W. Husinsky

Velocity distributions and sputtering yields of chromium atoms under argon, oxygen and carbon ion bombardment

The velocity distribution of sputtered ground-state Cr atoms under 15 keV Ar, O and C ion bombardment has been measured using Doppler-Shift Laser Fluorescence Spectroscopy (DSLFS). Auger analysis of the target surface showed that a clean carbon-free surface could only be obtained under simultaneous Ar ion bombardment and annealing of the Cr target at high temperatures (500 ° C). In the case of Ar ion bombardment we have observed a shift of the velocity distribution to higher values with increasing oxygen partial pressure and, furthermore, a strong density decrease of sputtered Cr ground-state atoms by a factor greater than 50 compared with the results for clean Cr targets. No measurable shift and a density decrease by a factor of only about 2 has been observed for Cr ground-state atoms sputtered by Ar ions and an increased hydrogen background pressure. Contrary to the exposure of Cr targets to an oxygen background atmosphere, under oxygen ion bombardment of up to a few μA/cm2 the density and velocity distributions of sputtered Cr ground-state atoms were comparable with those produced by Ar ion bombardment of Cr targets without oxygen background. A strong density decrease was again observed for simultaneous oxygen exposure and oxygen ion bombardment. The results obtained for carbon ion bombardment in general agree with those for oxygen ion bombardment. n nThe sputtering yield of Cr has been measured for clean and oxygen-covered targets by means of a quartz microbalance (thickness monitor). For clean Cr targets a sputtering yield of 4.2 atoms/ion has been obtained. For increasing oxygen background pressure the sputtering yield decreases. For oxygen pressures of 1 × 10−7 and 1 × 10−6 mbar sputtering yields of 3.8 and about 1, respectively, are obtained.

Time-of-flight investigation of the intensity dependence of laser-desorbed positive ions from SrF2

The intensity dependence of the total and specific yields of positive ions desorbed from SrF2 under 193 nm and 308 nm excimer-laser irradiation has been investigated by the time-of-flight method. The following positive ion species have been detected: F+, Sr+, Sr++, SrF++ and SrF2+. The Sr+ and SrF+ emission yields are found to increase as En, where E represents the laser energy per pulse. The exponent n is related to defect-initiated neutral particle emission and gas-phase ionization. The influence of surface damage on this power dependence is investigated. The F+ emission yield showed a quite different behaviour compared to that of the Sr+ and SrF+ emission. At both wavelengths the total positive ion emission yields saturate at a certain laser energy. In the saturation regime the SrF+ emission vanishes and alternative emission of F+ and Sr+ was observed at both wavelengths, but the total emission yield in the saturation regime (F+ + Sr+) remained constant. A Scanning Electron Microscope (SEM) was used to investigate the damage spots after laser irradiation for thermal effects.

Electronic transitions in surface and near-surface radiation effects☆

Abstract Studies of surface effects arising from electron, heavy particle or photon irradiation of dielectric surfaces are increasingly focusing on the electronic interactions by which energy is absorbed, localized, and transformed or transferred prior to the ultimate dissipation of the incident energy — through, for example, ejection of atoms or molecules from the dielectric. Recent experiments in our laboratories illustrate the varied roles played by electronic transitions in determining the flow of electronic energy during the bombardment of dielectric surfaces by photons, electrons and heavy particles. Specific examples include: the effects of surface overlayers and adsorbed hydrogen in retarding substrate desorption; substrate-temperature- and energy-resolved studies of photon-stimulated desorption from alkali halides; and electronic level-hybridization effects in the sputtering of metal oxides by argon ions. These simple model systems are a critical testing ground for studying the mechanisms of surface radiation damage in more complex materials because of the wealth of information available about their electronic and geometric structure, and because the character and modes of formation of their permanent electronic defects are well understood.

Laser-Induced Desorption of Positive Ions from Wide Band Gap Insulators

The intensity dependence of the yield and energies of positive ions desorbed from SrF2, CaF2 and MgO under 193 nm and 308 nm excimer laser irradiation has been investigated by the Time-of-Flight method. Absorption of visible or UV photons in large band gap materials can take place by multiphoton excitation [1] of valence band electrons, defects or particle inclusions [2]. The creation of an electron-hole plasma in the interaction volume of the laser with the target [3,4] is the most typical mechanism of non-thermal laser annealing, leading to ablation. Holes may contribute to positive ion desorption by the formation of a positive ion in an excited and antibonding state on the surface with subsequent desorption [5] or two holes can be localized on the surface and lead to desorption [6]. At a material and wavelength dependent laser intensity, the flux of desorbing particles can become so high that a plasma will be formed at the surface. It is known from other experiments that ions emitted under laser irradiation can gain high kinetic energies [11,12], while measurements on CaF2 showed that emitted neutral species have nearly pure thermal energy distributions [13]. The origin of these energies are plasmoid extension effects first introduced by Bykovskii [14] and experimentally found by Akhsakhalyan [12]. In the present work we have addressed these questions analyzing the ion emission for two different laser wavelengths by measuring their thresholds, ion yield dependence with laser intensity and the energies of the emitted ions.

Infrared free-electron laser photoablation of diamond films

We report first IR free-electron laser experiments to compare and elucidate the effects of surface-localized vibrational excitation versus bulk vibrational excitation on the ablation of polycrystalline diamond. The measured ablation yield values as a function of laser intensity indicate the existence of two separate thresholds. The lower intensity thresholds is identified as the ablation threshold, and the higher intensity threshold is associated with the formation of a plasma plume. The wavelength dependences of both thresholds indicate that eh C-H absorption occurring at surfaces and grain boundaries does not play a significant role in the ablation process. However, both thresholds are lower when the laser is resonant with the two-phonon bulk absorption band. These findings are consistent with the model that a rapid laser- induced phase transition to graphite is responsible for the low-intensity ablation of diamond at and above the first threshold.

