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Oscillator strength (or f-value) is a fraction expressing the contribution of quantum mechanics to the probability that a particular atom absorbs an approaching photon, Each possible transition (specific pair of electron shells) has a unique oscillator strength, e.g., that for the Lyman alpha transition is 0.416.
Absorption cross section and absorption coefficient are other values that characterize the probability of absorption. They incorporate the oscillator strengths associated with a medium's chemical constituency, as well as the states of excitation, states of ionization, etc.