Volume 29, Issue 3, March 2003
Index of content:
29(2003); http://dx.doi.org/10.1063/1.1542436View Description Hide Description
29(2003); http://dx.doi.org/10.1063/1.1542437View Description Hide Description
We review different approaches to measure the transport of F atoms and ions in rare-gas matrices and compare the experimental results to simulations. Static measurements on sandwich structures and co-doped matrices yield rather long travel ranges beyond 2 nm, in accord with early classical simulations which predict a channeling of the F atoms in rare gas matrices. Nonadiabatic simulations show a rapid energy loss, fast nonadiabatic dynamics, and only short travel ranges of typically 1 unit cell. The rapid energy loss, fast nonadiabatic transitions and the time scale for direct dissociation (∼250 fs) are verified by femtosecond pump–probe experiments. It remains a challenge to account for the long-range migration when nonadiabatic processes are allowed in simulations, and to measure the long-distance flights directly by ultrafast spectroscopy.
Coherent motion and anomalous transport properties of exciton and hole polarons with intrinsic vibrational structure29(2003); http://dx.doi.org/10.1063/1.1542438View Description Hide Description
It is shown that intrinsic vibrational degrees of freedom, inherent in two-atom exciton and hole polarons, drastically affect their transport properties in wide-band dielectrics (rare-gas solids and alkali halides). A fast excitonic energy transport and a comparatively high hole mobility, experimentally observed and attributed to two-site polarons tightly bound with the lattice, cannot be explained by the conventional theory of small-radius polarons that predicts their negligibly weak diffusion, exponentially small in the ratio of the binding energy to temperature. The theory developed below with allowance for the intrinsic vibrational structure of two-site polarons describes qualitatively a large relevant set of experimental data which seem anomalous from the viewpoint of the conventional theory.
29(2003); http://dx.doi.org/10.1063/1.1542439View Description Hide Description
Experimental results on excess electron transport in solid and liquid phases of Ne, Ar, and solid mixture are presented and compared with those for He. The muon spin relaxation technique in frequently switching electric fields was used to study the phenomenon of delayed muonium formation: excess electrons liberated in the ionization track converge upon the positive muons and form Mu atoms. This process is shown to be crucially dependent upon the electron’s interaction with its environment (i.e., whether it occupies the conduction band or becomes localized in a bubble of tens of angstroms in radius) and upon its mobility in these states. The characteristic lengths involved are and the characteristic times range from nanoseconds to tens of microseconds. Such a microscopic length scale sometimes enables the electron to spend its entire free lifetime in a state which may not be detected by conventional macroscopic techniques. The electron transport processes are compared in: liquid and solidhelium (where the electron is localized in a bubble); liquid and solidneon (where electrons are delocalized in the solid, and the coexistence of localized and delocalized electron states in the liquid was recently found); liquid and solidargon (where electrons are delocalized in both phases); orientational glass systems (solid mixtures), where our results suggest that electrons are localized in an orientational glass. This scaling from light to heavy rare gases enables us to reveal new features of excess-electron localization on a microscopic scale. Analysis of the experimental data makes it possible to formulate the following tendency of the muon end-of-track structure in condensed rare gases. The muon–self-track interaction changes from isolated-pair (muon plus the nearest track electron) in helium to multipair (muon in the vicinity of tens of track electrons and positive ions) in argon.
29(2003); http://dx.doi.org/10.1063/1.1542440View Description Hide Description
The low-temperature electron transport on semiconductor surfaces has been studied using an ultrahigh-vacuum, variable temperature scanning tunneling microscope(STM). The STMspectroscopy performed at various temperatures has made it possible to investigate the temperature dependence (300 K to 35 K) of the surface conductivity of three different semiconductor surfaces: highly dopedn-type Si(100), p-type Si(100), and hydrogenated C(100). Low temperature freezing of specific surface electronic channels on the highly doped Si(100) and moderately dopedp-type Si(100) surfaces could be achieved, whereas the total surface conductivity on the hydrogenated C(100) surface can be frozen below only 180 K.
