Phonon spectroscopy of tunneling states in NaCl : <f>OD−</f>

It is known since long that <f>OH−</f> and <f>OD−</f> defects form tunneling states with <f>〈1 0 0〉</f> symmetry in certain alkali halide crystals. Surprisingly, recent measurements of the dielectric susceptibility and of the infra-red absorption indicated that the tunnel splitting of the heavier isotope <f>OD−</f> is larger by a factor of 1.4 compared to that of <f>OH−</f>, although one would expect the opposite behavior from the difference in mass of hydrogen and deuterium. These surprising findings are yet only shown for those levels accessible in dielectric experiments. We have investigated NaCl:<f>OD−</f> by means of phonon spectroscopy with superconducting tunnel junctions in order to obtain additional information on the level scheme of <f>OD−</f> tunneling states in NaCl. Since the selection rules pertinent to dielectric measurements are different from those in elastic experiments, it becomes possible to study different transitions within the level scheme. Indeed, we find a resonance, which clearly can be attributed to <f>OD−</f> defects. A comparison with earlier measurements by Kinder and Windheim on <f>OH−</f> in NaCl leads to the conclusion that the anomalous isotope dependence is also found for the levels which are involved in phonon transitions, which confirms that the isotope effect is indeed connected with the tunnel splitting. [Copyright &y& Elsevier]