strongly reduced partitioning of salt between the complex and the surrounding solution. Changes in the stability of actin-lysozyme complexes are of biomedical interest because their formation has been reported to contribute to the persistence of airway infections in cystic fibrosis by sequestering antimicrobials such as lysozyme. We present x-ray microscopy results that argue for the existence of actin-lysozyme complexes in cystic fibrosis sputum and demonstrate that, for a wide range of salt conditions, charge-reduced lysozyme is not sequestered in ordered complexes while retaining its bacterial killing activity.

The SialonsMLn[Si4−xAlxOxN7−x] withM = Eu, Sr, Ba andLn = Ho-Yb – Twelve Substitution Variants with theMYb[Si4N7] Structure Type

The oxonitridoalumosilicates (so-called sialons) MLn[Si4−xAlxOxN7−x] with M = Eu, Sr, Ba and Ln =Ho, Er, Tm, Yb were obtained by the reaction of the respective lanthanoid metal, the alkaline earth carbonates or europium carbonate, resp., AlN, “Si(NH)2” and MCl2 as a flux in a radiofrequency furnace at temperatures around 2100 °C. The compounds MLn[Si4−xAlxOxN7−x] are relevant for the investigation of substitutional effects on the materials properties due to their ability of tolerating a comparatively large phase width up to x ≈ 2.0(5). The crystal structures of the twelve compounds were refined from X-ray single crystal data and X-ray powder data and are found to be isotypic to the MYb[Si4N7] structure type. The compounds crystallize in space group P63mc (no. 186, hexagonal) and are made up of chains of so-called starlike units [N[4](SiN3)4] or [N[4]((Si,Al)(O,N)3)4], respectively. These units are formed by four (Si,Al)(N/O)4 tetrahedra sharing a common central nitrogen atom. The structure refinement was performed utilizing an O/N-distribution model according to Paulings rules, i.e. nitrogen was positioned on the four-fold bridging site and nitrogen and oxygen were distributed equally on both of the two-fold bridging sites, resulting in charge neutrality of the compound. The Si and Al atoms were distributed equally on their two crystallographic sites, referring to their elemental proportion in the compound, due to being poorly distinguishable by X-ray methods. The chemical compositions of the compounds were derived from electron probe micro analyses (EPMA).

Single Molecule Observations of the Adsorption Sites of Methyl Isocyanide on Pt(111) by Low-Temperature Scanning Tunneling Microscopy

Scanning tunneling microscopy (STM) has been used to directly investigate the local structure of methyl isocyanide (CNCH3) adsorbed on Pt(111). At low coverages, CNCH3 is preferentially adsorbed at on-top sites, in agreement with earlier deductions based on vibrational spectroscopy. When dosed at low coverages at 50 K, the molecules tend to adsorb near other CNCH3 molecules with preferred distances of a and a, where a = 2.78 A is the lattice constant of Pt. Annealing the surface to 120 K, however, results in a more uniform separation of the molecules. At higher coverages, the CNCH3 molecules are observed to occupy both on-top and two-fold bridge sites. On the basis of STM image analysis, CNCH3 forms an ordered layer of (2 x 3) periodicity at 0.33 ML. Additional details on the structures of CNCH3 adsorbed at the on-top and two-fold bridge sites are provided by density functional theory (DFT) calculations. At a coverage that saturates the first layer (0.33 ML), the occupation ratio for the on-top and two-fold bridge bonded CNCH3 is 1:1, which is consistent with the results obtained from the combined use of experimental reflection absorption infrared spectroscopy (RAIRS) data and DFT calculations.

