= Multiphoton photoemission
The single-<photon> <photoelectric threshold frequency> is a regime-dependent statement. In <multiphoton photoemission>, multiple <photons> can supply the escape <energy> in one emission process. An ideal $n$-<photon> <energy> balance would instead allow $K_{\max}=nh\nu-\Phi$ when $nh\nu\geq\Phi$, before additional losses or field effects. Thus increasing <light intensity> below the single-<photon> threshold cannot open that single-<photon> channel, but can make other channels relevant. The simple balance specifies neither their rates nor the illumination needed for them to dominate. Heating can also change the surface and emission mechanism. These cases require a different model from independent single-<photon> emission at a fixed <work function>.
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