Laser evaporation source

The Pd clusters have been produced by a recently developed high-frequency laser evaporation source, ionized, then guided by ion optics through differentially pumped vacuum chambers and size-selected by a quadrupole mass spectrometer [16-18]. The monodispersed clusters have been deposited with low kinetic energy (0.1-2 eV) onto a MgO thin film surface. The clusters-assembled materials obtained in this way exhibit peculiar activity and selectivity in the polymerization of acetylene to form benzene and aliphatic hydrocarbons [30]. 

The reactivity of the clusters can then be studied by various experimental techniques, including fast flow reactor kinetics in the postvaporization expansion region of a laser evaporation source [21, 22], ion flow tube reactor kinetics of ionic clusters [23, 24], ion cyclotron resonance [25, 26], guided-ion-beam [27], and ion-trap experiments [28-30]. Which of these techniques is applied depends on the charge state of reactants (neutral, cationic, anionic), on whether the clusters are size-selected before the reaction zone, on single or multiple collisions of the clusters with the reactants, on the pressure of a buffer gas if present, and on the temperature and collision energy of the reactant molecules. 

With the surface ionization source it is generally assumed that the reactant ion internal state distribution is characterized by the source temperature and that the majority of the reactant ions are in their ground electronic state. This contrasts with the uncertainty in reactant state distributions when transition metal ions are generated by electron impact fragmentation of volatile organometallic precursors (10) or by laser evaporation and ionization of solid metal targets (11). Many examples... 

The apparatus and techniques of ion cyclotron resonance spectroscopy have been described in detail elsewhere. Ions are formed, either by electron impact from a volatile precursor, or by laser evaporation and ionization of a solid metal target (14), and allowed to interact with neutral reactants. Freiser and co-workers have refined this experimental methodology with the use of elegant collision induced dissociation experiments for reactant preparation and the selective introduction of neutral reactants using pulsed gas valves (15). Irradiation of the ions with either lasers or conventional light sources during selected portions of the trapped ion cycle makes it possible to study ion photochemical processes... 

The ion source is an essential component of all mass spectrometers where the ionization of a gaseous, liquid or solid sample takes place. In inorganic mass spectrometry, several ion sources, based on different evaporation and ionization processes, such as spark ion source, glow discharge ion source, laser ion source (non-resonant and resonant), secondary ion source, sputtered neutral ion source and inductively coupled plasma ion source, have been employed for a multitude of quite different application fields (see Chapter 9). 

It is characteristic of such a laser ion source that the experimental conditions for LIMS can be optimized with respect to a stoichiometric evaporation and effective ionization of solid sample material by varying the laser power density as demonstrated in Figure 2.20. Under certain experimental conditions fractionation effects can be avoided. Stoichiometric laser evaporation and ionization of analyzed material is found at a laser power density between 109Wcm 2 and 1010Wcm-2. In this laser power density range, the relative sensitivity coefficients of the chemical elements (RSC = measured element concentration/true element concentration) are nearly one for all the... 

Bigger clusters have been formed, for instance, by the expansion of laser evaporated material in a gas still under vacuum. For metal-carbon cluster systems (including M C + of Ti, Zr and V), their formation and the origin of delayed atomic ions were studied in a laser vaporization source coupled to a time-of-flight mass spectrometer. The mass spectrum of metal-carbon cluster ions (TiC2 and Zr C j+ cluster ions) obtained by using a titanium-zirconium (50 50) mixed alloy rod produced in a laser vaporization source (Nd YAG, X = 532 nm) and subsequently ionized by a XeCl excimer laser (308 nm) is shown in Figure 9.61. For cluster formation, methane ( 15% seeded in helium) is pulsed over the rod and the produced clusters are supersonically expanded in the vacuum. The mass spectrum shows the production of many zirconium-carbon clusters. Under these conditions only the titanium monomer, titanium dioxide and titanium dicarbide ions are formed. 

Fig. 1.1. Details of the high-frequency iaser evaporation source. Shown are the rotary motor, which drives the planetary gear assembly for turning the target, and the thermalization chamber with exchangeable expansion nozzie. The iaser-produced plasma expands into this thermalization chamber. A heiium gas puise is then introduced by a piezo-driven pulsed valve and synchronized with the iaser puise into the same volume. The metal-gas mixture then expands through the nozzie into the vacuum leading to cluster formation. In contrast to conventional sources, the laser beam is coaxial to the molecular beam axis. The bellow is used to aiign the source along the optical axis of the ion optics...

FIG. 5. Schematic overview of the set-up with the laser vaporization source and the nozzle for supersonic expansion used in the production of clusters at Goteborg. The figure shows the laser vaporization source with the target material, the laser beam for evaporation, the small volume where a plasma of atoms and ions exist and the region where the clusters are formed in the expansion through the nozzle. 

Biomimetic nanocrystalline apatite coatings were deposited on titanium substrates by matrix-assisted pulsed laser evaporation (MAPLE), a technique with potential application in tissue engineering (Visan et al., 2014 Caricato etal., 2014). The targets were prepared from nano-sized, poorly crystalline apatite powders, analogous in composition to mineral bone. For the deposition of thin films, a KrF excimer laser source was used (A = 248 nm,rFWHM < 25 ns). Analyses of the deposited films showed that the structural and chemical nature of the nanocrystalline precursor apatite was preserved. Hence, MAPLE may be a suitable technique for the congruent transfer of a delicate material such as nanohydroxyapatite. 

The source can be heated by a variety of techniques. Resistance heating can be used, employing W, Mo, or Ta heater wire or tape. These have the advantage that their low vapour pressures do not cause contamination of the deposit. The source can be contained in a crucible made of boron nitride or titanium diboride. Induction heating can also be used to heat a susceptor source contained in a nonsusceptor crucible. Electric-arc and laser-beam sources can also be used to heat the evaporant. 

A good example of the power of synchrotron source radiation is found in the study of the thin-film superconductors that are required for applications of high-temperature superconductors (HTSC) in microelectronics technology [77]. An example of such work [78] involves the analysis of HTSC films produced by laser evaporation of elements in the Y-Ba-Cu-0 system. Ten films were simultaneously deposited with various target-to-substrate distances allowing study to be made of the laser plasma expansion in vacuo. A very important advantage of this method is that it allows extremely low detection limits to be achieved. As an example, in the referenced study [78] the authors claim a detection limit of 5 x lO atoms/cm. ... 

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