Principles of Absorption Spectroscopy

Atoms and molecules can absorb light/photons over a large range of wavelenths, ranging from the UV (A 400nm, hu 6.2 tV), through the visible (/I = 400—700nm, 1.6—6.2eV), to the IR 

The Beer-Lambert law (or Beer s law) is the linear relationship between absorbance A and number 

Laser Chemistry Spectroscopy, Dynamics and Applications Helmut H. Telle, Angel Gonzalez Urena Robert J. Donovan 2007 John Wiley Sons, Ltd ISBN 978-0-471-48570-4 (HB) ISBN 978-0-471-48571-1 (PB) 

See Box 6.1 for a more rigorous derivation of Beer s law. In addition, Box 6.1 makes reference to ihs. molar absorption coefficient a, which is widely used in high-density environments, such as liquids (the coefficient is also known as the extinction coefficient). Its units are (concentration x length), or more conveniently it is expressed in htres per mole per centimetre. However, in the gas phase, with its low particle densities, the cross-section notation is more convenient. 

By measuring the amount of light that a sample absorbs, and applying Beer s law one can determine the unknown concentration of an analyte atom or molecule. The linearity of the Beer-Lambert law is 

Because there are so many possible transitions, each differing from the others by only a sUght amount, each electronic transition consists of a vast number of lines spaced so closely that the spectrophotometer cannot resolve them. Rather, the instrument traces an envelope over the entire pattern. What is observed from these types of combined transitions is that the UV spectrum of a molecule usually consists of a broad band of absorption centered near the wavelength of the major transition. 

FIGURE 7.3 Electronic transitions with vibrational transitions superimposed. (Rotational levels, which are very closely spaced within the vibrational levels, are omitted for clarity.) 

The term log (/q//) is also known as the absorbance (or the optical density in older literature) and may be represented by A. The molar absorptivity (formerly known as the molar extinction coefficient) is a property of the molecule undergoing an electronic transition and is not a function of the variable parameters involved in preparing a solution. The size of the absorbing system and the probability that the electronic transition will take place control the absorptivity, which ranges from 0 to 10 . Values above 10 are termed high-intensity absorptions, while values below 10 are low-intensity absorptions. Forbidden transitions (see Section 7.1) have absorptivities in the range from 0 to 1000. 

The instrument design just described is quite suitable for measurement at only one wavelength. If a complete spectrum is desired, this type of instrument has some deficiencies. A mechanical system is required to rotate the monochromator and provide a scan of all desired wavelengths. This type of system operates slowly, and therefore considerable time is required to record a spectrum. 

A modem improvement on the traditional spectrophotometer is the diode-array spectrophotometer. A diode array consists of a series of photodiode detectors positioned side by side on a silicon crystal. Each diode is designed to record a narrow band of the spectrum. The diodes are connected so that the entire spectmm is recorded at once. This type of detector has no moving parts and 

7o = intensity of light incident upon sample cell 1 = intensity of light leaving sample cell c = molar concentration of solute I = length of sample cell (cm) e = molar absorptivity 

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The basic principles of absorption spectroscopy are summarised below. These are most obviously applicable to UV and IR spectroscopy and are simply extended to cover NMR spectroscopy. Mass Spectrometry is somewhat different and is not a type of absorption spectroscopy. 

The ultra-violet light induces electromagnetic excitation in molecules absorbing the laser energy, and by subsequently applying the principle of absorption spectroscopy, kinetic and spectroscopic information relating to the electronically excited states of various energetic molecules have been derived. The systems studied to-date include s-TNB, s-TNT, triphenyl-amine, and mono- as well as di-nitronaphthalenes (Ref 13, 14, 20, 21 28)... 

The measurement of vibrational optical activity requires the optimization of signal quality, since the experimental intensities are between three and six orders of magnitude smaller than the parent IR absorption or Raman scattering intensities. To date all successful measurements have employed the principles of modulation spectroscopy so as to overcome short-term instabilities and noise and thereby to measure VOA intensities accurately. In this approach, the polarization of the incident radiation is modulated between left and tight circular states and the difference intensity, averaged over many modulation cycles, is retained. In spite of this common basis, there are major differences in measurement technique and instrumentation between VCD and ROA consequently, the basic experimental methodology of these two techniques will be described separately. 

Fourier transform methods have revolutionized many fields in physics and chemistry, and applications of the technique are to be found in such diverse areas as radio astronomy [52], nuclear magnetic resonance spectroscopy [53], mass spectroscopy [54], and optical absorption/emission spectroscopy from the far-infrared to the ultraviolet [55-57]. These applications are reviewed in several excellent sources [1, 54,58], and this section simply aims to describe the fundamental principles of FTIR spectroscopy. A more theoretical development of Fourier transform techniques is given in several texts [59-61], and the interested reader is referred to these for details. 