Comparison of S, Pt, and Hf adsorption on NiAl(1 1 0)

First-principles periodic slab density-functional theory (DFT) calculations with a plane-wave basis are used to predict the properties of S, Pt, and Hf adsorption on NiAl(1 1 0). Stable adsorption sites are identified, and adsorbate binding energies and structures are predicted. We find that while S adsorbs in a threefold site, the metals prefer to adsorb in the Ni–Ni twofold bridge site. The latter finding is consistent with scanning tunneling microscopy experiments for adsorption of various transition metals on NiAl(1 1 0) by Ho and coworkers. S is predicted to easily diffuse between threefold sites. We find that Pt and Hf both induce significant changes in the local surface structure, changing twofold bridge sites into fourfold coordination sites by drawing next-nearest-neighbor atoms nearly equidistant with the nearest-neighbor atoms. We find Pt favors interaction with Al slightly more than Ni, while Hf shows a particularly strong affinity for Ni compared to Al. We also predict that Hf may diffuse one-dimensionally along Ni rows with a barrier of ≈0.6 eV.

Theoretical study of oxygen adsorption at the Fe(1 1 0) and (1 0 0) surfaces

Electronic, magnetic and structural properties of atomic oxygen adsorbed in on-surface and subsurface sites at the two most densely packed iron surfaces are investigated using density functional theory combined with a thermodynamics formalism. Oxygen coverages varying from a quarter to two monolayers (MLs) are considered. At a 1/4 ML coverage, the most stable on-surface adsorption sites are the twofold long bridge sites on the (1 1 0), and the fourfold-hollow sites on the (1 0 0) surface. The presence of on-surface oxygen atoms enhances the magnetic moments of the atoms of the two topmost Fe layers. Detailed results on the surface magnetic properties, due to O incorporation, are presented as well. Subsurface adsorption is found unfavored. The most stable subsurface O, in tetrahedral positions at the (1 0 0) and octahedral ones at the (1 1 0) surface, are characterized by substantially lower binding than that in the on-surface sites. Subsurface oxygen increases the interplanar distance between the uppermost Fe layers. The preadsorbed oxygen overlayer enhances binding of subsurface O atoms, particularly for tetrahedral sites beneath the (1 1 0) surface.

Atomic structure of theTe∕Si(100)‐(2×1)surface

We investigated the atomic structure of the $\mathrm{Te}∕\mathrm{Si}(100)\text{\penalty1000-\hskip0pt}(2\ifmmode\times\else\texttimes\fi{}1)$ surface using the density functional theory. A new model of reconstruction for this surface, based on a two-layers scheme, is proposed. In this model, Te atoms are situated in twofold bridge sites in the first two layers. Small shifts of Te atoms, with respect to the perfect bridge positions, are noticed in one of the two layers and probably give rise to a slightly disordered surface. This model is in agreement with all the experimental data available and in particular with the experimentally observed Te coverage of one monolayer. Comparison between total energies obtained for this surface and the $\mathrm{Te}∕\mathrm{Si}(100)\text{\penalty1000-\hskip0pt}(1\ifmmode\times\else\texttimes\fi{}1)$ surface shows a better stability for the $(2\ifmmode\times\else\texttimes\fi{}1)$ in agreement with what has been reported in the literature.

Comparison of experimental and calculated infrared spectra of aminocarbynes on the Pt(111) surface

Infrared spectra of the three aminocarbynes CNH2, CNHCH3, and CN(CH3)2 attached to Pt, Pt7H6, Pt2, Pt9H14, and Pt4 clusters are calculated from density functional theory and compared with the corresponding experimental spectra measured on the Pt(111) surface. The different clusters provide models for adsorption at on-top, twofold bridge, and threefold bridge sites. For the Pt2 and Pt9H14 models of the twofold bridge site, the calculations successfully reproduce the observed frequency shifts for C13, N15, and D substitutions and yield fair agreement with the observed relative intensities. In the case of CNHCH3, the calculations for one of two possible conformers give better agreement with the experimental spectra. No improvement was achieved with the larger Pt9H14 cluster compared with the Pt2 model, indicating that the differences between the experimental and calculated spectra for the latter cluster were not due to finite cluster size effects. The calculated spectra for the different adsorption site models indicate that the experimental spectra are most consistent with the twofold bridge site.

