M. Akbulut

Elastic and inelastic processes in the interaction of 1–10 eV ions with solids: ion transport through surface layers

Abstract We review the escape depth of secondary ions (or neutrals) desorbing from solid surfaces under the impact of electrons, photons or ions. We survey ion (or neutral) transport through many materials, but most are wide band gap insulators such as rare-gas solids and molecular solids. We address the issue of low-energy (

Adsorption and reaction of water on oxidized tungsten: thermal desorption and electron stimulated desorption measurements

Abstract The adsorption and decomposition of water, H 2 18 O, on an 16 O-oxidized W(100) surface have been examined over a wide temperature range (25–700 K) with thermal desorption spectroscopy (TDS), low energy ion scattering (LEIS) and electron stimulated desorption (ESD), and ESD ion angular distribution (ESDIAD). TDS is used to determine the coverage and the range of desorption temperature of H 2 18 O, and to identify desorption products from the oxidized W(100) surface, while ESD and ESDIAD are used to monitor the surface chemistry of H 2 18 O on the oxidized W(100) surface. ESD and ESDIAD data show no evidence for diffusion of H 2 18 O on the oxidized W(100) surface between 25 K and 120 K. TDS demonstrates that the majority of water adsorbed in the first monolayer at 25 K remains molecular and desorbs with a peak temperature of ∼ 155 K. However, both TDS and ESD measurements indicate that a very small percentage of H 2 18 O (∼8% of a monolayer) dissociates upon adsorption at 25 K to form adsorbed 16 OH and 18 OH. No stable OH species remains on the oxidized W(100) surface above about 350 K.

Transmission of low energy (<10 eV) 16O+ ions through condensed ammonia and water overlayers

We have studied the transmission of low energy (<10 eV) 16O+ ions through ultrathin films of condensed molecular solids, NH3 and H218O, in order to address the fundamental scattering processes that occur in the desorption of ions from below the surface of solids. 16O+ ions with a peak energy of ∼7 eV and a narrow angular distribution [full‐width at half‐maximum (FWHM) ∼15°] are generated by means of electron stimulated desorption (ESD) from an 16O oxidized W(100) surface and their yield, energy and angular distribution are measured with a digital ESDIAD (ESD ion angular distribution) detector. Ultrathin NH3 and H218O films of known thickness are condensed on the oxidized surface at 25 K and changes in the ion yield, energy and angular distribution are observed as a function of coverage. We find that adsorption of only 0.5 monolayer of H218O is enough to suppress the 16O+ ion emission by a factor of 100, while three monolayers of NH3 are necessary for equivalent suppression of the 16O+ ion emission. The an...

ELECTRON STIMULATED DESORPTION FROM PF3 ADSORBED ON PT. II. NEGATIVE IONS

We have studied electron stimulated desorption (ESD) of negative ions from PF3 molecules adsorbed on a Pt substrate over a wide electron energy range (0–175 eV). ESD from adsorbed PF3 gives rise to several negative ion fragments: F− (predominantly), F−2, P−, and PF−. The F− yield produced in the electron energy range 0–15 eV proceeds via dissociative electron attachment (DEA); the F− yield exhibits a peak around 11.5 eV with an onset around 7.5 eV. At electron energies above ∼15 eV, F− ions are produced via dipolar dissociation (DD). We have found that the F− ions produced from a 1 ML PF3/Pt surface via a DEA process with 11.5 eV electron impact desorb with a peak kinetic energy of ∼0.7 eV, while the F− ions generated via DD by 175 eV electron impact desorb with a peak kinetic energy of ∼1.2 eV. The F−2 yield curve also shows a peak at ∼11.5 eV; the onset of the F−2 yield from adsorbed PF3 is ∼9 eV. The F−2 yield in the electron range 9–15 eV is initiated via DEA. The P− signal from PF3 adsorbed on Pt has...

Depth of origin of ions in desorption induced by electronic transitions (DIET): ion transmission through ultrathin films

Measurements to address a fundamental issue in transport of low energy (< 10 eV) ions through the surface layers of a solid are described. Our goal is to identify the dominant energy-transfer and charge-transfer processes that limit the survival probability of ions excited below the surface. Our approach is to study the interaction of low energy positive and negative ions (O+, F+, F−) with ultrathin films of condensed gases (Ar, Kr, Xe, H2O, NH3) ranging from fractional monolayer to six monolayers in thickness. The ions are produced by electron stimulated desorption from an appropriate substrate (e.g., oxidized W(100) for O+, PF3 on Ru(0001) for F+, F−). The ions desorb from the surface with well defined energy (< 10 eV) and angular distributions, and their yield, mass/energy and desorption angle are measured using a digital, time-of-flight ESDIAD detector (electron stimulated desorption ion angular distribution). The gases are condensed at < 25 K onto the crystal substrate, and their film thickness is determined by means of thermal desorption spectroscopy. We find that 10% of the O+ ESD signal can be transmitted through 1.6 atomic monolayers (ML) of Ar, 2.9 ML of Kr and 4.0 ML of Xe. In contrast, the O+ signal is attenuated to < 1% by 0.5 ML of H2O. We attribute the attenuation of O+ in rare gas films mainly to elastic backscattering, whereas the attenuation of O+ by H2O and NH3 films is dominated by charge transfer neutralization. F+ ions are almost completely attenuated by 1 monolayer of Xe, while F− ions experience a four-fold increase in yield when the substrate is covered by 1 monolayer of Xe. We discuss these results in terms of charge and energy transfer models, and draw conclusions about the depth of origin of ions produced in DIET (desorption induced by electronic transitions) processes.

