# Nondegenerate two-photon absorption rate

1. ### kenkhoo

8
Hey guys,

a quick one. I've noticed that you can compute the two photon absorption coefficient by
$\beta = \frac{2 \hbar \omega}{I^2}W$
where W is the transition rate for TPA. (wiki link)

Now my question is what about nondegenerate two photon absorption? by that i mean two different frequencies of light being absorbed, what will be the equation linking beta and W?