Lasers spectroscopic sources

For the visible and near-ultraviolet portions of the spectmm, tunable dye lasers have commonly been used as the light source, although they are being replaced in many appHcation by tunable soHd-state lasers, eg, titanium-doped sapphire. Optical parametric oscillators are also developing as useful spectroscopic sources. In the infrared, tunable laser semiconductor diodes have been employed. The tunable diode lasers which contain lead salts have been employed for remote monitoring of poUutant species. Needs for infrared spectroscopy provide an impetus for continued development of tunable infrared lasers (see Infrared technology and RAMAN spectroscopy). 

Laterally inhomogeneous films and patterned structures of microelectronic and optoelectronic applications require small measuring spots. Today s measurements in 50 pm X 50 pm areas are standard for p-spot spectroscopic ellipsometers used in fa-blines. Areas more than ten times smaller can be analyzed by use of discrete-wave-length ellipsometers equipped with laser-light sources. 

It may be apparent to the reader at this stage that, when lasers are used as spectroscopic sources, we can no longer think in terms of generally applicable experimental methods. A wide variety of ingenious techniques have been devised using laser sources and it will be possible to describe only a few of them here. 

Studies on complex systems, such as metalloen-zymes, increasingly require the measurement of a minor spectral component in a far larger assembly. Basic aspects of optical spectroscopic techniques that make them particularly effective are the speed, sensitivity, and linearity of light detectors as well as the intensity, stability, and precision of conventional and laser light sources. 

Optimization of internal engine combustion in respect of fuel efficiency and pollutant minimization requires detailed insight in the microscopic processes in which complex chemical kinetics is coupled with transport phenomena. Due to the development of various pulsed high power laser sources, experimental possibilities have expanded quite dramatically in recent years. Laser spectroscopic techniques allow nonintrusive measurements with high temporal, spectral and spatial resolution. New in situ detection techniques with high sensitivity allow the measurement of multidimensional temperature and species distributions required for the validation of reactive flow modeling calculations. The validated models are then used to And optimal conditions for the various combustion parameters in order to reduce pollutant formation and fuel consumption. 

Istvan, K. Keresztury, G. Szep, A. Normal Raman and surface enhanced Raman spectroscopic experiments with thin layer chromatography spots of essential amino acids using different laser excitation sources. Spectrochim. Acta, Part A Mol. Biomol. Spectrosc. 2003, 59, 1709-1723. 

Platinum and palladium were among the first metals that were investigated in the molecular surface chemistry approach employing free mass-selected metal clusters [159]. The clusters were generated with a laser vaporization source and reacted in a pulsed fast flow reactor [18] or were prepared by a cold cathode discharge and reacted in the flowing afterglow reactor [404] under low-pressure multicollision reaction conditions. These early measurements include the detection of reaction products and the determination of reaction rates for CO adsorption and oxidation reactions. Later, anion photoelectron spectroscopic data of cluster carbonyls became available [405, 406] and vibrational spectroscopy of metal carbonyls in matrices was extensively performed [407]. Finally, only recently, the full catalytic cycles for the CO oxidation reaction with N2O and O2 on free clusters of Pt and Pd were discovered and analyzed [7,408]. 

Another recent advance in electron-molecule resonances is their role in molecular autoionization and photoionization. Here they show up as an exit-channel effect. The increasing availability of synchrotron sources and the proliferation of high-resolution laser spectroscopic techniques are leading to expanded interest in these processes because of the necessity to interpret the resonance features for a greater variety of molecules of chemical interest. Electron-molecule shape resonances are also responsible for structure in inner-shell electron energy-loss spectra in the region around the core ionization threshold acting as a final-state interaction, the same resonances... 

The development of ion-guides has provided an ion-source that is both very fast and in principle independent of the chemical properties of the element under study [38]. This opens two regions of particular nuclear interest for laser spectroscopical studies nuclear size and moments of isomers and the shape transition at N=59, showing a coexistence of spherical and deformed bands in for example Sr and i Zr [39]. It is thus of interest to extend the systematic measurements performed in Rb and Sr... 

Laser spectroscopic studies of radioactive isotopes have proved to be a valuable source in obtaining nuclear properties. The continuing developments promise a way of meeting challenge in measuring nuclear properties far from stability. It may also be so that new isotopes are discovered by optical rather than nuclear methods. The extreme high precision measurements in ionic traps make rather small nuclear effects such as hype ne anomaly interesting for tx experimental and theoretical studies. In ad(Ution the theories of hyperfine structure and isotope shift are well understood, so that detailed information on nuclear properties can be extracted. 

