Absorption of Electromagnetic Radiations

Now let us examine the consequences of the imaginary component of the complex index of refraction. Recall that the solution to the wave equation required that k = Multiplying the top and bottom k = by c, and using the fact that c = 1/sq, we get 

So we see that k is also complex. Now the solution to the wave equation becomes 

This is the equation for a damped wave or an exponentially decaying wave. Thus the presence of an imaginary component in the complex index of refraction is to cause the propagating wave to lose energy. Thus the K is sometimes called the extinction coefficient. Since the intensity of fhe radiation I EE, we can write 

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Structure determination m modern day organic chemistry relies heavily on instrumental methods Several of the most widely used ones depend on the absorption of electromagnetic radiation... 

Absorption of electromagnetic radiation causes a molecule to be excited from Its most stable state (the ground state) to a higher energy state (an excited state)... 

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... 

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. 

Mass spectrometry is not based on absorption of electromagnetic radiation, but monitors what happens when a substance is ionized by collision with a high-energy electron. 

Spectrometer (Section 13.1) Device designed to measure absorption of electromagnetic radiation by a sample. 

Absorption of electromagnetic radiation in the visible and ultraviolet regions of the spectrum resulting in changes in the electronic structure of ions and molecules. 

The absorption of electromagnetic radiation by molecular species in solution in the UV/visible region is followed by relaxation from excited electronic states to the ground state mostly by a combination of radiationless processes. Vibrational relaxation, where the excess energy is rapidly dis-... 

The allowed transition in ESR is diagrammed in Figure 2. The ESR experiment is commonly conducted at a fixed frequency near 9.5 x 109 Hz by scanning through a magnetic field range until absorption of electromagnetic radiation is detected at H0. The value of H0 can then be used to calculate the electron g-factor. 

Di- -octylphthalate may also undergo photolysis in surface waters as a result of its absorption of electromagnetic radiation at wavelengths less than 290 nm. The estimated photolytic half-life of the compound in surface water is 144 days (EPA 1992a). Photolysis was predicted to be the most important removal mechanism after volatilization for di- -octylphthalate losses from oligotrophic lakes (Wolfe et al. 1980). 

Rule First of all, never make the mistake of calling it mass spectroscopy. Spectroscopy involves the absorption of electromagnetic radiation, and mass spectrometry is different, as we will see. The mass spectrometrists sometimes get upset if you confuse this issue. [21]... 

Even at the highest fields, the NMR experiment would not be practicable if mechanisms did not exist to restore the Boltzmann equilibrium that is perturbed as the result of the absorption of electromagnetic radiation in making an NMR measurement. These mechanisms are known by the general term of relaxation and are not confined to NMR spectroscopy. Because of the small magnitude of the Boltzmann excess in the NMR experiment, relaxation is more critical and more important in NMR than in other forms of spectroscopy. 

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