29(2003); http://dx.doi.org/10.1063/1.1542441View Description Hide Description
We review recent research on reactions (including dissociation) initiated by low-energy electron bombardment of monolayer and multilayermolecular solids at cryogenic temperatures. With incident electrons of energies below 20 eV, dissociation is observed by the electron stimulated desorption(ESD) of anions from target films and is attributed to the processes of dissociative electron attachment (DEA) and to dipolar dissociation. It is shown that DEA to condensed molecules is sensitive to environmental factors such as the identity of co-adsorbed species and film morphology. The effects of image-charge induced polarization on cross sections for DEA to are also discussed. Taking as example, the electron-induced production of CO within multilayer films of methanol and acetone, it is shown that the detection of electronic excited states by high-resolution electron energy loss spectroscopy can be used to monitor electron beam damage. In particular, the incident energy dependence of the CO indicates that below 19 eV, dissociation proceeds via the decay of transient negative ions (TNI) into electronically excited dissociative states. The electron-induced dissociation of biomolecular targets is also considered, taking as examples the ribose analog tetrahydrofuran and DNA bases adenine and thymine, cytosine and guanine. The ESD of anions from such films also show dissociation via the formation of TNI. In multilayermolecular solids, fragment species resulting from dissociation, may react with neighboring molecules, as is demonstrated in anion ESD measurements from films containing and various hydrocarbon molecules. X-ray photoelectron spectroscopy measurements reported for electron-irradiated monolayers of and on a passivated surface further show that DEA is an important initial step in the electron-induced chemisorption of fragment species.
29(2003); http://dx.doi.org/10.1063/1.1542442View Description Hide Description
Electron-stimulated desorption ion angular distribution (ESDIAD) and temperature-programmed desorption(TPD) techniques have been employed to study radiation-induced decomposition of fractional monolayerfilms physisorbed on Ru(0001) at 25 K. Our focus is on the origin of and ions, which dominate ESD from fractional monolayers. ions escape only in off-normal directions and originate from undissociated molecules. The origins of ions are more complicated. The ions from electron-stimulated desorption of molecularly adsorbed desorb in off-normal directions, in symmetric ESDIAD patterns. Electron beam exposure leads to formation of fragments, which become the source of positive ions in normal and off-normal directions. Electron exposure results in decomposition of the entire adsorbed layer.
29(2003); http://dx.doi.org/10.1063/1.1542443View Description Hide Description
Soft landing of mass-selected clusters in rare gas matrices is a technique used to preserve mass selection in cluster deposition. To prevent fragmentation upon deposition, the substrate is covered with rare gas matrices to dissipate the cluster kinetic energy upon impact. Theoretical and experimental studies demonstrate the power of this technique. Besides STM,optical absorption, excitation, and fluorescence experiments, x-rayabsorption at core levels can be used as a tool to study soft landing conditions, as will be shown here. X-ray absorption spectroscopy is also well suited to follow diffusion and agglomeration of clusters on surfaces via energy shifts in core level absorption.
Element-specific and site-specific ion desorption from adsorbed molecules by deep core-level photoexcitation at the K-edges29(2003); http://dx.doi.org/10.1063/1.1542444View Description Hide Description
This article reviews our recent work on the ion desorption from adsorbed and condensed molecules at low temperature following the core-level photoexcitations using synchrotron soft x-rays. The systems investigated here are adsorbed molecules with relatively heavy molecular weight, containing third-row elements such as Si, P, S, and Cl. Compared with molecules composed of second-row elements, the highly element-specific and site-specific fragment-ion desorptions are observed when we tune the photon energy at the dipole-allowed resonance. On the basis of the resonance Auger decay spectra around the ionization thresholds, the observed highly specific ion desorption is interpreted in terms of the localization of the excited electrons (here called “spectator electrons”) in the antibonding σ* orbital. In order to separate the direct photo-induced process from the indirect processes triggered by the secondary electrons, the photon-stimulated ion desorption was also investigated in well-controlled mono- and multilayer molecules. The results confirmed that the resonant photoexcitation not in the substrate but in the thin films of adsorbates plays a significant role in the realization of the highly specific ion desorption.
