Laser irradiance

Surface photochemistry can drive a surface chemical reaction in the presence of laser irradiation that would not otherwise occur. The types of excitations that initiate surface photochemistry can be roughly divided into those that occur due to direct excitations of the adsorbates and those that are mediated by the substrate. In a direct excitation, the adsorbed molecules are excited by the laser light, and will directly convert into products, much as they would in the gas phase. In substrate-mediated processes, however, the laser light acts to excite electrons from the substrate, which are often referred to as hot electrons . These hot electrons then interact with the adsorbates to initiate a chemical reaction. 

A number of diamagnetic defects are also beheved to exist in vitreous siUca. Because there is no direct way to study these species, their identification is either done indirecdy, such as by uv absorption, or by employing esr after the material has been made paramagnetic using ionizing or laser irradiation. 

The molecular extinction coefficients (at various wavelengths) of the four main components of the irradiation are shown in Table 5. The absorption of light above 300 nm is favored by tachysterol. A yield of 83% of the previtamin at 95% conversion of 7-dehydrocholesterol can be obtained by irradiation first at 254 nm, followed by reirradiation at 350 nm with a yttrium aluminum garnet (YAG) laser to convert tachysterol to previtamin D. A similar approach with laser irradiation at 248 nm (KrF) and 337 nm (N2) has also been described (76). 

The possible mechanism of ionization, fragmentation of studied compound as well as their desoi ption by laser radiation is discussed. It is shown that the formation of analyte ions is a result of a multi stage complex process included surface activation by laser irradiation, the adsoi ption of neutral analyte and proton donor molecules, the chemical reaction on the surface with proton or electron transfer, production of charged complexes bonded with the surface and finally laser desoi ption of such preformed molecules. 

Hydrogen to uranium all isotopes Yes, on a scale of few micrometers depth Yes, depending on the laser irradiance... 

The LIMS technique is rarely used for quantitative elemental analysis, since other techniques such as EPMA, AES or SIMS are usually more accurate. The limitations of LIMS in this respect can be ascribed to the lack of a generally valid model to describe ion production from solids under very brief laser irradiation. Dynamic range limitations in the LIMS detection systems are also present, and will be discussed below. 

The material evaporated by the laser pulse is representative of the composition of the solid, however the ion signals that are actually measured by the mass spectrometer must be interpreted in the light of different ionization efficiencies. A comprehensive model for ion formation from solids under typical LIMS conditions does not exist, but we are able to estimate that under high laser irradiance conditions (>10 W/cm ) the detection limits vary from approximately 1 ppm atomic for easily ionized elements (such as the alkalis, in positive-ion spectroscopy, or the halogens, in negative-ion spectroscopy) to 100—200 ppm atomic for elements with poor ion yields (for example, Zn or As). 

Other topics recently studied by XPS include the effects of thermal treatment on the morphology and adhesion of the interface between Au and the polymer trimethylcy-clohexane-polycarbonate [2.72] the composition of the surfaces and interfaces of plasma-modified Cu-PTFE and Au-PTFE, and the surface structure and the improvement of adhesion [2.73] the influence of excimer laser irradiation of the polymer on the adhesion of metallic overlayers [2.74] and the behavior of the Co-rich binder phase of WC-Co hard metal and diamond deposition on it [2.75]. 

Raman spectroscopy of matrix-isolated molecules carries some difficulties conneeted with the possibility of local heating of the matrix under laser irradiation. Besides, because of the relatively low intensity of Raman bands, higher concentrations of the species to be studied are needed in the matrix (the ratio of matrix gas to reagent = 100-500). As a result, the effective isolation of reactive intermediates is prevented. 

The depths of the ablated cylinder parts indicate that the TCPP-doped P4VP-cylinder parts are selectively ablated away to a larger extent by 100-shot laser irradiation. However, the laser irradiation of 100 shots results in a morphological change of the PS-matrix parts. That is, the PS parts were also ablated in part. 

Peng et al. [150] prepared AgAu nanoalloys via three different procedures by using laser-induced heating (i) mixture of Au nanoparticles and Ag(I) ions irradiated by a 532 nm laser, (ii) mixture of Au and Ag nanoparticles irradiated by a 532 nm laser, and (iii) mixture of Au and Ag nanoparticles irradiated by a 355 nm laser. In procedures (ii), nanoalloys with a sintered structure were obtained. The morphology of the obtained nanoalloys depended not only on the laser wavelength but also on the concentration of nanoparticles in the initial mixture. Large-scale interlinked networks were observed upon laser irradiation when the total concentration of Ag and Au nanoparticles in the mixture increased. 

