Laser power density

Surface analysis by non-resonant (NR-) laser-SNMS [3.102-3.106] has been used to improve ionization efficiency while retaining the advantages of probing the neutral component. In NR-laser-SNMS, an intense laser beam is used to ionize, non-selec-tively, all atoms and molecules within the volume intersected by the laser beam (Eig. 3.40b). With sufficient laser power density it is possible to saturate the ionization process. Eor NR-laser-SNMS adequate power densities are typically achieved in a small volume only at the focus of the laser beam. This limits sensitivity and leads to problems with quantification, because of the differences between the effective ionization volumes of different elements. The non-resonant post-ionization technique provides rapid, multi-element, and molecular survey measurements with significantly improved ionization efficiency over SIMS, although it still suffers from isoba-ric interferences. 

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... 

Figure 2.20 Relative sensitivity coefficients as a function of laser power density in UMS.

A serious problem in LA-ICP-MS described in the literature on many occasions is the time-dependent elemental fraction (so-called ablation fractionation ) occurring during laser ablation and the transport process of ablated material, or during atomization and ionization processes in the inductively coupled plasma.20-22 Numerous papers focus on the study of fraction effects in LA-ICP-MS as a function of experimental parameters applied during laser ablation (such as laser energy, laser power density, laser pulse duration, wave length of laser beam, ablation spot size,... 

An important parameter in order to avoid elemental fractionation in laser ablation ICP-MS is the laser power density ( ) which is a function of laser energy, of laser pulse duration and focusing conditions, as described in the following equation ... 

Figure 9.56 Cluster ion formation in laser plasma in dependence of laser power density . (C. Seifert, j. S. Becker and H. J. Dietze, Int.J. Mass Spectrom. Ion Proc., 184, 121(1988). Reproduced by permission of Elsevier)...

Fig. 3. Transient absorption signals recorded at 715 nm after photoionisation of ethylene glycol at 263 nm with three laser power densities. Inset the same signals after normalisation.

In order to study the influence of electron concentration on the observed dynamics, we performed experiments with different laser power densities. As an illustration, the transient absorption signals recorded at 715 nm in ethylene glycol upon photoionisation of the solvent at 263 nm with three different laser power densities are presented in Fig.3. As expected for a two-photon ionization process, the signal intensity increases roughly with the square of the power density. However, the recorded decay kinetics does not depend on the 263 nm laser power density since the normalised transient signals are identical (Cf. Fig.3 inset). That result indicates that the same phenomena occur whatever the power density and consequently that the solvation dynamics are independent of the electron concentration in our experimental conditions i.e. we are still within the independent pair approximation as opposed to our previous work on hydrated electron [8]. 

Samples were characterized by X-ray diffraction, magnetic susceptibility and chemical analysis with some results summarized in Table 1. The electrical resistivity measurements were made down to 80 K using a four-probe method. Raman scattering experiments used the excitation line A = 514.5 nm of an Ar+ laser in a quasi-backscattering geometry. The laser power of 5 mW was focused to a 0.1 mm diameter spot on the (010) surface. The averaged laser power density amounts to 6 105 W/m2 which is much less compared to earlier Raman studies in manganites [12-15],... 

Figure 4. Ionization signal as a function of the laser power density on double-...

In the above photochemical systems we have assumed that only A absorbs a photon for the sake of simplicity. At very high laser power densities one should expect that the new intermediates could also be photochemically transformed to other intermediates or products and that some of the latter processes in the above changes could be due to photochemical changes. In these cases the lifetimes of the different species should be power dependent and their time dependent concentrations will have different from linear dependency on the power of the initial photolytic or the probe lasers. 

Figure 103. Raman spectrum of laser-grown SWCNTs measured with a 514.5 nm laser focused to -1 nm through a 50x objective. Laser power density was maintained below 3 kW/cm2 to avoid heating.

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