Table of contents

Photostimulated processes on surfaces are at the interface of photochemistry and surface science. This subject is, by nature, open to a large variety of concepts, experimental methods and applications. One of the key concepts associated with photostimulated processes on surfaces is the so-called `desorption induced by electronic transition' (DIET) process. After having been discovered in the 1960s, the study of DIET processes entered a highly active phase of development in the 1990s due to the tremendous evolution of photon facilities accompanied by the revolution of DIET at the nanoscale. Innovative photon sources have arisen from the advent of new laser sources (femtosecond lasers, high repetition rate lasers, VUV lasers) and the construction of numerous high brightness, second- and third-generation synchrotron radiation facilities all over the world. Applications of photostimulated processes on surfaces are actively pursued in photolithography, the fabrication of functionalized surfaces and in advanced materials, astrophysics and environmental science. These fundamental and applied developments have encouraged the editors to solicit articles for this issue. The organization of the issue presents photostimulated processes from metal (30%) and semiconductor (20%) surfaces as well as insulating surfaces and condensed gases on surfaces (50%). The processes have been induced using laser (50%) and synchrotron radiation (50%) sources. We are grateful to all the authors for their contributions, reviewing some of their previous work or including some new results.

In recent years, the formation of thin, well-ordered but complex surface oxides on late transition metals has been discovered. The driving force for this line of research has been the strong incentive to increase the partial pressure of oxygen from ultra-high vacuum to conditions more relevant for heterogeneous catalysis. Here we review the present status of the research field. Compared to oxygen adatom superstructures, the structure of the surface oxides has proven to be extremely complex, and the investigations have therefore relied on a combination of several experimental and theoretical techniques. The approach to solving the structures formed on close-packed surfaces of Pd and Rh is presented in some detail. Focusing on the structures found, we show that the surface oxides share some general properties with the corresponding bulk oxides. Nevertheless, of all surface oxide structures known today, only the two-dimensional surface oxides on Pd(100) and Pt(110) have the same lattice as the bulk oxides (PdO and PtO, respectively). In addition to two-dimensional oxides, including the O–Rh–O trilayers found on Rh, one-dimensional oxides were observed at ridges or steps of open surfaces such as (110) or vicinal surfaces. Finally, we briefly report on a few studies of the reactivity of surface oxides with well-known structure.

We propose a double-trimer model and a single-trimer model for reconstructions observed on the surface of the 3C-SiC(111) island and the 6H-SiC(0001) surface, respectively. We study their atomic and electronic structures by using first principles calculations within density-functional theory. The total energy calculations indicate that the double-trimer model and the single-trimer model are energetically more favourable than the previously reported DV model and Tri-Ad model, respectively. The simulated scanning tunnelling microscopic images for these two models agree fairly well with the experimental observations. The surface energy band structures of the single-trimer model and the Tri-Ad model were compared, which might provide a criterion for the further discrimination of the exact models by experimental evaluation.

The lower temperature and shorter timescale for microwave irradiation may be ascribed to the activating and facilitating effect of microwaves on solid-phase diffusion. Using the microwave-heating process, it is possible to achieve enhanced mechanical properties such as greater hardness, improved scratch resistance, and structure texturing. In the present investigation, thin films of pure TiO₂ and their respective perovskite-structured thin films, namely NiTiO₃ and CdTiO₃, have been prepared by simple and cost-effective sol–gel processing on Si(100) substrates and subjected to irradiation with different microwave powers for 10 min. X-ray diffraction (XRD), scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS) techniques have been employed to characterize the structural, morphological and elemental composition of the films. It is evident that there is a dramatic change in the structural and morphological properties of the films irradiated in microwaves compared to conventional annealing at high temperature. A plausible mechanism for the formation of the anatase phase of TiO₂ and perovskite structures with a rhombohedral phase formation at low microwave powers has also been discussed. This new innovative microwave heating could open a door for advanced technologies to cut process costs in the post-treatment of materials.

