A process by which a particle in the @[email protected] or an @[email protected] may undergo transition to a higher energy level by @[email protected] of a photon. For a given particle in the lower state of @[email protected] per second of such a transition in a field with a continuous spectrum is proportional to the @[email protected] density of the @[email protected] The proportionality constant is termed the transition @[email protected] for absorption. The transition @[email protected] for @[email protected] is defined in a similar way for the reverse @[email protected] process that is induced by the same radiation field.