The ionisation enthalpy of lithium is quite high. The photons are not in a position to reject electorns form the surface of lithium metal. Therefore, photoelectric effect is not noticed. However, both potassium and cesium have comparatively low ionisation enthalpies. This means that the electrons can quite easily be ejected from the surface of these metals when photos of certain minimum frequency (threshold frequencey `v^(0)`)strike against their surface.
