IMPURITY [NDUCED FAR INFRARED ABSORPTION OF CsI CRYSTALS L.GENZEL, W. PRETTL and E.SIEP
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چکیده
In perfect alkali halide crystals only the transverse optic phonons of zero wave vector absorb far infrared radiation in first order. The introduction of impurities in the lattice removes the translational invariance and almost all phonons become optically active. In addition to the absorption at the reststrahlen frequency, impurity induced absorption appears in the phonon band region and sometimes outside the bands as localized or gap modes [l]. In frequency regions with high density of states the coefficient of the impurity induced absorption is correlated to the density of phonon states in the perturbed crystal, which for low defect concentrations is nearly that of the host lattice. Thus this additional absorption gives information about the density of phonon states [2]. Besides this, weakly bound impurities can cause resonant band mode absorption in the low density region of the acoustic bands [3]. This paper deals with the far infrared absorption in the phonon bands of CsI doped with Li+, Na+, K’, Rb+ and Tl+. The single crystals were grown from u.p. material by the Czochralski-technique in air. The impurity concentration of the Tl+ doped crystals was determined by colorimetric chemical analysis [4]. In the other cases only the contents of impurities in the melt are quantitatively known. Their presence in the crystals was established by flame spectroscopy. The crystals were mounted in a cryostat and cooled by a cold finger or by helium exchange gas, which was in thermal contact with liquid helium or nitrogen. The temperature was measured with an Allen Bradley carbon resistor. The additional absorption was determined by comparison of the transmission of a doped and an undoped sample of equal thickness. The transmission measurements were performed by a com-
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تاریخ انتشار 2002