Laser induced order

Figure 8.3. Laser-induced ordering in an anisotropic liquid.

From Equation (8.16) one can see that the molecular orientational nonlinearity in the isotropic phase of a liquid ciystal is directly proportional to the laser-induced order parameter Q. In typical anisotropic liquids (e g., CS2 or liquid crystals at temperatures far above Tq), the value of Q may be obtained by a statistical mechanics approach. In the completely random system, the average orientation is described by a distribution function G) ... 

To put this in more quantitative terms, let us ascribe the laser-induced order parameter change a time dependence of the form... 

If the system under consideration is chemically inert, the laser excitation only induces heat, accompanied by density and pressure waves. The excitation can be in the visible spectral region, but infrared pumping is also possible. In the latter case, the times governing the delivery of heat to the liquid are those of vibrational population relaxation. They are very short, on the order of 1 ps this sort of excitation is thus impulsive. Contrary to a first impression, the physical reality is in fact quite subtle. The acoustic horizon, described in Section VC is at the center of the discussion [18, 19]. As laser-induced perturbations cannot propagate faster than sound, thermal expansion is delayed at short times. The physicochemical consequences of this delay are still entirely unknown. The liquids submitted to investigation are water and methanol. 

In order to relate material properties with plasma properties, several plasma diagnostic techniques are used. The main techniques for the characterization of silane-hydrogen deposition plasmas are optical spectroscopy, electrostatic probes, mass spectrometry, and ellipsometry [117, 286]. Optical emission spectroscopy (OES) is a noninvasive technique and has been developed for identification of Si, SiH, Si+, and species in the plasma. Active spectroscopy, such as laser induced fluorescence (LIF), also allows for the detection of radicals in the plasma. Mass spectrometry enables the study of ion and radical chemistry in the discharge, either ex situ or in situ. The Langmuir probe technique is simple and very suitable for measuring plasma characteristics in nonreactive plasmas. In case of silane plasma it can be used, but it is difficult. Ellipsometry is used to follow the deposition process in situ. 

In order to record excitation spectra, the radical ions must first be thermalized to the electronic ground state, which happens automatically if they are created in condensed phase (e.g. in noble-gas matrices, see below). In the gas-phase experiments where ionization is effected by collision with excited argon atoms (Penning ionization), the unexcited argon atoms serve as a heat bath which may even be cooled to 77 K if desired. After thermalization, excitation spectra may be obtained by laser-induced fluorescence. 

Fluorescence detection can be up to four orders of magnitude more sensitive than UV absorbance, especially where laser induced excitation is used, mass detection limits being as low as 10-20—10 21 mole. Pre- and post-column derivatization methods are being developed to extend the applicability of fluorescence detection to non-fluorescent substances. Several types of electrochemical and mass spectrometric detector have also been designed. Detector characteristics are summarized in Table 4.21. 

Another method which uses capillary electrophoresis with laser-induced fluorescence detection can also be employed to detect zearalenone (Maragos and Appell 2007). In order to analyse trace amounts of zearalenone in plants, a sensitive, quick and accurate method, the enzyme-linked immunosorbent assay (ELISA) was developed by Chen et al. 1989. 

When compared to fluorescence detectors for HPLC, the design of a fluorescence detector for CE presents some technical problems. In order to obtain acceptable sensitivity, it is necessary to focus sufficient excitation light on the capillary lumen. This is difficult to achieve with a conventional light source but is easily accomplished using a laser. The most popular source for laser-induced fluorescence (LIF) detection is the argon ion laser, which is stable and relatively inexpensive. The 488-nm argon ion laser line is close to the desired excitation wavelength for several common fluorophores. The CLOD for a laser-based fluorescence detector can be as low as 10 12 M. 

Collet E, Lemee-Cailleau MH, Buron-Le Cointe M, Cailleau H, Wulff M, Luty T, Koshihara S, Meyer M, Toupet L, Rabiller P, Techert S (2003) Laser-induced ferroelectric structural order in an organic charge-transfer crystal. Science 300 612-615... 

After a delay of several ps, the luminescence of Eu " is already very weak, and narrow long-lived lines of trivalent RE dominate in the spectrum. The lines at 589, 617, 651, and 695 nm (Fig. 4.1c) have never been detected in natural apatite by steady-state spectroscopy. According to their spectral position they may be ascribed to Eu ", but they are different from known lines in synthetic apatites activated by Eu (Jagannathan and Kottaosamy 1995 Morozov et al. 1970 Piriou et al. 1987 Piriou et al. 2001 Voronko et al. 1991). In order to clarify this problem we studied artificially activated samples by laser-induced time-resolved luminescence spectroscopy. 

As was already mentioned, the origin of this band was ambiguous, hi order to clarify this we studied the irradiation influence on laser-induced time-resolved luminescence of the following samples (Gaft et al. 2003b) ... 

Our study of sedimentary apatite from Israel proved that laser-induced time-resolved luminescence is a perspective tool for evaluation of sedimentary phosphate ores with high dolomite content (Gaft et al. 1993b). The idea was based on the fact that natural apatite contains several characteristic luminescence centers, which enables us to differentiate it from dolomite. The most widespread characteristic luminescence center in sedimentary apatite is uranyl (U02) with a typical vibrational green band luminescence under nitrogen laser excitation (Fig. 8.13a,b). Nevertheless, it appears that such luminescence is absent in phosphate rock samples from Florida, evidently because of extremely low uranium concentration (Fig. 8.13c,d). hi order to find potential liuninescence centers, ICP-MS analyses of Florida phosphates was accompHshed. From discovered REE, theoretically Dy + is the best candidate... 

In order to discuss an essential feature of the laser-induced structural change, we mnst first consider the structure of amorphous films. Almost all available structural data indicate that the main constituent of a-Sbj Sei j (x < 0.05) is the chain mol-ecnle, althongh molecules with Sb branching sites may be contained in minority. That is, we assume that the local structure of a-Se containing Sb additives resembles the hexagonal (trigonal) Se structure. Accordingly, in atomic structural terms, a-Sbj Sci withx < 5at.% may be characterized as a quasi-one-dimensional chain structnre. 

Under UV-laser irradiation, photosensitive multifunctional acrylate resins become rapidly cross-linked and completely insoluble. The extent of the reaction was followed continuously by both UV and IR spectroscopy in order to evaluate the rate and quantum yield of the laser-induced polymerization of these photoresist systems. Two basic types of lasers emitting in the UV range were employed, either a continuous wave (C.W.) argon-ion laser, or a pulsed nitrogen laser. 

Figure 6. Dependence of the polymerization quantum yield ( ) on the light-intensity (Iq) in the laser-induced polymerization of epoxy-acrylate photoresists (— expected variation of on I0 for a half-order kinetic law).

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