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A model is presented which describes the optical modes of a mixed crystal. The model emphasizes a realistic use of the local electric field and is completely defined by the macroscopic parameters of the pure end members. The concentration dependence of the optical frequencies can be categorized into four distinct classes in contrast to the usual two classes, "one-mode" and "two-mode". We suggest that experimental evidence already exists which supports this new classification scheme.Es wird ein Modell behandelt, welches die langwelligen optischen Phonon-Moden von Mischkristallen beschreibt. Das Modell beinhaltet eine realistische Behandlung des lokalen elektrischen Feldes und ist vollstandig durch die makroskopischen Parameter der reinen Endkristalle definiert. Die Konzentrationsabhangigkeit der optischen Frequenzen zeigt eine Unterscheidung in vier unterschiedliche Klassen im Gegensatz zu den ublichen zwei Klassen, namlich den Ein-Moden-und Zwei-Moden-Mischkristallen. Es wird gezeigt, daB bereits experimentelle Evidenz fur das neue Klassifikations-Schema existiert.

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SynopsisNew techniques in laser Raman spectroscopy are used to obtain spectra of aqueous solutions of lysozyme for frequency shifts as small as 5 ern-'. In addition, Raman measurements are made on two crystalline forms of hen egg white lysozyme. The spectra obtained from the solution and from the crystal are found to be similar for frequencies above 100 cm-'. However, a low-frequency band at 25 cm-' observed in crystalline lysozyme is not found in the solution, indicating that this band cannot be attributed to an internal molecular vibration.

Far-infrared measurements, between 3 and 450 cm ', of absorption spectra of highly oriented films of Li-DNA and Na-DNA in the temperature range 5 -300 K are reported. Five lowfrequency infrared-active vibrational modes are observed. The lowest infrared-active mode at 45 cm ' for Li-DNA and 41 cm ' for Na-DNA, as observed for the first time, is found to soften upon sample hydration. Studies of the hydration-induced absorption at the low-frequency end of the spectrum also show a pronounced absorption band at about 10 cm ' that is attributed to relaxation processes. Based on a simple lattice-dynamics model, the description of low-lying vibrational modes of DNA is presented. Eigenvectors of the lowest infrared-active and Raman-active modes are calculated. In addition, the model semiquantitatively describes the influence of hydration on mode frequencies.