Stress‐Induced Dichroism of the Vibrational Lines and Phonon Sidebands of SH⁻ Centres in CsBr

Abstract

Paraelastic alignment of SH-centres in CsBr leads t o a stress-induced dichroism of the infrared absorption lines and their phonon sidebands. The principal values of the elastic dipole tensor are determined from quantitative infrared measurements under uniaxial stress. The centres turn out t o be (l00) oriented and have a disk-shaped elastic dipole tensor.The stress-alignment is used to determine the polarization of t h e phonon sidebands with respect to the molecular axis. Those irreducible representations, by which phonons and localized modes mainly couple to the intramolecular vibration, are derived with the help of group-theoretical considerations. Selection rules for the appearance of part,icular phonons in the sidebands are discussed.Paraelastische Ausrichtung von SH--Zentren in CsBr fuhrt zu einem druckinduzierten Dichroismus der IR-Absorptionslinien und deren Phononenseitenbanden. Die Eigenwerte des elastischen Dipoltensors werden aus quantitativen IR-Messungen unter einachsigem Druck bestimmt. Es zeigt sich, daD die Zentren <loo)-orientiert sind und einen scheibenformigen elastischen Dipol haben. Die Ausrichtung der Zentren unter Druck wird benutzt, urn die Polarisationsrichtung der Phononenseitenbanden bzgl. der Molekulachse zu bestimmen. Mit Hilfe von gruppentheoretischen Betrachtungen werden diejenigen irreduziblen Darstellungen ermittelt, mit denen die Phononen und lokalisierten Moden hauptsiichlich a n die intramolekulnre Schwingung ankoppeln. Auswahlregeln fur das Auftreten bestimmter Phononen in den Seitenbanden werden d i s k d e r t .

Electric Field Induced Splitting of the Stretching Vibration Lines of SH−, SeH−, and TeH− Centres in Alkali Halides

“…It is known from paraelectric and paraelastic studies that OH -centres are predomi-nant81y (100) oriented in all alkali halides with NaCl structure with the exception of K I and NaBr (( 110) orientation), whereas they are (111) oriented in CsBr [l]. SH-, SeH-, and TeH-centres, however, are (111) oriented in potassium and rubidium halides (according to the present paper) and are (100) oriented in CsBr [18].…”

“…SH-, SeH-, and TeH-centres, however, are (111) oriented in potassium and rubidium halides (according to the present paper) and are (100) oriented in CsBr [18].…”

“…I n order to explain why no effect occurs in cesium halides, one could assume that the displacements of those nearest neighbour ions in the direction of the molecular axis are decisive for the line shifts. I n the case of cesium halides the centres are (100) oriented [18] and the neighbours in the direction of the molecular axis are anions. They are displaced by the electric field in the same direction as the impurity itself so that no shift of the vibrational frequency occurs.…”