Spectrum, electromagnetic

Molecular spectroscopy involves the study of the absorption or emission of electromagnetic radiation by matter the radiation may be detected directly, or indirectly through its effects on other molecular properties. The primary purpose of spectroscopic studies is to understand the nature of the nuclear and electronic motions within a molecule. 

In the classical theory of electrodynamics, electromagnetic radiation is emitted when an electron moves in its orbit but, ac cording to the Bohr theory of the atom, 

It has been established beyond any reasonable doubt that the absorption and the emission of energy in the electromagnetic spectrum take place in distinct separate pockets or photons. The relationship 

However, the relationship between wavelength and frequency may be expressed as follows  

The radiant power of a beam is designated by its intensity of radiation, which in turn is directly proportional to the number of photons per second that are propagated in the beam. 

Monochromatic Beam A beam that carries radiation of only one distinctly separate wave length is known as monochromatic. 

Polychromatic or Heterochromatic A beam that carries radiation of several wavelengths is termed as polychromatic or heterochromatic. 

The Great Physicists from Galileo to Einstein, pp. 156 157, Dover Publications, New York (1961). 

Hertz was able to identify these electromagnetic rays as an invisible form of light by demonstrating that they displayed the wavelike phenomena of reflection, refraction, polarization, and interference. From interference measurements, he showed that the wavelength of the waves received by the detector was 66 cm. 

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In the electromagnetic spectrum, the ultra violet region is between that of X-rays and visible light. This corresponds to the energies hv ot one hundred to a few tens of electron-volts (wavelengths from 180 to 400 nm). 

Spectroscopy, or the study of the interaction of light with matter, has become one of the major tools of the natural and physical sciences during this century. As the wavelength of the radiation is varied across the electromagnetic spectrum, characteristic properties of atoms, molecules, liquids and solids are probed. In the... 

MW frequency of 10 Hz. There are various considerations that influence the choice of the radiation frequency. Higher frequencies, which require higher magnetic fields, give inlierently greater sensitivity by virtue of a more favourable Boltzmaim factor (see equation (b 1.15.11)). However, several factors place limits on the frequency employed, so that frequencies in the MW region of the electromagnetic spectrum remain favoured. One limitation is the sample size at frequencies around 40 GHz the dimensions of a typical... 

The positions of hnes or bands in the electromagnetic spectrum may be expressed either as wave lengths (X) or as frequencies (v). The units employed in the measurement of wave lengths are ... 

The wave lengths of the various parts of the electromagnetic spectrum of immediate interest are ... 

The electromagnetic spectrum showing the colors of the visible spectrum. 

The determination of an analyte s concentration based on its absorption of ultraviolet or visible radiation is one of the most frequently encountered quantitative analytical methods. One reason for its popularity is that many organic and inorganic compounds have strong absorption bands in the UV/Vis region of the electromagnetic spectrum. In addition, analytes that do not absorb UV/Vis radiation, or that absorb such radiation only weakly, frequently can be chemically coupled to a species that does. For example, nonabsorbing solutions of Pb + can be reacted with dithizone to form the red Pb-dithizonate complex. An additional advantage to UV/Vis absorption is that in most cases it is relatively easy to adjust experimental and instrumental conditions so that Beer s law is obeyed. 

The so-called peak power delivered by a pulsed laser is often far greater than that for a continuous one. Whereas many substances absorb radiation in the ultraviolet and infrared regions of the electromagnetic spectrum, relatively few substances are colored. Therefore, a laser that emits only visible light will not be as generally useful as one that emits in the ultraviolet or infrared ends of the spectrum. Further, witli a visible-band laser, colored substances absorb more or less energy depending on the color. Thus two identical polymer samples, one dyed red and one blue, would desorb and ionize with very different efficiencies. 

A dye molecule has one or more absorption bands in the visible region of the electromagnetic spectrum (approximately 350-700 nm). After absorbing photons, the electronically excited molecules transfer to a more stable (triplet) state, which eventually emits photons (fluoresces) at a longer wavelength (composing three-level system.) The delay allows an inverted population to build up. Sometimes there are more than three levels. For example, the europium complex (Figure 18.15) has a four-level system. 

Neodymium and YAG Lasers. The principle of neodymium and YAG lasers is very similar to that of the ruby laser. Neodymium ions (Nd +) are used in place of Cr + and are often distributed in glass rather than in alumina. The light from the neodymium laser has a wavelength of 1060 nm (1.06 xm) it emits in the infrared region of the electromagnetic spectrum. Yttrium (Y) ions in alumina (A) compose a form of the naturally occurring garnet (G), hence the name, YAG laser. Like the ruby laser, the Nd and YAG lasers operate from three- and four-level excited-state processes. 

The electromagnetic spectrum measures the absorption of radiation energy as a function of the frequency of the radiation. The loss spectrum measures the absorption of mechanical energy as a function of the frequency of the stress-strain oscillation. 

In the electromagnetic spectrum, the energy absorbed makes up the difference between two allowed energy states in the absorber. In the loss spectrum the frequency absorbed closely matches the frequency of dissipative modes of molecular motion in the sample. 

The electromagnetic spectrum is a quantum effect and the width of a spectral feature is traceable to the Heisenberg uncertainty principle. The mechanical spectrum is a classical resonance effect and the width of a feature indicates a range of closely related r values for the model elements. 

Theoretical analysis of certain features in the electromagnetic spectrum yields basic molecular parameters such as bond lengths and bond stiffness. We shall see presently that the mechanical spectra can be related to molecular parameters and not just modelistic characteristics as we have used until now. 

The scattering of visible light by polymer solutions is our primary interest in this chapter. However, since is a function of the ratio R/X, as we saw in the last section, the phenomena we discuss are applicable to the entire range of the electromagnetic spectrum. Accordingly, a general review of the properties of this radiation and its interactions with matter is worthwhile before a specific consideration of scattering. 

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