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Why are potassium and caesium, rather li...

Why are potassium and caesium, rather lithium used in photoelectric cells?

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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.
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Statement-1: Potassium and caesium are used in photo-electric cells. Statement-2: Potassium and caesium emit electrons on exposure to light above certain minimum frequency.