J.-J. Shin

Interaction of wide band gap single crystals with 248 nm excimer laser radiation. IV. Positive ion emission from MgO and NaNO3

We report quadrupole mass‐selected, time‐of‐flight measurements of Mg+ from polished, single crystal MgO and Na+ from cleaved, single crystal NaNO3 exposed to 248 nm (5 eV) laser radiation. A large fraction of the ions emitted from these materials have energies well above the energy of the incident photon. As the fluence is raised from low values, the ion intensities show thresholdlike behavior with a high‐order fluence dependence (roughly sixth order). At still higher fluences, the fluence dependence of Mg+ from MgO decreases to roughly second order. We attribute these emissions to weakly bound ions adsorbed atop surface electron traps; when the underlying vacancy is photoionized, the adsorbed ion is electrostatistically ejected at high energy. We argue that several photons are required to ionize a surface electron trap beneath an adsorbed ion, accounting for the high‐order fluence dependence and satisfying conservation of energy. (Several 5 eV photons are required to produce a 10 eV ion.) We show that a...

Effect of tribological wear on ultraviolet laser interactions with single crystal NaNO3 and CaCO3

We report measurements of the neutral and ion emissions accompanying laser irradiation of cleaved and abraded NaNO3 and CaCO3. In both materials, abraded surfaces yield especially intense positive ion emissions during laser irradiation at low fluences (i.e., fluences well below those required for surface damage and/or the formation of a fluorescent plume). Abraded NaNO3 also yields extremely intense neutral emissions (NO, O2) derived from NO3− decomposition. Measurements of neutral molecules released during abrasion show significant anion‐derived emissions from both materials (CO2 from CaCO3; NO from NaNO3). We attribute the effect of abrasion on the laser‐induced emissions to the production of easily photoionized electron traps during abrasion. Such traps are expected to strongly enhance photoinduced ion emission from both materials. Because the NO3− ion undergoes dissociative electron attachment, photoionizable defects also enhance NO emission during laser irradiation. In contrast, the CO32−anion does n...

Electron stimulated neutral and ion emission from single crystal NaNO3

Abstract Quadrupole mass spectrometry of the neutral products accompanying irradiation of single crystal NaNO3 with a pulsed beam of 1–3 keV electrons shows intense NO and O2 emissions and smaller amounts of Na and NO2. Time-of-flight and intensity measurements as a function of pulse width and temperature are consistent with NO emission via a thermally assisted, electronic emission mechanism. The activation energy for the thermally assisted process is about 0.11 eV -similar to the excitation energies for internal vibrational modes of the NO3− anion. The emission of neutral O2 involves fairly complex reactions with dissociation products and is also thermally assisted at temperatures above 380 K. Mass-selected ion emission measurements indicate copious H+ emission, where the temperature dependence of the H+ intensity is similar to that of NO. We propose that H+ emission is limited by the density of appropriate surface defect sites, and that these sites are directly related to NO3− dissociation.

Laser-induced emission of neutral atoms and molecules from electron-irradiated NaNO3

We report measurements of the neutral NO, O2, and Na emissions accompanying pulsed laser irradiation of single crystal NaNO3 at 248 nm as a function of exposure to 2500 eV electrons. Electron irradiation strongly enhances the emission intensities, allowing for observable emissions at much lower laser fluences. We present preliminary results on the dependence of these emissions on electron dose and laser fluence and employ time-of-flight measurements to determine the peak surface temperature during the laser pulse. The surface temperature measurements are consistent with near-surface heating by laser absorption at e-beam induced defects. This increase in surface temperature plays a significant role in enhancing the observed neutral species intensities due to thermally activated steps in the emission process.

The role of defects in laser induced positive ion emission from ionic crystals

Abstract The energy distributions of positive ions produced by exposing single-crystal MgO to pulsed 248 nm excimer laser light at fluences of 200–1100 mJ/cm 2 were determined by combined quadrupole mass spectrometry and time-of-flight techniques. The dominant ionic species is Mg + , although small amounts of Mg 2+ , MgO + , and Mg 2 O + are also observed. In particular, the Mg + and Mg 2+ energies distributions each show two peaks, with the energies of the Mg 2+ peaks at significantly higher energies. Ion trajectory simulations (accounting for Coulomb forces only and assuming no surface relaxation) suggest that Mg 2+ and Mg + adsorbed at sites directly atop surface F-centers (oxygen vacancies with two trapped electrons) would be ejected upon photo-ionization of the F-center. The experimentally observed kinetic energies and angular distributions agree well with the energies and angles predicted by these simulations.

