Stoichiometric laser

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... 

Pressure effects on the energy transfer between f elements of the same kind were studied by Merkle et al. (1981) for the case of Nd3+-Nd3+ pairs in Ndx Y xP50i4 (x = 1,0.1). This material was studied in detail because of its potential use as a stoichiometric laser material. An outstanding property is a very weak concentration quenching of the luminescence. The total luminescence decay rate of the 4F3/2 multiplet in Ndx Y xP50i4 (x = 1,0.1) underpressure is shown in fig. 17. Obviously the stoichiometric compound shows a much larger increase of the decay rate than the doped compound. 

In analogy to the observed behaviour in LIMS, in LA-ICP-MS increasing fractionation effects are observed with decreasing laser power density for <10 Wcm . These fractionation effects increase significantly if the laser power density is lower than lO Wcm. A stoichiometric laser ablation of sample material is observed at a laser power density between 10 Wcm and 10 °Wcm in the author s laboratory. 

This is the second most extensively exploited lanthanide laser ion. Stimulated emission is observed for pulsed and CW operation, for nine different transitions ranging from 0.55 to 2.9 fim, and in many crystalline hosts including the first stoichiometric laser material (HoFs-Devor et al., 1971) and thin films (van der Ziel et al., 1973). The most common laser transition is however low... 

Rare earth crystals used for stoichiometric lasers. 

Danielmeyer, H.G., 1975, Stoichiometric laser materials, in Queisser, H.J., ed., Festkor-perprobleme (Advances in Solid State Physics), vol. XV (Pergamon/Vieweg, Braunschweig), pp. 253-277. 

Vector profiles of (a) cold vortex ring and (b) burning vortex ring (Dq = 60mm, P = 0.6MPa, stoichiometric mixture). Insets show the relative position of the PIV laser sheet to (a) the vortex ring and (b) the flame. 

The product was identihed by a number of spectroscopic methods. Dioxygen uptake was measured by spectrophotometric titration. MALDI-TOF-MS (matrix-assisted laser desorption/ionization-time of flight-mass spectrometry), an MS method particularly suited to determining molecular masses of biopolymers and synthetic materials with relative masses up to several hundred kilodaltons, determined that the product contained stoichiometric amounts of the heme starting material, the copper complex, and dioxygen in a 1 1 1 ratio. 

One technique which has produced thin, but not epitaxial films of BaPbj.xBixOg, and which shows good promise is the use of laser evaporation methods (40). Since the compound is efficiently transported stoichiometrically from the target to the substrate at a high rate and does not require a vacuum, this method may be superior to sputtering techniques. 

The solution behavior of poly(amic acids) was until recently, probably the least understood aspect of the soluble polyimide precursor. However, the advent of sophisticated laser light scattering and size exclusion chromatography instrumentation has allowed elucidation of the solution behavior of poly(amic adds). In the early days of polyimide chemistry, when most molecular weight characterization was based on viscosity determinations, a decrease in viscosity was associated with molecular weight degradation [15, 28, 29]. Upon combination of the two monomers an increase in the viscosity to the stoichiometric equivalence point is observed, followed by a decrease in the solution viscosity as a... 

The hydroxyl concentration profile for a stoichiometric CH -air flame is presented in Figure 8. Here the maximum mole fraction observed and the predicted mole fraction are equal to better than 10% accuracy. The abscissas of the theoretical and the experimental results were matched by setting the theoretically predicted temperature equal to the measured hydroxyl rotational temperature. At all positions in the flame the hydroxyl 2j[(v,=o) state exhibited a Boltzmann distribution of rotational states. This rotational temperature is equal to the N2 vibrational temperature to within the +100 K precision of the laser induced fluorescence and laser Raman scattering experiments. An example of this comparison is given in Figure 9. 

A resistor completed the circuit and signals were measured across the resistor. The laser was a flashlamp pumped tunable dye laser with a pulse duration of -1 ps and a peak power of several kW the bandwidth was 0.014 nm in the neighborhood of 589 nm. We used a stoichiometric H2-02 Ar flame of 1800 K, shielded with a mantle flame of identical composition. In the inner flame a 2500 pg/ml NaCl solution was nebulized. An extensive description of the experiment can be found elsewhere (7). 

The utilily of PLD for Ihin film synlhesis is due in large part to Ihe unique characteristics of Ihe laser ablation process. As indicated above, laser ablation is a nonequilibrium process that enables stoichiometric evaporation of elements from a target source. In addition, it is also possible to control the energy of evaporated species in PLD, and thus control film growlh on Ihe subslrale surface. The underlying basis for these features of laser ablation and their utihty in thin film synthesis are described below. 

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