Radiation, electromagnetic intensity

In absorption spectroscopy a beam of electromagnetic radiation passes through a sample. Much of the radiation is transmitted without a loss in intensity. At selected frequencies, however, the radiation s intensity is attenuated. This process of attenuation is called absorption. Two general requirements must be met if an analyte is to absorb electromagnetic radiation. The first requirement is that there must be a mechanism by which the radiation s electric field or magnetic field interacts with the analyte. For ultraviolet and visible radiation, this interaction involves the electronic energy of valence electrons. A chemical bond s vibrational energy is altered by the absorbance of infrared radiation. A more detailed treatment of this interaction, and its importance in deter-... 

In the wave picture of electromagnetic radiation, the intensity I of the light transmitted by any material depends on the value of the electric field , according to the equation... 

A Hertzian dipole emits electromagnetic radiation. Its intensity S is proportional to the square of the absolute value of the second time derivative of the induced dipole moment 5 oc< >. Thus, the first of the terms in Equation 2.4-5 describes... 

The dipole-dipole potential between atoms induced by far-off resonance electromagnetic radiation of intensity I, wave-vector ki = hi, y, and polarization e = z (along the z-axis) is [Thirunamachandran 1980 Craig 1984]... 

Infrared (IR) thermography is a nonintrusive, two-dimensional technique for the whole-field measurement of surface temperature. An infrared scanning radiometer (IRSR) detects the electromagnetic energy radiated in the IR spectral band by the surface of interest and converts it into an electronic signal. For an object whose surface temperature is T, the monochromatic radiation of intensity Ex emitted by the surface can be written using Planck s law as... 

This chapter deals with basic considerations about absorption and emission of electromagnetic waves interacting with matter. Especially emphasized are those aspects that are important for the spectroscopy of gaseous media. The discussion starts with thermal radiation fields and the concept of cavity modes in order to elucidate differences and connections between spontaneous and induced emission and absorption. This leads to the definition of the Einstein coefficients and their mutual relations. The next section explains some definitions used in photometry such as radiation power, intensity, and spectral power density. 

The first volume contains the basic physical foundations of laser spectroscopy and the most important experimental equipment in a spectroscopic laboratory. It begins with a discussion of the fundamental definitions and concepts of classical spectroscopy, such as thermal radiation, induced and spontaneous emission, radiation power, intensity and polarization, transition probabilities, and the interaction of weak and strong electromagnetic (EM) fields with atoms. Since the coherence properties of lasers are important for several spectroscopic techniques, the basic definitions of coherent radiation fields are outlined and the description of coherently excited atomic levels is briefly discussed. 

Brightness temperature The definition is not unique great care is needed to decipher the intention of a given author. The temperature at which a blackbody radiator would radiate an intensity of electromagnetic radiation identical to that of the planet for a specific frequency, frequency bandwidth, and polarization under consideration is one definition of brightness temperature. A second definition is that it is the intensity of radiation under consideration divided (normalized) by the factor jlk). The normalization factor dimensionally scales... 

Two additional wave properties are power, P, and intensity, I, which give the flux of energy from a source of electromagnetic radiation. 

As discussed earlier in Section lOC.l, ultraviolet, visible and infrared absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, as well as the intensity of the absorption, is determined by the chemical environment of the absorbing moiety. Eor example, benzene has several ultraviolet absorption bands due to 7t —> 71 transitions. The position and intensity of two of these bands, 203.5 nm (8 = 7400) and 254 nm (8 = 204), are very sensitive to substitution. Eor benzoic acid, in which a carboxylic acid group replaces one of the aromatic hydrogens, the... 

The vibrational motions of the chemically bound constituents of matter have fre-quencies in the infrared regime. The oscillations induced by certain vibrational modes provide a means for matter to couple with an impinging beam of infrared electromagnetic radiation and to exchange energy with it when the frequencies are in resonance. In the infrared experiment, the intensity of a beam of infrared radiation is measured before (Iq) and after (7) it interacts with the sample as a function of light frequency, w[. A plot of I/Iq versus frequency is the infrared spectrum. The identities, surrounding environments, and concentrations of the chemical bonds that are present can be determined. 

Spectrometers are designed to measure the absorption of electromagnetic radiation by a sample. Basically, a spectrometer consists of a source of radiation, a compartment containing the sfflnple through which the radiation passes, and a detector. The frequency of radiation is continuously varied, and its intensity at the detector is compar ed with that at the source. When the frequency is reached at which the sample absorbs radiation, the detector senses a decrease in intensity. The relation between frequency and absorption is plotted as a spectrum, which consists of a series of peaks at characteristic frequencies. Its interpretation can furnish structural information. Each type of spectroscopy developed independently of the others, and so the data format is different for each one. An NMR spectrum looks different from an IR spectrum, and both look different from a UV-VIS spectrum. 

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