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Orbit, evel,, , , , r, , , , Peo Metastable state 3, We have seen that When g, , er ener State. ae cl 3, , B JOWST SHEUY State By to q pore) IE given tolan electron tn ert mee, EER, , ime of about 1078 5. In Certain sub; nce Te bey rere an memes =, ate Bim For the atom which is beiwer’’ Like Ruby crystal there is an intermediate enctBY, a electron from the energy Sue ee the energy levels onde, Thisae, jumps” to the energy state Py "2 does not jump" directly back to the energy state Ps, mm ics ae cide ter electron can Temain in this energy level Em for a, , an in By i, , grate. that in Eo. This Shergy state IS eallesi eta, | Seer, optical pumping an i, ana ig and Population inversion, , At ordinary temper:, , Uitable ¢, , ow), , ature the, , number of, pulation of upper energy st of ele, , ee ctrons in the higher energy state, 1.é., the, ates, in, , atomic system is considerably smaller than that iu, ground state, , an, , , , west energy state, /,, , e., the, , Consider a material whose, state (ii) Ee, table state in between F., , the lo, , “toms can reside in three different states (i) E1, the ground, , Mm which atom can reside only for 10~® s and (iii) Em, the, 1 and 5 in which the atom can reside for about 10-° s. et, = nergy, hy = Ep - }, is incident on the atoms which are in, the ground state. The atoms are raised from the ground state Fy to the excited state Ep., , : ack to Ey as in the case of or dinary atoms. They “decay, to the metastable state F,,.. In this metast;, , the life time of E;,, is much longer than, , , , in excited state, , metas, sume that photons of e, , , , us as, , , , These excited atoms do not fall b;, , able state the atoms remain for longer time. Since, , , the atoms reach the state Ey, much faster than, m contains more atoms than the state Hy. This, is known as population inversion. The process by which it is obtained is called optical, pumping, , , , they leave the state E,,. Thus, the state B, , (vi) Emission of radiation, , Once population inversion has been reached, the atoms in metastable state Hm are, , bombarded by photon of energy, hy = B,, — F resulting in a stimulated emission giving, an intense coherent beam in the direction of the incident photons., , , , \V Ns, optical, , pumping, , , , absorption __ stimulated emission, , Fig. 4