|Abstract:||The Hall mobility of electrons released from F centers was measured in several different alkali halides in the temperature range of 7°K to 100° K using an improved technique. In KBr, the mobility increases from about 100 cm2/volt sec at 85°K to as high as l5,000 cm2/volt sec at 6°K. The mobility is primarily due to optical mode scattering between 85°K and 40°K with a Debye temperature of about 240°K. The mobility between 40°K and 25°K appears to be influenced by acoustical mode scattering, and below 25°K, the mobility varies approximately as the inverse square root of the F-center density.
If part of the F centers are converted to F' and a centers by irradiation in the F band at 120°K, the mobility at 6°K is found to decrease about an order of magnitude. The mobility is found to increase toward the original values as the F1 and α centers are destroyed by bleaching light. Simultansously, the Fn photoresponse if found to first increase to a maximum and then decrease to zero.
A few tests on zone refined KCl gave low temperature mobilities in lightly colored crystals about three times larger than found in Harshaw material.
The temperature dependence of the mobility in KI is quite similar to that found in KBr. Optical mode scattering dominates above 40 0 K with a Debye temperature of 222°K ± 20°K. Acoustical mode effects are present below 40°K, and the mobility at 6°K is about the same as that found in KBr with the same F-center density.
In NaCl, the low temperature mobility is about 1000 cm2/volt sec independent of the F-center density. A steep decrease in mobility begins at about 40°K due to optical mode scattering.