Multiphoton Absorption and Coherence

We study the probability for multiphoton absorption processes induced by both laser and thermal light acting on an atomic system. When the thermal radiation, suitably filtered, has the same frequency spectrum and the same total energy as the laser packet, we find that the ratio between the relative probabilities for $s$-photon absorption can be expressed by ${(4\ensuremath{\pi})}^{\frac{\ensuremath{-}1}{2(s\ensuremath{-}1)}}s!{s}^{\frac{1}{2}}{(\frac{{\ensuremath{\tau}}_{c}}{\ensuremath{\tau}})}^{s\ensuremath{-}1},$ where $\ensuremath{\tau}$ is the time during which the atomic system is illuminated by thermal radiation with coherence time ${\ensuremath{\tau}}_{c}$. This shows that multiphoton absorption processes depend on the statistical properties of the incident light, and that the probability induced by the laser packet is certainly greater than that induced by the thermal light if $sl\frac{\ensuremath{\tau}}{{\ensuremath{\tau}}_{c}}$.