Reflectance anisotropy spectroscopy (RAS) has been used, together with scanning tunnelling spectroscopy (STS), to investigate the optical and electronic properties of nanodots formed by depositing Ag on the Si(111)–3 × 1-Ag surface. One-dimensional (1D) arrays of nanodots were grown on a single-domain (3 × 1)-Ag surface and the anisotropic optical response in the 0.5–5 eV range measured by RAS. Aligned, elongated Ag islands were also grown on this surface to compare their properties with those of the nanodots. STS of the Ag islands showed distinct metallic behaviour, whereas the nanodots revealed a bandgap of ∼0.6 eV, indicating that the surface of the dots has a non-metallic character, similar to that of the Si(111)–3 × 1-Ag surface. RAS also showed substantial differences between the structures, with a large infrared anisotropy for the metallic Ag islands consistent with anisotropic Drude-like intraband transitions, whereas the nanodots gave a negligible response in this spectral region. The RAS results in the infrared spectral region are consistent with the difference in the metallic character of the nanodots and islands, as determined by STS.

We present a scanning tunnelling microscopy (STM) investigation of Co silicide cluster and island formation on Si(111)-(7 × 7). We identify sub-nanometre clusters, which we propose to be the precursors to Co silicide nucleation and growth. The observation of metastable phases and various domains on the surface suggest significant Si rearrangement during silicide formation. Surface diffusion of both Si and type-1 clusters plays a vital role in the nucleation of Co silicide islands. With the help of density functional theory calculations, we propose a model for the surface structure of atomically flat CoSi₂(111)-(2 × 2) islands.

In this work, barium and oxygen interaction on the Ni(110) surface is investigated, by means of soft x-ray photoemission spectroscopy (SXPS), mainly at submonolayer and monolayer coverages. The results show that oxygen interacts with barium and the substrate as well forming BaO and NiO respectively. The formation of both oxides is consistent with previous Auger electron spectroscopy (AES) results. The oxidation of barium on the surface induces negative binding energy shifts of the Ba low core atomic levels 4d, 5s and 5p. Both initial and final state effects are shown to be necessary in order to explain these peculiar energy shifts. This interpretation is based on correlating the core level binding energy shifts with previously recorded AES transition line shifts, also taking into account the changes of the work function of the surface. The analysis shows that the extra-atomic relaxation energy decreases on going from metallic Ba to BaO. This implies that the reduction of the free electron screening more than counteracts the increase in relaxation energy due to the polarizable O²⁻ anions.

Nanometre-sized (hereafter n-) Pb particles embedded in an Al matrix were prepared by ball milling, and the surface melting behaviour was studied by the internal friction technique using a dynamic mechanical analyser. There is an internal friction peak appearing around Pb melting point and the height of the internal friction peak decreases with the increase of Pb particle size. Surface melting of n-Pb particles accounts for the internal friction peak. The study may throw light on the nature of surface melting in condensed matter physics. In addition, the present work makes a beneficial attempt at exploring internal friction as an experimental method for studying surface melting.

The structural, electronic and magnetic properties of bulk Gd and of the Gd(0001) surface have been investigated using ab initio calculations based on density-functional theory (DFT) and on DFT+U calculations. In agreement with earlier work we find that the neglect of the strong correlation of the 4f electrons leads to the wrong prediction of an antiferromagnetic ground state of hexagonal close-packed (hcp) Gd, as well as to substantial errors in the magnetic moments and exchange splitting. If the strong on-site Coulomb repulsion in the 4f band is described by a Hubbard-like term added to the DFT Hamiltonian, an improved description of the structural, electronic and magnetic properties of both bulk Gd and of the Gd(0001) surface is achieved. The enhancement of the exchange coupling and of the magnetic ordering temperature at the surface is investigated using a simple model. The adsorption of hydrogen on the Gd(0001) surface and its diffusion into deeper layers has been investigated. It is shown that H adsorption eliminates the electronic surface state which is partly responsible for the enhanced magnetism at the clean surface and leads to the formation of H-induced electronic states below the bottom of the valence band.