Density functional calculation of a potential energy surface for alkane thiols on Au(1 1 1) as function of alkane chain length

Density functional theory calculations of alkane thiols on Au(1 1 1) have been carried out as a function of the alkane chain length from 1 to 12 carbons using a slab geometry with periodic boundary conditions. Geometry optimized structures were obtained and the potential energy was calculated as a function of the binding site and distance from the surface. While the binding site of minimum energy is only subtly different for different chain lengths there is a pronounced difference in the Au–S bonding and side chain interactions of the methyl group of methane thiol compared to ethane thiol or any longer alkane thiol self-assembled monolayer. For example, the barrier to lateral motion along the surface is lowest for motion through fcc sites for methane thiol while the twofold bridge sites present the lowest barrier to ethane thiol and all longer alkane thiols. The hexagonal close-packed (hcp) and face-centered cubic (fcc) threefold sites were found to differ in energy by less than 1 kJ/mol and therefore only fcc sites were considered in the study. According to the calculated potential energy surfaces the top sites have weaker binding than either the bridge or threefold sites in all cases. The calculations show that ethane thiol is a reasonable model for longer alkane chains.

Cluster model DFT study of acetylene adsorption on the Cu (100) surface

The density functional theory and the cluster model approach have been used to study the adsorption of the acetylene molecule on the (100) surface of copper. Five possible adsorption sites have been considered: parallel twofold bridge, perpendicular twofold bridge, threefold hollow, diagonal fourfold hollow and aligned fourfold hollow sites. For each case, optimized geometries have been calculated. Vibrational frequencies have been calculated for the two energetically most favored adsorption sites. The results show clearly that on the (100) surface of copper the acetylene molecule adsorbs preferably on a fourfold hollow site. These theoretical results are in good agreement with recent Scanning Tunneling Microscopy results.

CO adsorption on the multiple-site Ru(112̄1) surface: The role of bonding competition

The chemisorption and dissociation of CO on Ru(112̄1) were investigated by using high-resolution electron energy-loss spectroscopy and thermal desorption spectroscopy. Three different adsorption states of CO can be distinguished. The most strongly bound β-state, characterized by a C–O stretch frequency of 166 meV, is attributed to CO adsorbed in a fourfold hollow site of the Ru(112̄1) unit cell. This state occurs only at low total coverage and dissociates at T&amp;gt;300 K. A more weakly bound state is α1-CO with a stretch frequency of 240–255 meV, attributed to CO on-top bonded to first and second layer Ru atoms. This species converts to β-CO at moderate total coverage and T&amp;gt;360 K, increasing the amount of dissociated CO. The α1-CO species dominates the coverage regime up to 1.5 ML. The α2-CO species is most weakly bound in the coverage range up to 2 ML and is characterized by a stretch frequency of 220 meV. It is proposed to be located in twofold bridge sites. The ratio of on-top to bridge bonded CO is equal to three at saturation. The finite existence range for β-CO is rationalized by a bond competition effect, due to neighboring α1-CO species destabilizing the β-state at increasing coverage. Consequently β-CO converts to α1-CO under these conditions. A decrease of the β-state coverage via dissociation of CO may initiate the reverse process of α1- to β-CO conversion.

Dark and photoreactions of acetates on TiO 2( [formula omitted]) single crystal surface

The surface reaction of acetates on TiO 2(1 1 0) single crystal has been monitored in dark and under UV by XPS. XPS O(1s) data indicated that surface saturation is obtained at ≈0.5 monolayer coverage consistent with STM results [J. Chem. Phys., 106 (1997) 2924, Surf. Sci. 433–435 (1999) 322]. Annealing the surface results in a steady decrease of surface acetates that disappeared by ≈600 K. UV illumination at room temperature, and in the absence of diatomic oxygen in the gas phase, did not decrease the intensity of the XPS C(1s) and O(1s) signals attributed to surface acetates. On the other hand, UV illumination in presence of oxygen (≈10 −7–10 −6 Torr of diatomic oxygen) has resulted in a noticeable decrease of surface acetates. These combined results indicate that both the two-fold bridging oxygen and the three fold lattice oxygen atoms are not directly involved in the photooxidation reaction of acetates on TiO 2(1 1 0) single crystal. The cross-section of the photodepletion of acetates was estimated equal to 0.6–0.8×10 −17 cm 2.