Low energy (<5 eV) F+ and F− ion transmission through condensed layers of water

We report on the transmission of F+ and F− ions through ultrathin films of condensed water at 20 K, and compare the experimental results with theoretical calculations. The F+ and F− ions are produced by ESD (electron stimulated desorption) of a PF3 monolayer adsorbed on a Ru(0001) surface (PF3/Ru(0001) surface). We find two surprising results: (a) the off-normal F+ signal is attenuated to ∼1% by only ∼2.5 monolayer (ML) of H2O, while a much thicker layer, ∼10 ML of H2O, is necessary for equivalent attenuation of the F− ion emission, and (b) 1 ML of H2O increases the emission of F− ions and causes a dramatic change in the ion angular distribution. The striking changes in the angular distribution of F− ions transmitted through condensed H2O films indicate that elastic scattering is an important process in determining the attenuation of F− by H2O. No direct evidence for any kind of ion–molecule chemical reaction or collision induced dissociation reaction has been found. The strong attenuation of F+ without s...

Transmission of low-energy (<10 eV)O+ ions through Au films on TiO2(110)

In an attempt to identify the fundamental processes that influence ion transport through metallic surface layers, we have studied the transmission of O+ ions through discontinuous Au films adsorbed on TiO2(110). A low energy (< 10 eV) O+ ion beam is generated via electron stimulated desorption when an Au-dosed TiO2(110) substrate is bombarded with a focused 250 eV electron beam. Low energy ion scattering data indicate that Au evaporated under ultrahigh vacuum conditions at ∼ 300 K forms three-dimensional clusters on TiO2(110). As the Au coverage increases, the formation of Au clusters on TiO2(110) blocks a fraction of the TiO2 surface and the O+ yield is attenuated. However, for high coverages (≥30% Au covered substrate) the O+ signal decreases at a faster rate than the TiO2 open area fraction. We attribute the attenuation of the O+ yield for high Au coverages mainly to blocking of O+ by Au clusters, to deflection of trajectories by the image force between ions and Au clusters, and to charge transfer between desorbing O+ and neighboring Au clusters.

ELECTRON STIMULATED DESORPTION FROM PF3 ADSORBED ON PT. I. POSITIVE IONS

We have studied electron stimulated desorption (ESD) of positive ions from PF3 molecules adsorbed on a Pt substrate over a wide electron energy range (0–175 eV). Electron bombardment of 1 ML PF3 adsorbed on the Pt surface gives rise mainly to an F+ signal, whereas ESD from 6 ML thick PF3 film (thick PF3 layer) leads to P+, PF+, and PF+2 signals, in addition to F+. We find that the onset for F+ desorption from the 1‐ML PF3/Pt is at ∼26.5 eV, while the F+ threshold from the thick PF3 layer is ∼28.5 eV. The P+ appearance potential from the thick PF3 layer is ∼23 eV. The ESD F+ ion energy distribution has a peak energy of ∼4 eV for all electron impact energies and a full width at half maximum (FWHM) of ∼3 eV. The P+ ions desorb with a peak energy of ∼2 eV under 55 eV electron impact; the FWHM of the P+ energy distribution is ∼2 eV. We suggest that the near threshold P+ formation from PF3 corresponds to the excitations of the 6a1 level, while the F+ threshold for adsorbed PF3 on the Pt surface is due to the ex...

Transmission of low energy (< 10 eV) oxygen ions through ultrathin xenon films

In studies of desorption induced by electronic transitions (DIET) such as electron or photon stimulated desorption, it is important to know whether the desorbing species originate solely from the outermost surface layer, or also from layers beneath the surface. In order to gain better understanding of the charge transfer, elastic scattering, and other inelastic processes involved in this issue, we are currently performing a series of experimental studies of the transmission of low energy ions (∼ 7 eV) through ultrathin films (submonolayer to multilayer) of condensed gases. Here we report on the first quantitative measurements of the yield, angle, and energy of oxygen ions after transmission through ultrathin films of xenon. In our novel approach, a focused 300 eV electron beam bombards a target at 25 K consisting of an oxidized tungsten (100) crystal with adsorbed overlayers of xenon. In the absence of the xenon, O+ ions desorb in a sharp beam normal to the surface, as measured in a velocity and angle resolving ESDIAD apparatus (electron stimulated desorption ion angular distribution). When Xe layers are present, some oxygen ions penetrate several monolayers of xenon without significant change in energy and angle while others seem to be scattered by large-angle elastic scattering or to be attenuated from the O+ beam. The work presented is the first experimental study of the depth of origin of desorbing ions in DIET processes.

Depth of origin of ESD ions: enhancement of F− from PF3Ru(0001 upon passage through xenon overlayers

Abstract We have studied the electron stimulated desorption of F− from a monolayer of PF 3 Ru (0001) ; the ions pass through ultrathin overlayer films of Xe. Surprisingly, we find the F− yield to be 4 times higher in the presence of a Xe overlayer one monolayer thick than in the absence of Xe. This enhancement is accompanied by a dramatic change in the ion angular distribution: While F− ions desorb from PF 3 Ru (0001) in a hexagonal array of off-normal beams, they desorb in a broad normal distribution in the presence of a Xe overlayer. We discuss a model which attributes the increase in F− to a decrease in the neutralization probability of F− with the surface, and the change in angle to elastic scattering.