NARROWING 853 given by Van Vleck 33 ; in fact, there exists a similar, although not nearly so simple relationship between the two as that between the intensity of the sidebands and Van Vleck's form of the second moment.It would be of interest to verify experimentally not only the narrowing of the center line but also the existence of the sidebands. The fact that this has not been possible so far is due to the relatively small intensity of the sidebands. In order to permit their individual 33 See reference 3, Eq. (29).
The advantages of chromium ions in aluminum oxide crystals (ruby) for double resonance experiments have been pointed out 1 ' 2 and utilized in several microwave-optical experiments. 3 " 5 Recently, stimulated emission 6 and coherence 7 of the characteristic "#/' fluorescence at 6943 A has been observed in pink ruby (-0.07% Cr +++ by weight). We wish to report the observation of stimulated emission at wavelengths 7010 A and 7040 A from transitions in red ruby (-0.7% Cr +++ by weight) which arise 8 from exchange coupling between neighboring chromium ions.The observations were made by monitoring the fluorescence of a - §-in. xl^-in. red crystal rod following an intense excitation flash. By virtue of the broad optical absorption bands and the efficient radiationless transfer to sharp excited states, these levels accumulate an excess population which may satisfy the requirements 9 for stimulated optical emission. At high excitation energy the fluorescence at 7010 A and 7040 A exhibits the initial decrease from the natural radiative lifetime which is characteristic of stimulated emission. However, the decay curves show significant differences from the previous results with the R x line. Figure 1 is a comparison of the R v line in pink ruby, the 7010A line in red ruby, and the R x line in red ruby, all under similar conditions of excitation at 77°K. The decay curves at low excitation are all similar in appearance to Fig. 1(c), and in two cases the shapes change progressively with increasing excitation until they reach the curves shown in Fig. 1(a) and Fig. 1(b). A plot of the areas under the fluorescence curves of R x and 7010 A as a function of flash energy is shown in Fig. 2.Our interpretation of the data is as follows: It is apparent that the R x line in red ruby does not exhibit stimulated emission. This follows from the fact that the absorbed flash energy is not sufficient to overcome the ground-state population, and is corroborated by the approximately linear relationship between R x and flash energy in Fig. 2. (The increased linewidth in red ruby would not change conditions enough to prevent stimulated emission from occurring.) The behavior of the 7010A line (and 7040A line) is a little more complicated. The fact that it exhibits enhanced emission while the R x line in this sample does not is a result of the relatively unpopulated terminal state for the 7010A transition (negligible optical absorption for this wavelength at 77°K). Since the number of atoms radiating 7010A light saturates at intermediate energy (Fig. 2), it appears that the total number of equivalent ion pairs which have this line in their spectrum is excited at this energy. This number 1 Millisecond FIG. 1. Fluorescence decay curves of pink and red ruby taken at 77°K under approximately the same high excitation conditions.95
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