Ion desorption from molecules condensed at low temperature: A study with electron-ion coincidence spectroscopy combined with synchrotron radiation (Review)29(2003); http://dx.doi.org/10.1063/1.1542445View Description Hide Description
This article reviews our recent work on photostimulated ion desorption (PSID) from molecules condensed at low temperature. We have used electron–ion coincidence (EICO) spectroscopy combined with synchrotron radiation. The history and present status of the EICO apparatus is described, as well as our recent investigations of condensed and Auger electron photon coincidence (AEPICO) spectra of condensed at the ionization showed that desorption was stimulated by O:KVV Auger processes leading to two-hole states (normal-Auger stimulated ion desorption (ASID) mechanism). The driving forces for desorption were attributed to the electron missing in the O–H bonding orbitals and the effective hole–hole Coulomb repulsion. The normal ASID mechanism was also demonstrated for condensed The desorption at the resonance of both condensed and condensed was found to be greatly enhanced. Based on the AEPICO spectra the following four-step mechanism was proposed: (1) the transition, (2) extension of the HO–H distance within the lifetime of the state, (3) spectator Auger transitions leading to states, and (4) desorption. The enhancement of the desorption yield was attributed to the repulsive potential surface of the state. At the resonance of condensed on the other hand, the yield was found to be decreased. The AEPICO spectra showed that the desorption was stimulated by spectator Auger transitions leading to states. The decrease in the yield was attributed to a reduction in the effective hole–hole Coulomb repulsion due to shielding by the electron. Photoelectron photon coincidence (PEPICO) spectra of condensed showed that the core level of the surface responsible for the desorption was shifted by 0.7 eV from that of the bulk The desorption from condensed was also investigated. In a study of condensed using PEPICO spectroscopy, site-specific ion desorption was directly verified; that is, and desorption was predominant for the photoionization at the - site, while and desorption was predominantly induced by the photoionization at the - site. These investigations demonstrate that EICO spectroscopy combined with synchrotron radiation is a powerful tool for studying PSID of molecules condensed at low temperature.
29(2003); http://dx.doi.org/10.1063/1.1542446View Description Hide Description
Absolute yields of the photo-induced desorption at the surface of solid rare gases are studied in the excitonic excitation region. Both metastable and total desorption yields depend strongly on excitation energy and film thickness of rare gas solids. The absolute desorption yields and their dependence on film thickness are quantitatively reproduced by a simulation based on the diffusion of excitons in the bulk and the kinetic energy release by a cavity ejection mechanism and an excimer dissociation mechanism followed by internal sputtering.
29(2003); http://dx.doi.org/10.1063/1.1542447View Description Hide Description
We study the creation of biexcitons in neon films on a metal substrate by one-photon processes. We demonstrate that photon-stimulated desorption of ions is a perfect tool for the investigation of these excitation processes, which possess very low cross sections. We show that the principle of the equivalent-core approximation which is well known from inner shell experiments can also be applied to the neon biexciton case. Comparing the equivalent-core molecules and we find that neon biexcitons can be described well in a Frenkel picture.
29(2003); http://dx.doi.org/10.1063/1.1542448View Description Hide Description
The lattice defect formation induced by electronic excitation in solid Ne is studied using the selective vacuum ultraviolet spectroscopy method. The samples are excited with synchrotron radiation in the range of excitonic absorption The dose dependence of the intensity distribution in the band of atomic-type self-trapped excitonluminescence is analyzed. Direct evidence of the formation and accumulation of point lattice defects in solid Ne via the excitonic mechanism is obtained for the first time. A model of permanent lattice defect formation is discussed.