STUDY OF 2ND-ORDER EXCITATIONS IN ALPHA-IODIC ACID CRYSTAL BY MEANS OF POLARITON K-SPECTROSCOPY

Polariton spectroscopy was used for studying the dispersion of linear and non-linear susceptibilities of an alpha-iodic acid crystal in the vicinity of second-order vibrations (wavenumber range 1000-1200 cm-1). The measurements were carried out by means of k-spectroscopy (spectroscopy in the space of wavenumbers). Here k-spectroscopy was applied to the resonant case when the polariton frequency lies close to one of the lattice resonances and therefore the angular line shape of scattering is not Lorentzian. To measure the crystal parameters at this frequency, the angular spectrum was analysed and a procedure was developed for angular line shape processing that allows the following values to be calculated: the real and imaginary parts of the dielectric function; the real and imaginary parts of the quadratic susceptibility; and the imaginary part of the cubic susceptibility. All these parameters were measured for an alpha-HIO3 crystal in the range containing a few second-order excitations and also a first-order vibration of the OH group at 1160 cm-1. The results allow some conjectures to be made regarding the nature of the second-order excitations.