I

Metastable state

by d E = d Iv d A dv dm dt in the cylinder in time dt. This equals the emission minus absorption in the cylinder. The absorbed energy is (c.f. (4.14))

where av is the opacity of the medium at frequency v. Let the amount of energy emitted per hertz at frequency v into unit solid angle from unit volume and per unit time be jv ([jv] = Wm—3 Hz-1 sterad-*). This is called the emission coefficient of the medium. The energy emitted into solid angle dw from the cylinder is then

Fig. 5.12. The operational principle of the maser and the laser. A metastable state (a state with a relatively long average lifetime) stores atoms where they accumulate; there are more atoms in the metastable state than in the ground state. This population inversion is maintained by radiatively exciting atoms to a higher excitation state ("pumping"), from which they spontaneously jump down to the metastable state. When the atoms are illuminated by photons with energies equal to the excitation energy of the metastable state, the photons will induce more radiation of the same wavelength, and the radiation is amplified in geometric progression the field, the motion is circular and therefore accelerated. The moving charge will radiate in the direction of its velocity vector. Such radiation is called synchrotron radiation. It will be further discussed in Chap. 15.

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