R. G. Albridge

Mechanisms of Electron- and Photon-Stimulated Desorption in Alkali Halides

Abstract The discovery that neutral excited- and ground-state atoms are efficiently desorbed from alkali halide surfaces by both electrons and photons has raised a host of questions about the ways in which energy is absorbed, localized and redistributed in electronically-stimulated desorption. We present recent measurements of electron- and photon-stimulated desorption of neutral alkali atoms from LiF and NaCI which attempt to answer these questions by providing details of energy dependence, velocity distributions and variations in the desorption yield as a function of surface temperature. The ground-state neutral desorption dynamics under both electron and photon irradiation are consistent with a model in which halogens are ejected from the surface in a focused collision sequence, while the neutral alkalis are desorbed thermally. However, the excited-state neutrals do not follow this pattern, and further measurements to understand excited-state neutral desorption will be necessary. The implications of the experimental data for an understanding of the desorption mechanism-such as correlations with specific core-level excitations and with mobilities of electronically-induced defects-will be discussed.

Free‐electron laser internal photoemission measurements of heterojunction band discontinuities

Internal photoemission (IPE) measurements of the conduction band discontinuities at GaAs/amorphous‐Ge interfaces have been carried out at the Vanderbilt Free‐Electron Laser (FEL). IPE is a simple and direct measurement capable of unprecedented accuracy. The FEL is uniquely equipped for semiconductor spectroscopy due to its brightness, tunability, and spectral range, 1–8 μm (0.15–1.2 eV). IPE uses FEL photons of sufficient energy to optically pump electrons over the conduction band discontinuity, producing a photocurrent. No complex modeling is required to extract the discontinuity from the experimental data, since it coincides with photocurrent threshold energy. Both linear and Fowler curve fits of the threshold region agree within an accuracy of ±5 meV. The IPE measurements are in agreement with previous conventional vacuum photoemission measurements that were limited to ±100 meV accuracy. Unlike conventional vacuum photoemission, IPE does not have the requirement that interfaces must be grown and measur...

Cathodoluminescence and photoluminescence from chemical‐vapor‐deposited diamond

Cathodoluminescence spectra from microwave‐plasma‐assisted chemical‐vapor‐deposited (CVD) diamond have been studied as a function of temperature at low electron energies. This investigation shows a pronounced difference between the luminescence spectra from CVD diamond films as grown and CVD diamond films annealed by rapid thermal processing at 1000 °C for one minute in an argon atmosphere. The relative intensities of the dominant features at 443 nm (2.8 eV) and 510 nm (2.4 eV) are very different for unannealed and annealed samples. We report measurements of the temperature dependence of these luminescence bands.

Optical absorption spectroscopy of defects in halides

Desorption of alkali atoms from, and defects formed in, alkali halide crystals stimulated by low-energy electron bombardment were investigated simultaneously by optical absorption-, mass spectroscopy-, and depth-profile surface sputtering- techniques. These techniques not only provide important information about the type, amount, and the spatial distribution of the defects formed in the alkali halide crystals, but also indicate which processes govern the emission rate of neutral alkali atoms during and after bombardment. The results show that at temperatures near and lower than room temperature, F-centers, small F-center clusters, and alkali metal clusters are formed during electron bombardment of the crystals

Simultaneous measurements of transmission optical absorption and electron stimulated Li desorption on LiF crystals

Abstract We report the first simultaneously performed transmission optical absorption and neutral lithium desorption yield experiments. Electron-stimulated desorption (ESD) of lithium atoms from lithium fluoride crystals was investigated with quadrupole mass spectroscopy. Correlations have been established between the desorption kinetics of lithium atoms and transmission optical absorption data (200–600 nm) obtained in situ as a function of irradiation time and temperature. The investigated temperature regime covered temperatures as low as room temperature up to 660 K. For temperatures lower than 640 K the data can be consistently explained by the assumption that lithium islands form on the surface of the crystals during bombardment and disintegrate after bombardment. The formation of lithium agglomerates is characterized by the occurrence of a very broad absorption band (maximum at 500 nm or higher). At higher temperatures the shape of the absorption spectrum changes in a way which can no longer be explained by simply assuming lithium islands on the surface.

Luminescence in silicas stimulated by low energy ions

Abstract Silicas manufactured by different methods and 800 A a: SiO2 thin films grown on silicon were used to investigate the near-surface bulk luminescence. The luminescence was monitored in situ during ion bombardment in an ultrahigh vacuum environment. All the ion energies were below 10 keV. Noble gas ions, He+, Ne+, and Ar+, were used to rule out the chemical reaction between incident ion and substrate. We found that the luminescence stimulated by ion impact has the same luminescence bands with different intensities in different silicas. This implied that the luminescence was an intrinsic process and was due to the same mechanism in variety of silicas. The experimental results also show that the luminescence intensity was proportional to the incoming ion energy. This indicated luminescence was generated by energy loss of energetic ions in a: SiO2. Since the luminescence intensity increased with decreasing the mass of the incoming ions at the same impact energy, the dominant energy dissipating channel to create the luminescence was electronic excitation (i.e. electronic energy loss).

Two-color optical technique for characterization of x-ray radiation-enhanced electron transport in SiO2

Damage enhanced electron transport, across thin oxides in x-ray irradiated Si/SiO2 samples, was measured via a contactless two-color laser technique. This method involves two steps: (1) optically stimulated electron injection into the oxide and (2) detection of transport, trapping, and recombination rates using time-dependent electric-field-induced second-harmonic generation arising from charge separation at the interface. Measured electron transport rates across an irradiated oxide are found to be substantially higher in comparison to unirradiated oxides. This effect is attributed to the presence of x-ray irradiation-induced defects that act as intermediate trapping sites facilitating enhanced electron tunneling through the oxide. The possible nature of the radiation-induced trapping sites is discussed.

Wavelength-selective laser ablation of diamond using hydrogen-impurity vibration modes

Abstract We have observed wavelength-dependent laser ablation of diamond films demonstrating the CH bond-stretching mode at 3.5 μm, which is important in the early stages of photodamage. In particular, there is a sharp drop in the ablation threshold near 3.5 μm. The measurements were made with the Vanderbilt Free-Electron Laser (FEL). We also observed the onset of morphological changes by using an atomic force microscope (AFM).

Broadband luminescence from particle bombardment of alkali halides

Abstract When measuring optical radiation due to electron and ion bombardment of alkali halides, we have observed a broadband radiation pattern whose origin is not well understood. Though CN has been suggested as a possible source [1,2], some problems with this interpretation are encountered. Conditions under which the peaks were produced and some parameters which affect their intensities are reported here. We also include a description of our attempts to determine the source of the bands.

Erosion and glow in the near-earth space environment

The phenomena of spaceglow and material erosion, which are observed on spacecraft that orbit the earth at low altitudes, are described. Also described are the physical environment of the low earth orbit and the methods by which laboratory research on these phenomena is conducted.