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why potassium and ceasium are used as electrodes in photoelectric cell..and not lithium and sodium?

Potassium and Cesium have low ionization energies. So, a low energy photon can also emit an electron from their atoms. Thus, they are used in photoelectric cells. Sodium and lithium have larger ionization energies as they are smaller in size and have higher effective nuclear charge. So, removing an electron from them requires higher energy and hence they are not used in photoelectric cells.

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 K and Cs has very low ionisation energy due to there large size and less effective nuclear charge. Due to this property, they are used as electrodes in photoelectric cells as they readily emit out electrons on striking of photons of threshold energy.

But, Na and Li are not used for this purpose as they have comparatively large ionisation energy due to their small sizes in the group.

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Pottasium (K) and Caesium(Cs) have much lower ionization enthalpy than that of Lithium and Sodium . As a result these metals on exposing to light lose electron much more readily and easily than Lithium and Sodium hence, Pottasium and Caesium rather than Lithium and Sodium are used in photoelectric cell.

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