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 $nh\nu \ge \Phi$, 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.