Abstract
Transition probabilities of a molecular system between different energy states, under the effects of a radiation field, are investigated by quantum-mechanical methods. Resonance conditions for absorption transitions and stimulated emission of radiation are found and discussed. Transitions are mostly due to the interaction of the field with the molecular electric dipole, but transitions involving the molecular magnetic dipole and higher order electric multipoles, as the electric quadrupole, can also occur. Selection rules related to the molecular geometrical symmetry are discussed. Einstein coefficients and line strengths are defined, and related to commonly measurable physical properties, as the integrated absorption coefficient. Spontaneous emission of radiation, with the relative Einstein coefficients, is finally discussed.
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