The energetic and structural properties of SiH₂ adsorbed on the single dimer vacancy (SDV) of the Si(100)-c(4 × 2) surface have been studied by using empirical tight-binding (ETB) total energy calculations. The adsorptions of SiH₂, SiH₂ with a hydrogen adatom (SiH₂+H) and SiH₂ with two hydrogen adatoms (SiH₂+2H) on the Si(100) surface with SDV structure have been studied. Three possible sites (A, B and C) are found in each case. Adsorbing of SiH₂ or H on the SDV makes the structure of the SDV change. When a unit of SiH₂ is adsorbed on the SDV, the A site is more stable than the B site. If SiH₂+H or SiH₂+2H is adsorbed on the SDV, the relative stability of the A site and B site reverses. The C site is found to be the least stable site in all cases.

Molecular bilayers of CH₃X (X = Br,I) on the Cu(110)–I surface are found to have a high degree of orientational order for the C–X bond axis. This orientational order is exploited in photodissociation experiments to select different dissociative excited states on the basis of photolysis wavelength and polarization. Results are presented from the photodissociation of CH₃Br at λ = 222 and 193 nm, and CH₃I at λ = 248 and 222 nm. The Φ^* branching ratio to yield either CH₃+X^*(²P_1/2) or CH₃+X(²P_3/2) is varied between 0.06 and 0.85 depending on the molecule, wavelength and polarization selected.

We present an overview of IR laser resonant desorption (LRD) studies with special focus on recent developments by our group. The resonant character of the laser desorption process mediated by O–H, C–H, and N–H vibrational modes in the 3 µm spectral range has been investigated on ice, PAH and ammonia samples, respectively. The desorption of cryogenic binary mixtures is also presented. An interesting wavelength-selective effect has been found by comparative IR/visible or IR/IR irradiation studies. At higher fluences and/or analyte concentration values a loss of selectivity occurs, with extensive co-desorption of both components and clustering phenomena. Insights on the desorption mechanisms involved are presented through an ejecta velocity distribution study. Finally, possible technological and analytical potentialities are discussed, in the fields of IR-MALDI, LRD depth profiling, pulsed laser cleaning and selective analysis of complex samples.

This paper reports on recent studies of photostimulated ion desorption (PSID) using electron ion coincidence (EICO) spectroscopy combined with synchrotron radiation. H⁺ desorption from H₂O dissociatively adsorbed on Si(111) and SiO₂/Si(111) surfaces (H₂O/Si(111) and H₂O/SiO₂/Si(111)) was studied for Si L-edge excitation. The Si 2p–H⁺ photoelectron photoion coincidence (PEPICO) and Si 2p photoelectron spectra of H₂O/Si(111) and H₂O/SiO₂/Si(111) show that H⁺ desorption probability increases as the number of positive charges at the Si site increases. The H⁺ desorption probability per Si 2p ionization for the Si⁴⁺ site was estimated and found to be 5–7 × 10⁻⁵. We proposed a mechanism that H⁺ desorption is induced by Si 2p photoionization accompanied by a Si LVVV double-Auger transition. This article also reviews recent EICO work on site-specific ion desorption of 1,1,1-trifluoro-2-propanol-d₁ (CF₃CD(OH)CH₃) adsorbed on Si(100) surfaces, and on the mechanisms of PSID of poly(tetrafluoroethylene) (PTFE) and TiO₂(110). Clear site-specific ion desorption was observed for the C 1s core excitation of a CF₃CD(OH)CH₃ sub-monolayer on Si(100). A spectator-Auger-stimulated ion-desorption mechanism was proposed for F⁺ desorption induced by a transition from F 1s to σ(C–F)^* of PTFE. O⁺ desorption induced by O 1s excitation of TiO₂(110) was attributed mainly to three-hole final states resulting from multi-electron excitation/decay. For O⁺ desorption induced by Ti core excitation of TiO₂(110), on the other hand, charge transfer from an O 2p orbital to a Ti 3d orbital, instead of the interatomic Auger transition, was proposed to be responsible for the desorption. These investigations demonstrate that EICO spectroscopy combined with synchrotron radiation is a useful tool for studying PSID.

