Norbert J. Sack

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 angle-resolved electron-stimulated desorption of CCl4 on Ru(0001)

The adsorption and electron-stimulated desorption of CCl4 on Ru(0001) have been studied at 100 K using a variety of surface analytical techniques, including thermal desorption spectroscopy, Auger electron spectroscopy, low-energy ion scattering, and mass and angle-resolved electron-stimulated desorption (ESDIAD). CC14 is found to dissociate partially for fractional monolayer coverages, and to adsorb molecularly above a coverage of 13%. Upon electron bombardment, Cl+ ions are found to desorb with their angular distribution directed along the surface normal; this implies that CC14 orients with one C-C1 bond normal to the surface. The total yield of Cl+ increases linearly with increasing molecular CC14 coverage up to one monolayer, which implies that Cl+ stems mainly from molecular CCl4 and that interadsorbate quenching is not significantly affecting the desorption yield. Beyond a coverage of 1 ML, the Cl+ yield continues to increase, and starts leveling off after 2 ML. We estimate the yield from a thick layer of CCl4 to be of the order of 3 × 10−8 ions/electron. The angular distribution of the desorbing Cl+ ions widens with increasing CCl4 exposure. Besides Cl+, higher mass fragments of CCl4, such as CCl+, CCl2+ and CCl3+ are also found to desorb from multilayers upon electron bombardment.

Coverage dependent azimuthal orientations of PF3 on Ru(0001): adsorbate-substrate versus interadsorbate interactions in rotational dynamics

Abstract We present evidence for a coverage dependent rotational dynamics and azimuthal orientation of PF 3 Ru (0001) . ESDIAD data are interpreted to indicate that at low coverages at temperatures above 75 K PF3 rotates nearly freely about an axis normal to the surface, while the rotation is hindered at 20 K; the barrier height is 1±0.5 meV. At saturation coverage (0.33 ML) the rotation is suppressed at all temperatures below 300 K. Surprisingly, the azimuthal orientation at saturation coverage differs by 30° from that for the low coverage at 20 K, which we explain on the basis of geometrical packing arguments and competing interaction potentials.

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...

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.

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.

Angle-resolved DIET of negative ions from halogenated molecules on Ru(0001)

Abstract We present data on the electron stimulated desorption (ESD) of negative ions from PF3 and CCl4 adsorbed on Ru(0001) at 100 K. We use a two-dimensional digital ESDIAD detector with time-of-flight capability which allows mass and angle resolved ion detection. We find F− and F2− to desorb from PF3, yielding hexagonal ESDIAD patterns, and Cl− and Cl2− from CCl4, desorbing in a broad normal emission. The ratio of F2− to F− and of Cl2− to Cl− is more than a factor 10 higher than in electron induced gas phase dissociation. We compare the negative ion yields to those of positive ions.