Ratio of the maximal (typically the initial) rate of photoadsorption of entities \(\ce{B}\) (molecules, atoms, or ions) on a surface to the incident (superscript 0, zero) photon flux, \(q_{\rm{p},\lambda}^{0} = {\rm{d}}N_{\rm{p},\lambda}^{0}/{\rm{d}}t\) at a given wavelength. Both rates should be measured under otherwise identical conditions (preferably as initial rates) in order to avoid concentration and/or surface area effects.