In addition to the fiber sensors, there have been numerous reports on remote fiberless optical detection of pollution from stationary sources. Mobile remote sensors can have cost advantages over on-site instruments and also are much more versatile. Pollutants in smokestack emissions have been identified by irradiation with a laser from a mobile unit equipped with a telescope, a monochromator, and low-noise detection electronics. The gases and particles in the plume scatter the laser light in various directions. A fraction is scattered back to the receiver and analysed to detect the amount and type of gas in the plume. Many of these methods await their application to fiber-optic sensing schemes which are inherently safer than direct laser spectroscopic schemes. 

Semiconductor lasers are ideal excitation sources, and they have already been demonstrated to be remarkably versatile and useful in a number of spectroscopic applications, as indicated by Ishibashi and cowotkers (22,23). These solid-state devices are inexpensive, small, easy to use, long-lived, and require little maintenance. The problem with semiconductor lasers is that powerful singlemode lasers are only available at NIR wavelengths, although this is likely to change in the near future. Until this work, however, no report had been made of the use of diode lasers for SERS. Recently, we demonstrated NIR SERS with a diode-laser excitation source and investigated the characteristics of the technique (Angel, S.M. Myrick, M.L. unpublished data). 

The laser source is especially useful for sampling small areas, where the amount of sample vaporized is very small and the bulk of the sample is not destroyed. It can be used on living tissue without destruction of the tissue. Thus the laser source is a valuable addition to the list of spectroscopic sources with special applications in the microsample area. 

To date, the laser-driven sources based on harmonic generation and frequency mixing have been used in a number of experiments to measure radiative lifetimes and to record absorption and laser-induced fluorescence spectra. While some of the anti-Stokes Raman sources have demonstrated high intensity and are being applied to specific problems of impurity detection in plasmas, their general utility in spectroscopy awaits the development of systems with broad tunability. Here, several typical spectroscopic applications of the above VUV and XUV sources are given to illustrate their usefulness. 

The rovibronic spectrum presented in Fig. 26 was recorded at the highest power, of 5 x 10, obtained to date with the VUV laser-driven sources described here. This represents a factor of 3 to 5 improvement over spectroscopic resolution obtained with grating instruments in this wavelength region. The 25-0 band of Fig. 26 is one of 12 vibronic bands originating from v = 0 or 1 levels of the ground state to levels v = 23 to 31 of the excited state. For each of the 24-0, 25-0, 26-0, 26-1, and 27-1 bands, three rotational branches were clearly resolved. Three branches were less conspicuous in other bands or unobservable because of blending of lines, but all bands could be analysed in terms of P, Q, and R branches. 

In this contribution we present two laser spectroscopic methods that use coherent resonance Raman scattering to detect rf-or laser -induced Hertzian coherence phenomena in the gas phase these novel coherent double resonance techniques for optical heterodyne detection of sublevel coherence clearly extend the above mentioned previous methods using incoherent light sources. In the case of Doppler broadened optical transitions new signal features appear as a result of velocity-selective optical excitation caused by the narrow-bandwidth laser. We especially analyze the potential and the limitations of the new detection schemes for the study of collision effects in double resonance spectroscopy. In particular, the effect of collisional velocity changes on the Hertzian resonances will be investigated. 

The book is divided into seven distinct parts and these are subdivided into individual chapters. In Parts I -3 we present the general principles that underpin the operation of lasers, the key properties of laser radiation, the main features of the various laser sources, and an overview of the most commonly used laser spectroscopic techniques, together with the instrumentation and methods for data acquisition. In Parts 4-6 we address the principles of unimolecular, bimolecular, cluster and surface reactions, which have been probed, stimulated or induced by laser radiation. In the final part, Part 7, we summarize a range of practical laser applications in industry, environmental studies, biology and medicine, many of which are already well established and in routine use. 

Conventional optical absorption spectrometry has detection limits of between 0.01 and 1 mM for the actinides. Highly sensitive spectroscopic methods have been developed, based on powerful laser light sources. Time resolved laser fluorescence spectroscopy (TRLFS), based on the combined measurement of relaxation time and fluorescence wavelength, is capable of speciating Cm(III) down to 10 mol/L but is restricted to fluorescent species like U(VI) and Cm(III). Spectroscopic methods based on the detection of nonradiative relaxation are the laser-induced photoacoustic spectroscopy (LPAS) and the laser-induced thermal leasing spectroscopy (LTLS). Like conventional absorption spectroscopic methods, these newly developed methods are capable of characterizing oxidation and complexation states of actinide ions but with higher sensitivity. 

A comparison is made in Table 9.1 between a conventional line light source and a continuous single-mode dye laser both sources with representative data. In many spectroscopic experiments a decisive factor is by what power per unit area and spectral interval a sample can be irradiated. The designation I(i ) is used for this power density/frequency unit. In the comparison in... 

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