Population inversion
المؤلف:
Peter Atkins، Julio de Paula
المصدر:
ATKINS PHYSICAL CHEMISTRY
الجزء والصفحة:
496
2025-12-08
80
Population inversion
One requirement of laser action is the existence of a metastable excited state, an excited state with a long enough lifetime for it to participate in stimulated emission. Another requirement is the existence of a greater population in the metastable state than in the lower state where the transition terminates, for then there will be a net emission of radiation. Because at thermal equilibrium the opposite is true, it is necessary to achieve a population inversion in which there are more molecules in the upper state than in the lower. One way of achieving population inversion is illustrated in Fig. 14.28. The molecule is excited to an intermediate state I, which then gives up some of its energy non radiatively and changes into a lower state A; the laser transition is the return of A to the ground state X. Because three energy levels are involved overall, this arrangement leads to a three-level laser. In practice, I consists of many states, all of which can con vert to the upper of the two laser states A. The I ← X transition is stimulated with an intense flash of light in the process called pumping. The pumping is often achieved with an electric discharge through xenon or with the light of another laser. The con version of I to A should be rapid, and the laser transitions from A to X should be relatively slow. The disadvantage of this three-level arrangement is that it is difficult to achieve population inversion, because so many ground-state molecules must be converted to the excited state by the pumping action. The arrangement adopted in a four-level laser simplifies this task by having the laser transition terminate in a state A′ other than the ground state (Fig. 14.29). Because A′ is unpopulated initially, any population in A corresponds to a population inversion, and we can expect laser action if A is sufficiently metastable. Moreover, this population inversion can be maintained if the A′←X transitions are rapid, for these transitions will deplete any population in A′ that stems from the laser transition, and keep the state A′ relatively empty.
Fig. 14.28 The transitions involved in one kind of three-level laser. The pumping pulse populates the intermediate state I, which in turn populates the laser state A. The laser transition is the stimulated emission A →X.
Fig. 14.29 The transitions involved in a four level laser. Because the laser transition terminates in an excited state (A′), the population inversion between A and A′ is much easier to achieve.
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