Before describing the application of Nuclear magnetic resonance (NMR) spectroscopy to potentized homeopathic drugs we would first discuss the basic principles of NMR spectroscopy. This spectroscopy is a powerful tool providing structural information about molecules. Like UV-visible and infra red spectrometry, NMR spectrometry is also a form of absorption spectrometry. Nuclei of some isotopes possess a mechanical spin and the total angular momentum depends on the nuclear spin, or spin number 1. The numerical value of I is related to the mass number and the atomic number and may be 0, Vi, 1 etc. The medium of homeopathic... 

Infra-red (IR) spectroscopy functions to probe vibrational transitions (2000-50 000 nm 5000-200 cm i.e. wave number - typical IR spectroscopy units) in the singlet ground electronic state of molecules. The absorption principles of IR spectroscopy are identical to those of UV-visible and CD spectroscopy. Hence the Beer-Lambert law (Equation (4.3)) applies. Moreover, absorption band intensities are determined by the transition dipole moment and there are extensive perturbation and coupling effects. Overall though, values of molar extinction coefficients for vibrational transitions, are up to 10 times lower in magnitude... 

One of the most sensitive methods to detect volatile compounds released by the plants is that of laser photoacoustic spectroscopy (LPAS), which permits the identification of many molecules signalling plant defence mechanisms (e.g. see Harren and Reuss (1997)). The technique is based on the photoacoustic effect, i.e. the generation of acoustic waves as a consequence of light absorption the general principles of photoacoustic spectroscopy were briefly outlined in Section 5.3. 

The principle of optical spectroscopy involves the measurement of the amount of light (radiation) that is absorbed by the sample when the radiation interacts with the sample. The most basic method involves the determination of the fraction of the radiation that is actually transmitted through a sample. The aspects of the measurement, and their relationship to the actual absorption of radiation are illustrated in Fig. 56. In this example, 7o is the power of the incident radiation from the infrared light source, and I is the actual amount of radiation transmitted through the sample. The fundamental relationships are provided with Fig. 56, and these form the basis of a fundamental expression that is used to correlate the analytical spectrum with the amount(s) of material(s) present in a sample. This fundamental expression is a simple rendering of the Beer-Lambert-Bouguer law, which is used in one form or another in the quantitative determination of material composition. 

Fig. 5. Principle of Mossbauer spectroscopy. Top Set-up for measurement in transmission geometry., Bottom Overlap of Doppler-sKifted zero-phonon emission line with the zero-phonon absorption line. The shift A is given by eq. (10)l [Taken from Kalvius (1987).]...

Certain frequencies of the radiation will be absorbed when the matter is irradiated by infrared light. The energy is transferred into the vibration, and either light may be re-emitted or the energy may be transferred to the surrounding molecules, i.e. converted to heat. The absorption frequencies correspond to the frequencies of the normal modes. This is the principle of infrared spectroscopy. It should be noted that not all of the normal modes are infrared-active. Only an oscillating dipole can absorb the infrared light, provided that the frequency is... 

The detailed examination of the behavior of light passing through or reflected by an interface can, in principle, allow the determination of the monolayer thickness, its index of refiraction and absorption coefficient as a function of wavelength. The subjects of ellipsometry, spectroscopy, and x-ray reflection deal with this goal we sketch these techniques here. 

Section BT1.2 provides a brief summary of experimental methods and instmmentation, including definitions of some of the standard measured spectroscopic quantities. Section BT1.3 reviews some of the theory of spectroscopic transitions, especially the relationships between transition moments calculated from wavefiinctions and integrated absorption intensities or radiative rate constants. Because units can be so confusing, numerical factors with their units are included in some of the equations to make them easier to use. Vibrational effects, die Franck-Condon principle and selection mles are also discussed briefly. In the final section, BT1.4. a few applications are mentioned to particular aspects of electronic spectroscopy. 

A technique is any chemical or physical principle that can be used to study an analyte. Many techniques have been used to determine lead levels. For example, in graphite furnace atomic absorption spectroscopy lead is atomized, and the ability of the free atoms to absorb light is measured thus, both a chemical principle (atomization) and a physical principle (absorption of light) are used in this technique. Chapters 8-13 of this text cover techniques commonly used to analyze samples. 

EXAFS is part of the field of X-ray absorption spectroscopy (XAS), in which a number of acronyms abound. An X-ray absorption spectrum contains EXAFS data as well as the X-ray absorption near-edge structure, XANES (alternatively called the near-edge X-ray absorption fine structure, NEXAFS). The combination of XANES (NEXAFS) and EXAFS is commonly referred to as X-ray absorption fine structure, or XAFS. In applications of EXAFS to surface science, the acronym SEXAFS, for surface-EXAFS, is used. The principles and analysis of EXAFS and SEXAFS are the same. See the article following this one for a discussion of SEXAFS and NEXAFS. 

X-ray absorption spectroscopy is an important part of the armory of techniques for examining pure and applied problems in surface physics and chemistry. The basic physical principles are well understood, and the experimental methods and data analysis have advanced to sophisticated levels, allowing difficult problems to be solved. For some scientists the inconvenience of having to visit synchrotron radia-... 

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