In Raman measurements [57], the 514-nm line of an Ar+ laser, the 325-nm line of a He-Cd laser, and the 244-nm line of an intracavity frequency-doubled Ar+ laser were employed. The incident laser beam was directed onto the sample surface under the back-scattering geometry, and the samples were kept at room temperature. In the 514-nm excitation, the scattered light was collected and dispersed in a SPEX 1403 double monochromator and detected with a photomultiplier. The laser output power was 300 mW. In the 325- and 244-nm excitations, the scattered light was collected with fused silica optics and was analyzed with a UV-enhanced CCD camera, using a Renishaw micro-Raman system 1000 spectrometer modified for use at 325 and 244 nm, respectively. A laser output of 10 mW was used, which resulted in an incident power at the sample of approximately 1.5 mW. The spectral resolution was approximately 2 cm k That no photoalteration of the samples occurred during the UV laser irradiation was ensured by confirming that the visible Raman spectra were unaltered after the UV Raman measurements. 

In spite of the high polarity of PA6, identification of additives was also feasible in formulations of PA6/additive dissolutions, although with decreased sensitivity. Hostavin N 20, Irganox B 1171, Tinuvin 320 and Tinuvin 350 can be determined in PA6 in technical concentrations, although the sensitivity is less than for nonpolar polymers, such as polyolefins. This was tentatively explained as follows. In a nonpolar polymer matrix, the electronically excited polar additive molecule can easily be desorbed. In the polar polyamide matrix, desorption of the additives is hindered by strong polar interactions (e.g. hydrogen bridges) between the excited analytes and the polymer matrix. This hinders selective desorption of the additives by laser irradiation. However, in a polymer/additive matrix-modified solution, evaporated to dryness, the interactions between the polar... 

For PMMA/additive dissolutions, it was not possible to identify any additive characteristic mass peaks, either by direct laser desorption or with matrix-assistance (dithranol, DHBA or sinapinic acid, 4-hydroxy-3,5-dimethoxy-cinnamic acid). This has again been ascribed to very strong interaction between PMMA and additives, which suppresses desorption of additive molecules. Also, partial depolymerisation of pho-tolytically labile PMMA by laser irradiation may play a role, which leads to saturation of the detector by PMMA fragment-ions and disappearance of additive mass peaks below noise level. Meyer-Dulheuer [55] has also reported MALDI-TOFMS analysis of a coating/2-ethylhexyldiphenylphosphate sample. Quantitative determination of the additives by means of MALDI-ToFMS proved impossible. Possibly the development of reproducible (automated) sample handling procedures or thin films might overcome this problem. 

Polyvinyl chloride has been modified by photochemical reactions in order to either produce a conductive polymer or to improve its light-stability. In the first case, the PVC plate was extensively photochlorinated and then degraded by UV exposure in N2. Total dehydrochlorination was achieved by a short Ar+ laser irradiation at 488 nm that leads to a purely carbon polymer which was shown to exhibit an electrical conductivity. In the second case, an epoxy-acrylate resin was coated onto a transparent PVC sheet and crosslinked by UV irradiation in the presence of both a photoinitiator and a UV absorber. This superficial treatment was found to greatly improve the photostability of PVC as well as its surface properties. 

Figure 1. Three step procedure of the carbonization of PVC by UV and laser irradiation...

Figure 6. IR absorption spectra of chlorinated PVC before and after laser irradiation at 488 nm for 0.1 s in air...

The quantum yield of the carbonization process can be evaluated from the amount of HC1 evolved during the laser irradiation ... 

Since the UV degraded C-PVC still contains substantial amounts of the initial CHC1-CHC1 structure, one can expect the chlorine radicals evolved to also initiate the zip-dehydrochlorination of these structures. The resulting chlorinated polyenes will then be further destroyed by the laser irradiation, so that finally all the C-PVC polymer is converted into a purely carbon material within a fraction of a second. 

The black lines that appear on the C-PVC plate after scanning by the laser beam were found to consist essentially of carbon and were thus expected to exhibit some electrical conductivity. Indeed, when a low voltage was applied to both ends of the laser tracks, the tiny filament turned bright red and even incandescent when a potential over 30 V was applied. This clearly shows that the laser irradiation can transform an insulating polymer like C-PVC into a conductive material. Thus, it becomes possible to write high resolution conductive patterns with this light-pencil which can be easily visualised since they appear as well contrasted black tracks. 

In the present study it has been shown for the first time that chlorinated PVC can be readily transformed into a conducting polymer by simple laser irradiation in the presence of air. The resulting material consisted essentially of carbon and proved to be able to carry electrons, without any doping procedure. By focusing the laser beam down into the micron range, it becomes thus possible... 

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