Comet stimulated emission. When a comet approaches the

Chapter , Problem 66

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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 H2O molecules in the vapor to H atoms and OH molecules. The sunlight can also excite the OH molecules to higher energy levels. When the comet is still relatively far from the Sun, the sunlight causes equal excitation to the E2 and E1 levels (Fig. 40- 28a). Hence, there is no population inversion between the two levels. However, as the comet approaches the Sun, the excitation to the E1 level decreases and population inversion occurs. The reason has to do with one of the many wavelengthssaid to be Fraunhofer linesthat are missing in sunlight because, as the light travels outward through the Suns atmosphere, those particular wavelengths are absorbed by the atmosphere. As a comet approaches the Sun, the Doppler effect due to the comets speed relative to the Sun shifts the Fraunhofer lines in wavelength, apparently overlapping one of them with the wavelength required for excitation to the E1 level in OH molecules. Population inversion then occurs in those molecules, and they radiate stimulated emission (Fig. 40- 28b). For example, as comet Kouhoutek approached the Sun in December 1973 and January 1974, it radiated stimulated emission at about 1666 MHz during mid-January. (a) What was the energy difference E2 # E1 for that emission? (b) In what region of the electromagnetic spectrum was the emission?