| Abstract | Studies of the resonant interaction of radiation from a liquid‐nitrogen‐cooled ruby laser with a ruby crystal in the temperature range 4.2°–100°K are described. The interaction was measured by observation of the change which occurs in the Cr3+ ground state EPR absorption on laser irradiation. At low radiation intensities, the temperature dependence of this change is shown to be determined by the well‐known thermal tuning behavior of the R lines. Precise measurement of the relative laser and ruby‐sample absorption frequencies by this technique of double‐resonance spectroscopy is described. Under optical saturation conditions, additional temperature‐dependent pumping effects occur at low temperatures. In particular, as the ruby temperature is increased from 4.2° to 40°K, the laser‐induced change in EPR absorption is observed to undergo a large increase. This increase cannot be explained by a relative thermal shift of the laser and absorption frequencies which is known to be very small in this temperature range. The phenomena are attributed in part to the temperature‐dependent homogeneous linewidth which determines the widths of the holes burnt into the inhomogeneous strain‐broadened R lines. Calculations indicate that the related temperature‐dependent thermalization process which redistributes excited Cr3+ among the 2E levels quantitatively accounts for most of the increase. |
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