Spectroscopy dispersive

Figure 6.1. Common sampling geometries for Raman spectroscopy. Dispersive spectrometers are shown, but similar sample arrangements are used for FT-Raman.

At this point it is relevant to note the terminology employed in this chapter the expression laser-induced fluorescence (LIF) is used as a general term describing any fluorescence that is excited using a laser. A fluorescence excitation spectrum shows fluorescence emission yield as a function of excitation wavelength that is, it is similar to an absorption spectrum when that absorption results in radiative emission. It is noted that some authors reserve LIF as a synonym for fluorescence excitation spectroscopy. Dispersed fluorescence refers to dispersion of the emitted fluorescence light into its component wavelengths, that is, production of an emission spectrum. 

Amiot, C. and DuUeu, O., The Cs2 ground electronic state by Fourier transform spectroscopy dispersion coefficients, J. Chem. Phys., 117, 5155, 2002. 

Table 1 Environmentally Important Compounds Studied by Normal Raman Spectroscopy (Dispersive and FT), Resonance Raman Spectroscopy (Including Preresonance), and Surface-Enhanced Raman Spectroscopy... 

Both dispersive (D) and Fourier transform (FT) micro-Raman are commercially available [507-509]. The choice between D- and FT-Raman has been discussed cfr. Chp. 1.2.3). However, as the intrinsic superiority of interferometers in terms of high resolution and geometrical extent cannot be exploited in micro-Raman spectroscopy, dispersive spectral analysers are preferred in combination with the microscope. FT-Raman instruments equipped with a microscope have thus far been limited to a sensitivity far lower than that of modern Raman microspectrometers equipped with multichannel detectors, which employ visible excitation. Consequently, FT-Raman microscopes often yield deceiving results with poor sensitivity and spatial resolution (from 15... 

Lee S-Y 1995 Wave-packet model of dynamic dispersed and integrated pump-probe signals in femtosecond transition state spectroscopy Femtosecond Chemistry ed J Manz and L Wdste (Heidelberg VCH)... 

While a laser beam can be used for traditional absorption spectroscopy by measuring / and 7q, the strength of laser spectroscopy lies in more specialized experiments which often do not lend themselves to such measurements. Other techniques are connnonly used to detect the absorption of light from the laser beam. A coimnon one is to observe fluorescence excited by the laser. The total fluorescence produced is nonnally proportional to the amount of light absorbed. It can be used as a measurement of concentration to detect species present in extremely small amounts. Or a measurement of the fluorescence intensity as the laser frequency is scaimed can give an absorption spectrum. This may allow much higher resolution than is easily obtained with a traditional absorption spectrometer. In other experiments the fluorescence may be dispersed and its spectrum detennined with a traditional spectrometer. In suitable cases this could be the emission from a single electronic-vibrational-rotational level of a molecule and the experimenter can study how the spectrum varies with level. 

All nonlinear (electric field) spectroscopies are to be found in all temis of equation (B 1.3.1) except for the first. The latter exclusively accounts for the standard linear spectroscopies—one-photon absorption and emission (Class I) and linear dispersion (Class II). For example, the temi at third order contains by far the majority of the modem Raman spectroscopies (table B 1.3.1 and tableBl.3.2). 

The latter condition corresponds to the phase matching requirement already mentioned—the wavelength and direction of the material polarization wave must match those of the new EM wave as closely as possible. However, for all Class I spectroscopies, this condition is automatically achieved because of quadrature. In fact, this is tnie for all quadrature spectroscopies—the Class I spectroscopies being the principal such, but, as noted, it is a nontrivial requirement in the nonquadrature Class II spectroscopies, particularly in optically dispersive media. 

McCreery R L 1996 Instrumentation for dispersive Raman spectroscopy Modern Techniques in Raman Spectroscopy ed J J Laserna (New York Wiley)... 

One interesting new field in the area of optical spectroscopy is near-field scaiming optical microscopy, a teclmique that allows for the imaging of surfaces down to sub-micron resolution and for the detection and characterization of single molecules [, M]- Wlien applied to the study of surfaces, this approach is capable of identifying individual adsorbates, as in the case of oxazine molecules dispersed on a polymer film, illustrated in figure Bl.22,11 [82], Absorption and emission spectra of individual molecules can be obtamed with this teclmique as well, and time-dependent measurements can be used to follow the dynamics of surface processes. 

XPS X-ray photoelectron spectroscopy Absorption of a photon by an atom, followed by the ejection of a core or valence electron with a characteristic binding energy. Composition, oxidation state, dispersion... 

Yuzawa T, Kate C, George M W and Hamaguchi H O 1994 Nanosecond time-resolved infrared spectroscopy with a dispersive scanning spectrometer Appl. Spectrosc. 48 684-90... 

K. E. Peiponen, E. M. Vertiainen, and T, Asakura, Dispersion, Complex Analysis and Optical Spectroscopy. (Classical theory), Springer-Verlag, Berlin, 1999,... 

The focus of this chapter is photon spectroscopy, using ultraviolet, visible, and infrared radiation. Because these techniques use a common set of optical devices for dispersing and focusing the radiation, they often are identified as optical spectroscopies. For convenience we will usually use the simpler term spectroscopy in place of photon spectroscopy or optical spectroscopy however, it should be understood that we are considering only a limited part of a much broader area of analytical methods. Before we examine specific spectroscopic methods, however, we first review the properties of electromagnetic radiation. 

In the second broad class of spectroscopy, the electromagnetic radiation undergoes a change in amplitude, phase angle, polarization, or direction of propagation as a result of its refraction, reflection, scattering, diffraction, or dispersion by the sample. Several representative spectroscopic techniques are listed in Table 10.2. 

To examine a sample by inductively coupled plasma mass spectrometry (ICP/MS) or inductively coupled plasma atomic-emission spectroscopy (ICP/AES) the sample must be transported into the flame of a plasma torch. Once in the flame, sample molecules are literally ripped apart to form ions of their constituent elements. These fragmentation and ionization processes are described in Chapters 6 and 14. To introduce samples into the center of the (plasma) flame, they must be transported there as gases, as finely dispersed droplets of a solution, or as fine particulate matter. The various methods of sample introduction are described here in three parts — A, B, and C Chapters 15, 16, and 17 — to cover gases, solutions (liquids), and solids. Some types of sample inlets are multipurpose and can be used with gases and liquids or with liquids and solids, but others have been designed specifically for only one kind of analysis. However, the principles governing the operation of inlet systems fall into a small number of categories. This chapter discusses specifically substances that are normally liquids at ambient temperatures. This sort of inlet is the commonest in analytical work. 

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