Electric quadrupole Electromagnetic radiation

Electric-quadrupole transition, 123,127 Electromagnetic radiation, 114-117. See also Radiation, electromagnetic Electromagnetic spectrum, 115 Electronic energy, 57,64,148 Electronic spectra, 130, 296-314 of diatomics, 298-306 and molecular structure, 311 of polyatomics, 71-72, 73, 75, 306-314 selection rules for, 297-301, 306-307 Electronic structure of molecules, 56-76 Electron spectroscopy for chemical analysis (ESCA), 319-320 Electron spin resonance (ESR), 130, 366-381... 

The probability of a transition being induced by interaction with electromagnetic radiation is proportional to the square of the modulus of a matrix element of the form where the state function that describes the initial state transforms as F, that describing the final state transforms as Tk, and the operator (which depends on the type of transition being considered) transforms as F. The strongest transitions are the El transitions, which occur when Q is the electric dipole moment operator, — er. These transitions are therefore often called electric dipole transitions. The components of the electric dipole operator transform like x, y, and z. Next in importance are the Ml transitions, for which Q is the magnetic dipole operator, which transforms like Rx, Ry, Rz. The weakest transitions are the E2 transitions, which occur when Q is the electric quadrupole operator which, transforms like binary products of x, v, and z. 

One of the most important aspects in both experimental and theoretieal studies in molecular spectroscopy is, undoubtedly, the characterization of intensities induced by electromagnetic radiation. We are, of course, interested in obtaining information concerning infrared and Raman transitions which are driven by electric dipole and quadrupole operators, respectively. These transitions can be represented as... 

They quanta may carry away different angular momenta (L). IfL= 1,2,3,... the radiations are called dipole, quadrupole, octupole, etc., respectively. Gamma radiation with L = 0 does not exist because the electromagnetic waves have transversal nature (the photons have spin 1). Each multipolarity 2 is characterized by a specific angular distribution. The radiation may be electric or magnetic, depending on the term of the electromagnetic interaction that is responsible for the particular transition. 

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