**Oscillator strength** When a atom or molecule absorbs light it can undergo a transition from one quantum state to another. The oscillator strength is a dimensionless quantity to express the strength of a transition.

The oscillator strength f12 of a transition from a lower state |1m

_{1}> to an upper state |2m

_{2}> may be defined by:

f12 = c(E

_{1}-E

_{2})∑

_{m2}∑

_{α}|<1m

_{1}|R

_{α}|2m

_{2}>|²

where

c = a constant

E

_{1} = the energy of state 1m

_{1} E

_{2} = the energy of state 2m

_{2} ∑

_{m2} = the sum over all degenerate states 2m

_{2} ∑

_{α} = the sum over α=x,y,z

R

_{α} = R

_{x},R

_{y},R

_{z} R

_{x} = ∑

^{N}_{i=1} R

_{x} = the sum of all the x coordinates of the N atoms in the system.

The oscillator strength is related to the transition dipole moment:

http://www.oraxcel.com/cgi-bin/yabb2/YaBB.pl?num=1217361779 __Source:__ http://en.wikipedia.org/wiki/Oscillator_strength