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PHOTOCONDUCTIVITY OF PHOTOCHROMIC Gd:CaF2
1.D. L. Staebler and Z. J. Kiss, Appl. Phys. Letters (to be published).
2.The fact that these rare‐earth‐doped calcium fluroides did not behave in stable ways was first noted by Anderson and Kiss when they did a survey of the photoconductivity of the rare earths in The rare earths that were later found to be photochromic yielded irreproducible photoconductivity spectra which of course, depended on the state the material was in. C. H. Anderson and Z. J. Kiss, Bull. Am. Phys. Soc. 9, 87 (1964);
2.and (private communication).
3.Additive coloration or chemical reduction is brought about by baking the sample in calcium vapor. F. Luty, Z. Physik 134, 596 (1953).
4.The data in Fig. 1 are presented in photocurrent per incident photon form. For the “T” state absorption both the photocurrent per incident photon and photocurrent per absorbed photon appear to be exponential because the photocurrent is a much stronger function of energy than is the absorption and the scale used makes the difference appear even smaller. For the “L” state there is a problem because the total absorption is small and there is a strong overlap between active and inactive bands which makes the pertinent value for the absorption difficult to obtain. It is presently thought, however, that the curves are exponential in nature on a per absorbed photon basis.
5.There is circumstantial evidence that the carriers move through the conduction band rather than the valence band. The evidence is that from optical measurements made by D. L. Staebler (private communication) there is a greater concentration of in the T state and a greater concentration of in the L state. The electron added to the in the switching process can either be excited from the valence band directly or it can be excited to the conduction band from another impurity and then trapped by the No absorption band that can be interpreted as a valence band to transition has been found. The implication is then that the conduction band is involved. Since Ce and Gd are similar systems it is expected that conduction is via the conduction band in also.
6.W. B. Fowler, Phys. Rev. 135, A1725 (1964).
7.D. L. Staebler, S. E. Schnatterly, and W. Zernik, IEEE J. Quantum Electron. QE‐4, F‐7 (October 1968).
8.D. S. McClure and Z. J. Kiss, J. Chem. Phys. 39, 3251 (1963).
9.H. A. Weakliem and Z. J. Kiss, Phys. Rev. 157, 377 (1967).
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