Simultaneous Measurements of Optical Absorption and Electron Stimulated Li Desorption on LiF Crystals

It has been known for some time that ionizing radiation incident on alkali halide crystals lead to the formation of F- and H-centers [1]. When an F-center reaches the surface of the crystal, it results in the neutralization of an alkali ion. If the temperature of the crystal is high enough the neutral alkali atoms desorb thermally from the surface [2]. It has been observed in many experiments that large doses of energetic neutrons, ions, or X-rays lead to the formation of colloids (alkali agglomerations) in the bulk of the crystals [3,4]. However, agglomeration processes at or close to the surface of the crystals have only been investigated recently [5,6]. Electron energy loss spectroscopy, and Auger electron spectroscopy investigations have supplied evidence for the formation of alkali islands on the surface of alkali halides during electron bombardment [7]. In previous publications [5,6] the desorption kinetics of lithium atom delayed emission (i.e. emission following the cessation of electron bombardment) has been investigated. The results show the occurrence of a prompt and a delayed decay. The delayed decay takes seconds, whereas the prompt decay is faster than a few msec. Under certain circumstances the Li desorption rate even increases after the cessation of electron bombardment (“delayed maximum”). In all these publications conclusions have been drawn about the processes occuring during and after electron bombardment by monitoring the ground state yield of lithium desorbing from the surface. Clearly, this method alone is not capable of differentiating in which manner F-center clusters, lithium islands on the surface, or lithium clusters individually contribute to the observed desorption phenomena. Simultaneous correlated transmission optical absorption spectroscopy (which provides information about defect densities in the crystal) combined with measurements of the ESD of alkali atoms from alkali halides provides a new approach to this problem.

Electron-Stimulated Desorption of Neutral Ground-State Lithium Atoms from LiF and the Influence of Stored Defects

We have investigated the ESD of neutral ground-state Li atoms from LiF single crystal surfaces caused by 100eV electron irradiation at varied crystal temperatures between 300K and 800K. During irradiation desorption increases exponentially with temperature, limited only by the evaporation of the alkali metal layer on the surface. The obtained activation energy varies between 0.8eV and 1.6eV with current density and reflects the size of the metal clusters on the surface. Above 300°C a plateau is reached where the electron flux becomes the rate-limiting quantity. Above 450°C the evaporation of the crystal adds significantly to the desorption yield.

DIET of Neutral Excited State Hydrogen from Alkali Halide Surfaces

The desorption yields of excited hydrogen atoms from the surfaces of KC1, KBr, NaCl, NaF and LiF have been measured as a function of incident photon and electron energy, time of irradiation, and sample temperature. The measurements indicate that the adsorption of hydrogen which leads to the stimulated emission of neutral, excited-state hydrogen is contingent upon surface damage induced by the incident radiation. Specifically, H2 adsorbs dissociatively at surface sites containing a non-stoichiometric excess of alkali metal. This hydrogen may come from the gas phase or from a source which has accumulated in the bulk of these compounds during the course of the experiment. Incident electrons or photons can then induce a valence excitation to a neutral, antibonding state of the surface alkali hydride molecule leading to the desorption of atomic hydrogen.

Bombardment of Alkali and Alkali-Earth Halides by Ions and Electrons

For alkali and alkali earth halides ion as well as electron bombardment are efficient processes leading to continuous surface erosion [1,2]. While sputtering of metals under ion bombardment is well understood in terms of a collision cascade emission mechanism [3], for electron bombardment no such process can account for any particle emission due to the negligible momentum transfer. Electronic processes have been proposed to explain electron or photon induced desorption [4,5]. The key steps are the generation of an exciton, which is trapped at a halogen site. Then the lattice ion which is raised to an excited state, thermally relaxes to form a molecular-like state with a neighbouring halogen ion. In this new system the excited electron can make a non-radiative transition to the new ground state, and the energy is then released to the ions as kinetic energy directed along the crystal direction. Thus a replacement sequence along the line of halogen ions takes place, and if the sequence reaches the surface, emission of a halogen atom is caused. Therefore the expelled halogen atoms feature directional emission and hyperthermal energies [6]. The replacement sequence and the diffusion of defects to the surface lead to an alkali or alkali-earth enrichment on the surface. These excess metal atoms prevent further halogen emission or, if the vapour pressure is high enough may evaporate in random directions [6].

Ground-State and Excited Atom Production by Electron and Ion Bombardment of NaCl and CaF2

For alkali and alkali earth haliaes, ion, electron and photon bombardment are efficient processes leading to continuous surface erosion [1,2]. While sputtering of metals under ion bombardment is well understood in terms of a collision cascade emission mechanism based on momentum transfer due to elastic collisions [3], for electron or photon bombardment no such processes can account for any particle emission due to the negligible momentum transfer. Electronic processes have been proposed to explain such electron or photon induced desorption processes [4,5]. Selective emission of the halogen atoms and molecules is ascribed to the H-centre migration model leading to the formation of a metal overlayer on the surface. If the vapour pressure of the alkali/alkali earth metal layer at the target temperature is high enough these excess metal atoms will desorb thermally, thus leading to continuous particle emission. Otherwise the excess metal layer will eventually prevent any further halogen emission and the emission process will be only of transient nature [6]. We have compared the particle emission processes for ion and electron bombardment of NaCl and CaF2 to differentiate between a collision cascade induced contribution and one due to Desorption Induced by Electronic Transitions (DIET) under ion bombardment and, in addition, to compare DIET processes under ion and electron bombardment to obtain a better understanding of the desorption mechanisms involved.