FAR INFRARED ABSORPTION OF CsBr AND CsI CRYSTALS DOPED WITH OH- AND OD- W. PRETTL and E. SIEP

Alkali halide crystals doped with OHand ODions exhibit in the spectral range above the two phonon absorption of the fundamental lattice vibrations (200 500 cm-l) several absorption lines, which are explained as transitions between librational levels of the OHand ODion respectively [1,2]. The fundamental librational frequency occurs at about 300 cm-l for OHand 220 cm-l for OD-. This isotope shift corresponds approximately to the square root of the moments of inertia. In addition, the so called “non Devonshire” line at about 30 cm-l has been observed in several alkali halide crystals [3,4]. We have studied the far infrared absorption of CsBr:OH-, ODand CsI:OH-, ODup to 400 cm -l, In the absorption curves of CsBr:OHthe librational line, labelled A in fig. la, appears at 289 cm-l. This line and weaker librational sidebands at 320, 441, 444, and 545 cm-l were reported by Harrison and Ltity [5]. Among these sidebands the line at 320 cm-l was in the frequency range covered by our measurements, and we really found a rather weak sideband, B in fig. la at 314 cm-l, which might be equivalent to that reported in ref. [5]. The CsBr crystal doped with ODshows three lines: A’, B’ and C’ at 212, 228 and 328 cm-l respectively (fig. lb). It also contains OHions, which is indicated by the presence of the OHline A. The line A’ is the main librational line of the ODmolecule corresponding to the line A of OH-. This is concluded from the frequency shift WA/WA? = 1.36, which is in very good agreement with the expected isotope effect WOH-/wOD= i’eOD-/8 1.37 [2]. In the same way the line B at 2j!0-1 can be related to the sideband B at 314 cm-l in CsBr:OH-. The line C’ and the OHlibrational doublet at 441 and 444 cm-l [5] exhibit rather well Cs Br : 0H-~1.3~10~5moL’?A