Enhancement of theF- andV-Bands in Sodium Chloride Containing Calcium

Abstract

906 LETTERS TO THE EDITOR 71 ! U I I I u-L_ 0.0 OJ 0.2 0.3 0.4 Thickness (cm) FIG. 2. Transmission of Te crystals as a function of thickness for a few values of X.absorption constant (cm" 1 ), and x is the thickness in cm. The resulting values of k are shown in Fig. 3. The interpretation of these results suggests first of all that the increasing absorption beyond the edge toward shorter X is due to electrons making the transition from the filled to the conduction band. If we locate the absorption edge which occurs for the larger value of X at 3.6ju by associating it with the gap width as deter-FIG. 3. Experimental values of the absorption constant of Te crystals as determined from the slopes of the lines in Fig. 2.mined by conductivity measurements, i.e., 0.34 ev, 4 the criterion so established places the second edge at 3.3/z. This is an arbitrary method of locating the edge, since the values as determined by absorption and conductivity can be expected to agree only at low temperatures. Secondly, as in germanium and silicon, 5 the value of k as determined by assuming only free carrier absorption beyond the edge toward longer X is smaller by a factor of 10 3 than the experimental values. Finally, the index of refraction, n, as computed from the expressionis about 5.3±0.2 at 6/x, which is somewhat larger than the value found by Moss in tellurium films. It was impossible to detect any difference in the values of n for the two directions of polarization by this method. Preliminary measurements on prisms of crystalline tellurium yield values of n of this same magnitude.

New oxygen centres in KCl single crystals

“…The presence of the O(1) centre leads to a marked increase in the growth rate of F centres during the first stage of colouration. Similar increases have been observed for KC1 crystals doped with OH- [16] and H- [17] and also for divalent-cation-doped crystals [17 to 191. When an X-irradiated O(1)-doped crystal is exposed to light in the F band region, the decay of the F band is accompanied by the simultaneous decay and growth of several absorption bands in the uv region in a complex pattern.…”