Emission and absorption of radiation

28 2 ELECTROMAGNETIC RADIATION AND ITS INTERACTION WITH ATOMS AND MOLECULES 

Induced absorption, in which the molecular (or atom) M absorbs a quantum of radiation and is excited from m to n  

This is the familiar absorption process illustrated by the appearance of an aqueous solution of copper sulphate as blue due to the absorption of the complementary colour, red, by the solution. 

Spontaneous emission, in which M (in state n) spontaneously emits a quantum of radiation  

Almost all emission that we usually encounter, such as that from a sodium vapour or tungsten filament lamp, is of the spontaneous type. 

Induced, or stimulated, emission. This is a different type of emission process from that of type 2 in that a quantum of radiation of wavenumber v given by Equation (2.2) is required to induce, or stimulate, M to go from n to m. The process is represented by 

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ABSORPTION AND EMISSION OF RADIATION For the vibrational energy level ... 

The absorption and emission of radiation in the near ultraviolet (UV) and visible regions of the electromagnetic spectrum are associated with electronic (and associated vibronic) transitions involving n- and/or n-electron systems of molecules. Synthetic and natural polymers absorb in the UV region and particularly strong absorption spectra are recorded for polymers containing aromatic and heteroaromatic groups (e.g., poly(styrenes), poly(vinyl naphthalenes), poly(vinyl carbazoles)). 

Spectroscopy is the study of the absorption and emission of radiation by matter. The most easily appreciated aspect of the absorption of radiation is the colour shown by substances that absorb radiation from the visible region of the spectrum. If radiation is absorbed from the red region of the spectrum, the transmitted or unabsorbed radiation will be from the blue region and the substance will show a blue colour. Similarly substances that emit radiation show a particular colour if the radiation is in the visible region of the spectrum. Sodium lamps, for instance, owe their characteristic orange-yellow light to the specific emission of sodium atoms at a wavelength of 589 nm. 

In phosphors and in the ruby laser, light was absorbed and emitted by electrons localised on an impurity site, but in other optical devices, delocalised electrons emit the radiation. In the next section, therefore, we shall consider the absorption and emission of radiation in solids with delocalised electrons, particularly in semiconductors. 

Absorption and Emission of Radiation. Raman spectra of NaNs have been detd by Petrikalns Hochberg (Ref 41), Kahovec Kohlrausch (Ref 111) and by Sheinker Syikin (Ref 120). Moler (Ref 99) expressed the absorption spectra of aq NaN3 soln as log of extinction coeff vs wave length. Sheinker (Ref 127) noted that the UV absorption spectra of aq NaNj soln were markedly different from those of typical aliphatic azides. Infrared absorption spectra were reported by Lieber et al (Ref 129) and by Delay et al (Ref 105) in the range 3 to 19ft. From the intensities of bands observed, it was concluded by Delay et al that the sym form was more abundant than the unsym form in azides of Na, Cu, Aq and Hg but the reverse was true for azides of T1 and Pb... 

Mechanism of Absorption and Emission of Radiation of Photochemical Interest... 

Let neq and /ieq rad be the equilibrium spin population differences between the a and y3 levels in the absence and presence of radiation, respectively. Let pap Ppa be the probabilities per second for stimulated absorption and emission of radiation, (a) Show that... 

Then Langevin pointed out that the central nucleus mentioned by Rutherford had to contain electrons in order to explain the emission of (3-rays by radioactive atoms and Marie Curie elaborated the idea that within the atom there should be two kinds of electrons. The peripheral electrons responsible for the processes of absorption and emission of radiation and for conductivity of metals and, in addition, the electrons emitted in the (3 decay of some radioactive nuclei. 

There are several reasons for starting this account with a discussion of electromagnetic radiation. Historically, it was in this area that the quantum theory first developed. It is easier here to understand the evidence for the theory, and to appreciate some of its paradoxical consequences, than it is in the quantum theory of matter. The applications of the light-quantum hypothesis, as it was first called, also provide key pieces of evidence for the quantization of energy in atoms and molecules. Studies of the absorption and emission of radiation—the field of spectroscopy—and of the effect of light on chemical reactions—photochemistry—are very important areas of modem chemistry, in which the quantum nature of radiation is crucial. 

In 1900 Max Planck proposed a solution to the problem of black-body radiation described above. He suggested that when electromagnetic radiation interacts with matter, energy can only be absorbed or emitted in certain discrete amounts, called quanta. Planck s theory will not be described here, as it is highly technical. In any case, Planck s proposal was timid compared with the theory that followed. He supposed that quanta were only important in absorption and emission of radiation, but that otherwise the wave theory did not need to be modified. It was Einstein who took a more radical step in 1905 (the year in which he published his first paper on the theory of relativity and on several other unrelated topics). Einstein s analysis of the photoelectric effect is crucial, and has led to a complete change in the way we think of light and other radiation. 

Absorption and Emission of Radiation in Ordinary Molecular Fluids (Equilibrium Systems)... 

The Kinetics of Absorption and Emission of Radiation.—With Bohr s picture of the relation bet ween energy levels and discrete spectral lines in mind, Einstein gave a kinetic derivation of the law of black-body radiation, which is very instructive and which has had a great deal of influence. Einstein considered two particular stationary states of an atom, say the ith and jth (where for definiteness we assume that the ith lies above the jth), and the radiation which could be emitted and absorbed in going between those two states, radiation of frequency vi7, where... 

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