Comet stimulated emission. When a comet approaches the Sun, the increased warmth evaporates water from the ice on the surface of the comet nucleus, producing a thin atmosphere of water vapor around the nucleus. Sunlight can then dissociate $\mathrm{H}_2 \mathrm{O}$ molecules in the vapor to $\mathrm{H}$ atoms and $\mathrm{OH}$ molecules. The sunlight can also excite the $\mathrm{OH}$ molecules to higher energy levels.
When the comet is still relatively far from the Sun, the sunlight causes equal excitation to the $E_2$ and $E_1$ levels (Fig. 40.7a). Hence, there is no population inversion between the two levels. However, as the comet approaches the Sun, the excitation to the $E_1$ level decreases and population inversion occurs. The reason has to do with one of the many wavelengths-said to be Fraunhofer lines-that are missing in sunlight because, as the light travels outward through the Sun's atmosphere, those particular wavelengths are absorbed by the atmosphere.
As a comet approaches the Sun, the Doppler effect due to the comet's speed relative to the Sun shifts the Fraunhofer lines in wavelength, apparently overlapping one of them with the wavelength required for excitation to the $E_1$ level in $\mathrm{OH}$ molecules. Population inversion then occurs in those molecules, and they radiate stimulated emission (Fig. 40.7b). For example, as comet Kouhoutek approached the Sun in December 1973 and January 1974, it radiated stimulated emission at about $1666 \mathrm{MHz}$ during mid-January. (a) What was the energy difference $E_2-E_1$ for that emission? (b) In what region of the electromagnetic spectrum was the emission?
A ( FIGURE CAN'T COPY )
B( FIGURE CAN'T COPY )