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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 a photon, thereby increasing its state of excitation. Each transition has a unique oscillator strength, e.g., that for the Lyman alpha transition is 0.416.
Absorption cross section and absorption coefficient are other methods of characterizing the probability of absorption. They incorporate the oscillator strengths of associated with a medium's chemical constituency, as well as the states of excitation, states of ionization, etc.