in spectrochemistry
Also contains definitions of: transition probability for absorption, transition probability for stimulated emission
https://doi.org/10.1351/goldbook.R05060
A process by which a particle in the
ground state
or an excited state
may undergo transition to a higher energy level by absorption
of a photon. For a given particle in the lower state of probability
per second of such a transition in a field with a continuous spectrum is proportional to the spectral radiant energy
density of the absorption line
. The proportionality constant is termed the transition probability
for absorption. The transition probability
for stimulated emission
is defined in a similar way for the reverse radiative de-excitation
process that is induced by the same radiation field.