Photoacoustic spectroscopy, described

The technique employed is IR-FT photothermal beam deflection spectroscopy (PBDS). It is an off-shoot of photoacoustic spectroscopy (PAS) [1] and is based on the "mirage detection of the photothermal effect invented by Boccara et al. [2] and shown to result in a spectroscopic technique of remarkable versatility and utility. Some applications of "mirage spectroscopy," mainly in the visible, and theoretical treatments, have been described [3 6]. The method has now been developed for use in the IR. The spectrometer and techniques are described in detail elsewhere [7], but it will be useful to give a brief outline of the principles. 

Photoacoustic spectroscopy was used to determine intersystem crossing quantum yields, d>-lsc, by a method previously described elsewhere (20). These values of d sc show a solvent dependence and are 0.4 in ethanol and 0.6 in dioxane. This is consistent with singlet involvement in the chemistry of a-guaiacoxyacetoveratrone. 

In addition to the IR, Raman and LIBS methods previously discussed, a number of other laser-based methods for explosives detection have been developed over the years. The following section briefly describes the ultraviolet and visible (UV/vis) absorption spectra of EM and discusses the techniques of laser desorption (LD), PF with detection through resonance-enhanced multiphoton ionization (REMPI) or laser-induced fluorescence (LIF), photoacoustic spectroscopy (PAS), variations on the light ranging and detecting (LIDAR) method, and photoluminescence. Table 2 summarizes the LODs of several explosive-related compounds (ERC) and EM obtained by the techniques described in this section. 

Photoacoustic Spectroscopy.—Photoacoustic spectroscopy (PAS) and its applications have been recently reviewed. A single-beam i.r. PAS spectrometer has been constructed for the range 800—4000 cm using a broad-band carbon rod spectral source in preference to a laser. A double-beam in-time PAS instrument has been described, in which a single microphone was used to monitor both the... 

The various reflectance methods which are now widely available, such as attenuated total reflectance, specular reflectance, and diffuse reflectance spectroscopy, were also introduced. Photoacoustic spectroscopy, gas chromatography-infrared spectroscopy, temperature studies and microsampling techniques were also described. 

In this article, experiments with gases employing a resonant photoacoustic cell are reviewed. In the next chapter the physical processes occurring in an acoustic resonator are analysed. The third chapter describes experimental setups and components used in resonant photoacoustic spectroscopy. In chapter four the theory of acoustic resonators is outlined and the loss mechanisms which dampen the... 

The basics of photoacoustic spectroscopy (PAS) are described in Chapter 5. PAS is useful for examining highly absorbing samples that are difficult to analyze by other IR techniques. Minor or even no sample preparation is required here. The size and shape of the sample are not critical. PA spectra can be obtained from a wide variety of samples such as powders, polymer pellets, viscous glues, single crystals, and single fibres. 

Rather than a more formal title such as laser Debye-Sears ultrasonic absorption spectroscopy we frequently refer to this experiment as simply "looking at sound." This has the advantage of drawing attention to the complementarity of this experiment to photoacoustic spectroscopy (5) that is aptly described as "listening to light," Piezoelectric transducers, lock-in amplifiers, and minicomputers play key roles in each of these two experiments. The details of the minicomputerization of our "looking at sound" experiment have been given elsewhere (6). 

ABSTRACT. The construction and operation of a Michelson interferometer that permits Fourier transform photoacoustic spectroscopy of opaque and partially transparent samples at visible wavelengths is described. Multiplexing and throughput advantages are considered. A visible spectrum of Nd(III) doped laser glass is reproduced and potential kinetic applications are described. 

Other methods have also been developed but are harder to use routinely (higher size of the equipment, longer measurement time, higher cost, etc.). They will not be described here and the reader is referred to scientific literature for details. They include, for example, near-infirared [106], infrared [107], and optothermal infrared [108] spectroscopy, photoacoustic spectroscopy [109], as well as many other indirect methods taking advantage of the consequences of skin dehydration (D-Squame disk analysis, profilometry, viscoelastic properties of the skin, etc.). 

Many solid photodegraded samples are strongly crosslinked and do not lend themselves to routine transmission or reflection IR techniques. The samples are either almost insoluble, difficult to grind into a powder or of irregular shape. Photoacoustic spectroscopy (PAS) is a unique method because it is the only method that provides a direct measurement of infrared absorption by sample. Photoacoustic detection is often described as a last resort method, as it has certain idiosyncrasies and is limited to small samples. 

Concrete applications of micro reactors for chemical analysis, albeit so far not a core application, have been described [5]. Among other uses in chemical analysis, micro devices for gas chromatography, infrared spectroscopy, and photoacoustic detection are mentioned. 

The methods of nonlinear Raman spectroscopy, i. e. spontaneous hyper Raman scattering (based on the hyperpolarizability) and coherent nonlinear Raman scattering (based on the third-order-nonlinear susceptibilities) are discussed in detail in Sec. 3.6.1. In Sec. 3.6.2 the instrumentation needed for these types of nonlinear spectroscopy is described. In this section we present some selected, typical examples of hyper Raman scattering (Sec. 6.1.4.1), coherent anti-Stokes Raman. scattering (Sec. 6.1.4.2), stimulated Raman gain and inverse Raman spectroscopy (Sec. 6.1.4.3), photoacoustic Raman spectroscopy (Sec. 6.1.4.4) and ionization detected stimulated Raman spectroscopy (Sec. 6.1.4.5). 

Fig. 7 Raman gain = stimulated Raman gain spectroscopy (SRGS), inverse Raman = inverse Raman spectroscopy (IRS) or stimulated Raman loss spectroscopy (SRLS), coherent anti-Stokes Raman spectroscopy (CARS), photoacoustic Raman spectroscopy (PARS), or ionization-detected stimulated Raman spectroscopy (IDSRS). In the following sections, the various methods are briefly described. More detailed information can be found in books [59-61], reviews [45,46,57,58,62,63] and conference reports [64-73].

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