For the system of oxygen adsorbed at the step sites of a vicinal Pt(111) surface it has been demonstrated that diffusion of strongly bound atomic adsorbates can be induced electronically with femtosecond laser pulses (Stépán et al 2005 Phys. Rev. Lett. 94 236103). The dynamics of energy transfer between the initial electronic excitation of the metallic substrate and the adsorbate motion is examined by comparing experimental hopping rates with those obtained from different model calculations. For this purpose the adsorbate–substrate coupling is approximated by an electronic friction. The experiment shows a strong nonlinear dependence of the hopping rate on laser fluence (F¹⁵) and a coupling time of 1.5 ps between electronic excitation and diffusive motion as deduced by two-pulse correlation measurements. The consistent modelling of these findings requires the introduction of a friction coefficient that strongly depends on electron temperature. This result is interpreted in terms of an indirect excitation mechanism. It is proposed that anharmonic coupling between primarily excited perpendicular O–Pt vibrations and frustrated translations results in an effective coupling strength that increases with increasing electron temperature.

Recent progress towards a quantum theory of laser-induced desorption and related phenomena is reviewed, for specific examples. These comprise the photodesorption of NO from Pt(111), the scanning tunnelling microscope and laser-induced desorption and switching of H at Si(100), and the electron stimulated desorption and dissociation of CO at Ru(0001). The theoretical methods used for nuclear dynamics range from open-system density matrix theory over nonadiabatically coupled multi-state models to electron–nuclear wavepackets. Also, aspects of time-dependent spectroscopy to probe ultrafast nonadiabatic processes at surfaces will be considered for the example of two-photon photoemission of solvated electrons in ice layers on Cu(111).

Ion photostimulated desorption (PSD) is a specific surface sensitive process. This paper reviews ion PSD following core level excitation of molecules adsorbed on the Si(111) 7 × 7 and Si(100) 2 × 1 surfaces. Several aspects of ion PSD will be discussed; (i) the use of ion PSD as a tool for investigating the adsorption configurations of O₂ on Si(111) 7 × 7, (ii) the relevance of ion PSD for probing the physisorption–chemisorption transition of benzene molecules on Si(111), (iii) the comparison between ion PSD of methanol adsorbed on Si(111) and photofragmentation of the methanol molecule in the gas phase, (iv) the comparison between ion PSD of formic acid adsorbed on the Si(111) and Si(100) surfaces and (v) the chemical selectivity of ion PSD following core level excitation of NO dissociatively adsorbed on Si(111). Finally, the interplay between ion dynamics and electronic relaxation following core level excitation and Auger processes will be discussed in the case of O⁺ photodesorption from O₂ adsorbed on Si(111) 7 × 7, as investigated via isotope and temperature effects on both the intensities and kinetic energies of the desorbed ions.

The present work reviews laser-induced electronic processes of structural instability on covalent semiconductor surfaces. In particular, we concentrate on the mechanism of the instability that takes place at the intrinsic sites of reconstructed surface structures. In order to elucidate the primary processes involved, we focus our attention on experimental results obtained by scanning tunnelling spectroscopy studies, to reveal surface structural changes at the atomic level, and by post-ionization spectroscopy studies, to probe desorption processes with high sensitivity. First, the results obtained for reconstructed Si surfaces and {110} surfaces of III–V compound semiconductors are systematically surveyed. The instability is characterized by local bond rupture at intrinsic surface sites that show important common features. Also, we find significantly different aspects of the instability, which depend on the basic properties of surfaces. Based on the characteristics revealed by the experimental results, we then propose a mechanistic model based on the generalized two-hole localization mechanism, and demonstrate, through the quantitative analysis of typical experimental results, that the model describes all the important features quantitatively and consistently.

