Dipole radiation, definition

In the classical theory of scattering (Cohen-Tannoudji et al. 1977, James 1982), atoms are considered to scatter as dipole oscillators with definite natural frequencies. They undergo harmonic vibrations in the electromagnetic field, and emit radiation as a result of the oscillations. 

There have been many studies of high-energy radiation effects upon liquids and solution of organic systems. In many of these, energy transfer by the long-range dipole—dipole interaction may be important to some degree. Since the properties of most of these systems remains uncharacterised, it is difficult to be more definitive, but the articles by Mullin et al. [175], Miyazaki [196], Wada and Hatano [177], Jonah et al. [178], and Katsumura et al. [179] are all of possible interest and concern. 

Within the density-matrix formalism (Vol. 1, Sect. 2.9) the coherent techniques measure the off-diagonal elements pab of the density matrix, called the coherences, while incoherent spectroscopy only yields information about the diagonal elements, representing the time-dependent population densities. The off-diagonal elements describe the atomic dipoles induced by the radiation field, which oscillate at the field frequency radiation sources with the field amplitude Ak(r, t). Under coherent excitation the dipoles oscillate with definite phase relations, and the phase-sensitive superposition of the radiation amplitudes Ak results in measurable interference phenomena (quantum beats, photon echoes, free induction decay, etc.). 

If the angle between the absorption dipole direction and that of emission is varied to best fit the experimental data, the radiation patterns in Figs.4.12,13 can be fit to within a few percent. However, a definitive test of theory and experiment requires a prior determination of a in (4.18). 

Homonuclear diatomic molecules do not absorb or emit radiation due to vibrational state transitions. By definition, homonuclear diatomic molecules have no dipole moment and no changing dipole moment as the two atoms vibrate. Other methods are necessary to observe vibrational energy levels directly. 

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