~) Solid State Communications, Vol. 77, No. 3, pp. 199-205, 1991. 0038-1098/9153.00+.00 Printed in Great Britain. Pergamon Press plc "ANOMALOUS" EXCITATION IN HYDROGEN-BONDED MOLECULAR CRYSTALS -- A RAMAN SCATTERING STUDY OF SPECIFICALLY DEUTERATED ACETANILIDE (C6D5-CONH-CD3) J.L. Sauvajol, G. De Nunzio*, R. Almairac, J. Morel and M. Barthds Groupe de Dynamique des Phases Condensdes (U.A.C.N.R.S 233) Universitd Montpellier II, Sciences et Techniques du Languedoc Place E. Bataillon 34095 MONTPELLIER Cedex 5 #~i~ (Received by J. Joffrin,/May, 1990) (Revised form 18 October 1990) The focus of experimental and theoretical works about crystalline Acetanilide has been the =anomalous" temperature-dependent ir absorption and Raman peaks at about 1650 cm "1 and the multiband structure in the N-H stretch region. A lively discussion about the assignment of these "anomalous" bands has arisen and is still in progress. The present Raman experiments should be placed in this context as an attempt to identify the molecular degrees of freedom which originate the "anomalous" bands. In this aim Raman experiments have been performed on specifically deuterated Acetanilide [C6D5-CONH-CD3] single crystal in the low-frequency (phonon) and C=O stretching regions. On cooling a distinct band at about 1495 cm "1 increases in intensity. We assign this peak to the equivalent of the 1650 cm "1 band in Acetanilide. The temperature dependence of this Raman line was studied. The results are discussed in the light of the models proposed to explain the anomalous behaviour of the 1650 cm "1 Raman line in Acetanilide. 1. INTRODUCTION Crystalline Acetanilide is an orthorhombic crystal (space groupe Pbca) characterized by soft hydrogen bonded chains of molecules which run through the crystal parallel to the b axis 1, Acetanilide is an anharmonic solid with a one-dimensional character, thus it is a likely candidate in which to find non-linear excitations which can be studied in the framework of highly ordered crystals 2. On the other hand, the nearly-planar amide groups in Acetanilide display bond distances which are close to those found in polypeptides. So Acetanilide has been proposed as a relevant model system for the investigation of physical phenomena in natural polypeptides3. In this context much interest has been focused on the anomalous temperature dependence of the I.R and Raman peaks at about 1650 cm "1 in Acetanilide (C6H5-CONH-CH3). On cooling, a distinct band at * Permanent adress: Universit~ di Lecce, Dip. Fisica, C/O INFN, Via per Arnesano Lecce, Italy 1650 cm "1 with shift to lower frequencies of about 15 cm "1 from the C=O stretching mode increases in intensity 3 A lively discussion about the assignment of this "anomalous" band has arisen and is still in progress. Several models have been proposed to explain the origin of this band: trapped Davydov soliton state 3,4 polaron state s,s topological soliton 7, coupling between hydrogen-bonded proton motion and low-frequency vibrations s and Fermi resonance 9 Recently the temperature dependence of the Raman scattering from a single-crystal of fully deuterated Acetanilide (C6D5-COND-CD3) in the low-frequency (phonon) and C=O stretching regions was studied lo. Two results were emphasized: a) No anomalous band was detected in the C=O frequency range upon cooling. The same result was observed in C6H5-COND-CH 3 9 b) At low-frequency a mode around 106 cm -1 at T=80 K (mode D' in Ref. 10) with a singular 199