In this paper we review a number of works on photon stimulated ion desorption (PSID) of hydrogen from various diamond surfaces. In particular, we elaborate on the bonding configuration of adsorbed hydrogen (deuterium) and its neighbouring atoms, and its thermal stability in surface and sub-surface regions. Usage of hydrogen PSID as a characterization method involves detailed description of the mechanism and dynamics of hydrogen ion desorption initiated by a synchrotron generated photon beam. Several complimentary analytical techniques (near edge x-ray absorption fine structure spectroscopy, x-ray photoelectron spectroscopy, secondary electron emission measurements) were used to clarify the results of the PSID measurements. The studied systems include hydrogenated/deuterated single-crystal and polycrystalline diamonds, diamond films hydrogenated ex situ using microwave hydrogen plasma versus in situ hydrogenation by passing hydrogen gas through a hot tungsten filament, hydrogenated diamond films exposed to atomic and molecular oxygen gas, and ion beam damaged diamond films.

Our recent work on photon-stimulated desorption of alkali halide surfaces has been reviewed. Most of the experimental data are presented for the first time. This new material is supplemented with two examples of our work published previously (see Szymonski et al 1996 Surf. Sci. 363 229; 2002 Acta Phys. Pol. 33 2237). The results are discussed and compared with relevant experimental and theoretical work by other authors. In particular, we focus on two aspects of the studies: high resolution AFM imaging of desorbed surfaces, and measurements of mass selected fluxes of the desorbing atoms. It is found that, at initial stages of the UV-photon stimulated desorption, the process is occurring in a 'layer-by-layer' mode as a result of growth and linking of monatomic pits on the surface. This results in periodic changes of surface topography from atomically flat to rough with a period equal to the time needed for desorption of 1 ML. Such periodic changes of the surface topography affect the efficiency of the desorption and result in oscillatory dependence of the emitted particle yields versus photon fluence. Careful analysis of the yields reveals that during irradiation there is a substantial number of stable ground state F-centres accumulated under the surface. The number of accumulated defects is changing periodically with the photon fluence in anti-correlation with the desorption yields. Based on the mentioned results, we argue that the production of F-centres and their interaction with the surface are, in fact, the limiting factor for PSD. To test the validity of this statement a series of desorption experiments for the crystal co-irradiated with visible light (with a wavelength corresponding to the F-centre absorption band) and UV photons have been performed. A dramatic increase in the desorption yield, as well as pronounced changes in the surface erosion process, have been found.

This paper reviews our progress on the desorption induced by electronic transitions (DIET) in rare gas solids by selective excitation of valence excitons. Observation of metastable atoms desorbed by excitonic excitation gives us direct information on the exciton-induced desorption processes in rare gas solids. The validity of three desorption mechanisms, cavity ejection, excimer dissociation, and internal sputtering, is demonstrated by systematic measurements of kinetic energies and angular distributions of desorbed particles. The absolute yield of total and partial desorption was measured, which can lead us to the quantitative understanding of exciton-induced desorption processes.

Recent developments in the field of photochemistry at nanoparticulate surfaces will be reviewed. Data on laser induced diffusion, desorption and dissociation of molecules adsorbed at supported palladium nanoparticles in the size regime of a few tens to up to a few 10 000 atoms per island will be summarized. Nanosecond as well as femtosecond experiments including quantum state selective monitoring of the energy partitioning within the desorbing molecules will be presented. Interesting effects such as adsorbate induced roughening of the particles after coherent laser excitation will be reported. All phenomena exhibit a strong size dependence for the photochemistry below an average particle size of 80 Å and with remarkable changes within the population of different reaction paths below 45 Å aggregates. Defects as well as edges and kinks turn out to be important for pinning the electronically excited states.