Mechanisms of laser desorption of positive ions and plume formation on ionic crystals

Abstract In the use of high-intensity lasers in chemical analysis and materials processing, the formation of an ablation plume is of high importance. For wide bandgap insulators (e.g., oxides, halides, nitrides, carbides) irradiated with sub-bandgap photon energies, the route to plume formation is not well understood. For example, contrary to metals and semiconductors, inverse bremsstrahlung (IB) is not possible for a wide range of laser intensities on these materials due to insufficient photon and electron densities. We present an alternative path to plume formation on nominally transparent materials. We first examine the interaction of photo- and thermally-emitted particles from exposure to pulsed laser irradiation of surfaces that include photoelectrons, energetic positive ions, and neutral metal atoms and present a model for the energetic ion emission. We establish experimentally that there is overlap in space and time of significant portions of the distributions of these particles in the near surface region. We present a model for the collected motion of these particles, and show that as laser fluence is increased, we achieve sufficient densities, overlap and kinetic energies to result in the onset of plume luminescence and eventually ionization at fluences far below any IB or catastrophic breakdown process.

Positive Ion Emission Accompanying UV Irradiation of Single Crystal Mgo and NanO 3

Although MgO is much more resistant to radiolysis by 248-nm photons than NaNO 3 , the ion emission processes at low fluence have much in common: both materials yield high energy ions (> 5 eV kinetic energy) with a strongly nonlinear fluence dependence. We report time-of-flight measurements of quadrupole mass-selected Mg + from polished, single crystal MgO and Na + from cleaved, single crystal NaNO 3 . At fluences between 10 and 1000 mJ/cm 2 , the Mg + intensities show a strongly nonlinear fluence dependence which decreases to roughly second order behavior at fluences above 100 mJ/cm 2 . The Na + intensities display third or fourth order emission kinetics throughout the experimental range of fluences. We attribute these emissions to cations adsorbed atop surface electron traps where the cation is ejected when the underlying trap is photo-ionized. The potential energy of this defect configuration accounts for the observed ion kinetic energies. However, the direct photo-ionization of surface vacancy/adsorbed ion defects with 5 eV photons should not be possible. Thus we propose that emission requires the photo-ionization of nearby electron traps, followed by photo-induced charge transfer to the empty traps. We show that a sequence of single-photon absorption events [involving photo-ionization, charge transfer, and electron retrapping] accounts for the strongly nonlinear fluence dependence.

Desorption of positive ions and plume formation from laser irradiation of ionic crystals

In applications of high intensity lasers to materials processing, the formation of an ablation plume is of high importance. For wide bandgap insulators (e.g., oxides, halides, nitrides, carbides) irradiated with sub bandgap photon energies, the route to plume formation is not well understood. For example, contrary to metals and semiconductors, inverse bremsstrahlung (IB) is not possible for a wide range of laser intensities on these materials due to insufficient photon and electron densities. We present an alternative path to plume formation on nominally transparent materials. In this paper, we first review the interaction of photo- and thermally-emitted particles from exposure to pulsed laser irradiation of surfaces which include photoelectrons, energetic positive ions, and neutral metal atoms. We establish experimentally that there is overlap in space and time of significant portions of the distributions of these particles in the near surface region. We then present a model for the collected motion of these particles and show that as laser fluence is increased we achieve sufficient densities, overlap, and kinetic energies to result in the onset of plume fluorescence and eventually ionization at fluences far below any IB or catastrophic breakdown process.

Role of defects in the laser ablation of wide bandgap materials

In model wide bandgap materials such as single crystal alkali halides and MgO (nominally transparent), the absorption of laser radiation at 248 nm (5 eV photons) at modest fluences is defect dominated. We describe a technique for imaging the initial defect densities by their luminescence at low laser fluences and show a typical photoluminescence image of cleaved MgO. High defect densities are observed along many cleavage steps, consistent with previous observations of strong point-to-point variations in the ablative response of cleaved MgO surfaces. At fluences below those required for sustained emission, the composition of neutral emissions from the surface can also be strongly influenced by impurity defects, as shown by the intense emission of carbon oxides and the correspondingly weak emissions of atomic and molecular oxygen from arc-fused MgO. We also present evidence for defect-mediated ion emission at these low fluences.

Mechanisms of excimer laser desorption and ablation from wide band-gap materials: the role of defects

In single crystal MgO, which is a wide bandgap, nominally transparent material, exposure to single pulses of 248 nm excimer laser radiation results in substantial interaction including extensive biaxial deformation and cleavage. We present evidence that achieving intense emission of atomic, molecular, and ionic particles depends on point defect production by laser-induced deformation and fracture. Defect production via dislocation motion yields orders of magnitude increases in laser vaporization, including neutral cluster formation. The field of moving atoms associated with a variety of dislocation structures (e.g., jogs) in turn produce point defects. We have developed new methods using the excimer laser itself to image the photoluminescence generated by these defects. In MgO, this results in a distinct 400 nm emission band which is due to clusters of oxygen vacancies occupied by electrons (F-centers).<<ETX>>