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 (spectro scopy in the space of wavenumbers). Here k-spectroscopy was applied to the resonant case when the polariton frequency lies close to one of t he lattice resonances and therefore the angular line shape of scatteri ng is not Lorentzian. To measure the crystal parameters at this freque ncy, the angular spectrum was analysed and a procedure was developed f or angular line shape processing that allows the following values to b e 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 seco nd-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.