Ole Ellegaard

Laser ablation deposition measurements from silver and nickel

The deposition rate for laser ablated metals has been studied in a standard geometry for fluences up to 20J/cm2. The rate for silver and nickel is a few percent of a monolayer per pulse at the laser wavelengths 532 nm and 355 nm. The rate for nickel is significantly higher than that for silver at 532 nm, whereas the rate for the two metals is similar at 355 nm. This behaviour disagrees with calculations based on the thermal properties at low intensities as well as predictions based on formation of an absorbing plasma at high intensities. The deposition rate falls strongly with increasing pressure of the ambient gases, nitrogen and argon.

THE YB/AL(110) INTERFACE STUDIED BY ELECTRON-SPECTROSCOPY

Abstract Thin films of Yb overlayers on an Al(110) surface have been studied with different methods of electron spectroscopy. The applicability of using Auger electron spectroscopy (AES), reflection electron energy-loss spectroscopy (ELS) and X-ray photoelectron spectroscopy (XPS) for recognizing mixed-valent Yb in the Yb/Al interface is discussed. Comparison between Yb + O 2 and Yb/Al shows that core hole interactions play a strong role in the ionization process which involves an ionization of the 4d shell in Yb. Diffusion of Yb into Al(110) can be described as a two step process.

Ablation of volatile films by laser heating of substrates

Thermal ablation of frozen layers of solid nitrogen were studied experimentally by irradiating condensed layers on a quartz crystal microbalance with a nitrogen laser at the standard wavelength 337.1 nm. The nitrogen layer is transparent to the incoming laser light and the ablation (vaporization) is caused by heat transfer from the underlying silver substrate layer. Experimental data are compared to calculated data from a three-layer finite-difference heat conduction model. The model takes into account the strong dependence of material parameters on the temperature. Calculated data are presented for water ice as well. The consequences of thermal ablation of volatile films by laser heating of substrates in existing experiments and methods are discussed.

Ablation from metals induced by visible and UV laser irradiation

The deposition rate of laser-ablated silver has been determined for fluences between 0.5 and 15 J / cm 2 at the wavelengths 532 and 355 nm for a beam spot area of around 0.01 cm 2 . The ablated metal was collected on a quartz crystal microbalance. The rate at 5 J / cm 2 was about 4 x 10 13 Ag / cm 2 per pulse for 532 nm, and somewhat lower for 355 nm. The initial vaporization during the ablation has been studied numerically as well.

Sputtering of the most volatile solids: the solid hydrogens

Abstract Electronic sputtering of the three stable hydrogenic solids, H 2 , HD and D 2 by keV hydrogen and deuterium ions has been studied at the low-temperature setup at Riso. The yield of the sputtered particles has been determined for hydrogenic films of thicknesses ranging from 0.1 × 10 18 up to 10 × 10 18 molecules/cm 2 and for 4–10 keV H + , H 2 + , H 3 + and D 3 + ions. The yield increases with decreasing film thickness for solid hydrogen as well as for deuterium. This behavior agrees with the trend observed for other volatile gases. For thick films the yield decreases to a constant value. This thick-film yield can be approximated fairly well by a quadratic function of the stopping power except for a somewhat steeper dependence for proton-bombarded deuterium. The yield increases strongly with decreasing sublimation energy from one isotope to another. No existing theory can account for the dependence of the yield on the electronic stopping power as well as the sublimation energy.

Sputtering by excitonic and elastic processes from solid neon by He ion bombardment

Abstract Sputtering of solid neon by keV He+ ions and 0.2 keV electrons has been studied. We have measured the energy distributions of neon atoms emitted from neon during bombardment by these particles. The ion-induced distributions show features from knockon (elastic) as well as electronic (inelastic) sputtering processes. The electronic components have been identified on the basis of the distribution from electron impact.

Sputtering of thin and intermediately thick films of solid deuterium by keV electrons

Abstract Sputtering of films of solid deuterium by keV electrons was studied in a cryogenic set-up. The sputtering yield shows a minimum yield of about 4 D 2 /electron for 1.5 and 2 keV electrons at a thickness slightly larger than the average projected range of the electrons. We suggest that the yield around the minimum represents the value closest to a bulk-yield induced by electron bombardment. It may also include contributions from the mechanisms that enhance the yield for thin and very thick films.

UV laser irradiation of thin films of silver and solid nitrogen

Films of silver or solid nitrogen have been irradiated with a nitrogen laser at the standard wavelength 337.1 nm. In an existing vacuum setup the bare silver electrode of a quartz crystal microbalance served as a target for the pulsed laser. The crystal was kept below a cryostat at liquid‐helium temperature, so that films of solid nitrogen could be irradiated as well. The absolute ablation yield of silver was determined from the mass loss to be approximately 10−3 Ag‐atoms per photon for intensities around 80 MW/cm2. The corresponding laser‐induced desorption of the nitrogen films led to a yield of the order 10−2 N2 molecules per photon at intensities from 1 to 10 MW/cm2.

ANGULAR DISTRIBUTIONS AND TOTAL YIELD OF LASER ABLATED SILVER

Abstract The angular distribution of laser ablated silver has been measured in situ with a newly constructed setup with an array of microbalances. The distribution is strongly peaked in the forward direction corresponding to cospθ, where p varies between 5 and 9 for laser fluences from 2 to 7 J/cm2 at 355 nm for a beam spot of 0.015 cm2. The total deposited yield is of the order 1015 Ag-atoms per pulse.

Sputtering of solid neon and argon by medium mass ions

Abstract Frozen neon and argon have been bombarded by neon and argon ions at energies between 4 and 10 keV. The sputtering yields range from 2 × 104 atoms/ion for neon bombardment of solid neon to about 800 atoms/ion for that of solid argon. No existing treatment can account for the magnitude of the yield as well as the energy dependence of the yield.