Reactions of halogens (Cl₂,XeF₂) with metals (Cu) and semiconductors (Si, GaAs) are investigated. The main focus is put on light induced reactions, stimulated by synchrotron radiation in the spectral range from 200 to 50 nm, in comparison with the dark reaction. Growth of reaction products on the surface and the desorption of volatile compounds are studied. A set-up with a quartz microbalance was adopted to determine reaction rates in situ. The rates are very sensitive to sample preparation. In the system Cu/Cl₂, oxygen was found to especially slow down the reaction and much higher reaction rates than reported previously were observed for pure samples. Measurements with masks show the possibility of using desorption (also called light induced dry etching) to microstructure materials. Analysis of the irradiations with different wavelengths reveals a high spectral dependence of the reactions, which can therefore be controlled. The efficiency of the light induced non-selective reaction follows the gas phase absorption of the etching gases, whereas selective reactions, which are used to structure the materials, are induced in adsorbed halogens at different wavelengths. High efficiencies of single-photon events, due to chain reactions, with multiplication factors of the order of 10⁵, are observed. The resulting pit size has to be contrasted with the intended spatial resolution.

Inner-shell excitation possesses an attractive potential to induce selective chemical bond breaking due to its special localization and atomic selectivity. This paper reviews our recent works about site-selective (or site-specific) photon stimulated desorption (PSD) of core-excited ester compounds, poly(methyl methacrylate) (PMMA) thin films, condensed methyl isobutyrate (MIB) multilayers, methyl ester terminated self-assembled monolayers (SAMs), partially deuterided SAMs and ethyl ester terminated SAMs. Exact selective bond breaking was observed for all molecular systems except for MIB multilayers: COO–R (R = CH₃, C₂H₅) breaking in the , O 1s(OR) excitations and CO–OR in the excitation. Such resonant excitations take place within the functional group locally and the initial memory of the excited site and its localization character is effectively conserved during the PSD process even after subsequent Auger decays and ionic dissociation/desorption. Variation of selectivity observed here indicates the importance of the molecular environment and the contribution of the indirect dissociation process due to statistical energy relaxation and x-ray-induced electron stimulated desorption. The obvious polarization dependence measured only for selectively desorbing ions also supports the above characteristics. Neutral fragments desorbing from core-excited PMMA follow the profile of the total electron yield spectrum but not the partial ion yield of corresponding ions. This result indicates that ionic dissociation locally promoted by Coulomb repulsion plays an important role to reveal site-selectivity in chemical reactions induced by core-level excitation.

Techniques to produce light in the vacuum ultraviolet (VUV; 100 nm≤λ≤200 nm) and extreme ultraviolet (XUV; 10 nm≤λ≤100 nm) are reviewed. Special emphasis is placed on high harmonic generation (HHG). Experimental studies that have utilized HHG as well as the VUV and XUV surface photochemistry of H₂O and O₂ are reviewed. HHG is shown to be a promising technique for the investigation of the dynamics of electrons and surface photochemistry. Lateral interactions and adsorbate structure in adsorbed H₂O and O₂ layers are shown to have important roles in determining the fate of ions produced by dissociative photoionization at surfaces.

Following our earlier finding of temperature effects in the radiation damage of self-assembled monolayers (SAMs) we investigated in detail the x-ray-induced modification of aliphatic and aromatic thiolate-based SAMs formed from dodecanethiol and biphenylthiol on gold substrates as representative test systems. For six sample temperatures between 50 and 300 K we measured cross sections and saturation behaviour for the most prominent irradiation-induced reactions, including desorption of hydrocarbon fragments, breaking of the headgroup–substrate bond, and formation of cross-linking networks and secondary bonds of the headgroup atoms within the layer. The results are discussed.