PHOTOELECTRIC EFFECT (PEE)
Generally, when X-ray photon of say 100Kev photon is incident on lithium, NO PEE will take place as electron in Lithium atom is nearly free w.r.t X-ray photon energy so there is no Photo electric absorption probability. On the contrary, if same X-ray photon is made to incident on high Z material say lead, then definitely PEE will take place as the electron is in bound state w.r.t energy of X-ray photon. Similarly, when UV photon is made to incident on Li atom PEE will take place as w.r.t UV photon energy the electron in Lithium is in bound state.
Thus, we conclude that photoelectric effect takes place for bound electron but not for free electrons
REASON: It is because for photo electric effect to take place the CONSERVATION OF MOMENTUM IS MUST. And also conservation of SPIN should be there. So, for bound electron the momentum conservation takes place easily.
But the momentum is not conserved for free electrons resulting into effectively zero cross section for absorption of photon for photo electric effect to take place in case of free electrons.
Moderation Team
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