ON THE POSSIBLE USE OF OPTICAL ROTATION MEASUREMENTS FOR DETECTING WEAKLY IR ACTIVE MODES

Abstract
IR activity of normal modes of a crystal is usually investigated through reflection or transmission measurements. We propose here another way which should be useful for studying modes of weak IR activity. The method involves measuring the rotation angle θ, of the plane of polarization of a monochromatic beam of frequency ω, as the beam traverses a crystal of thickness d. By studying θ as a function of incident frequency ω one would observe a 'resonance' close to each IR active frequency ωj. The method would be feasible if there is appreciable transmission of the beam that is the oscillator strength of the mode is small. We suggest a system, namely, LIKSO₄, where this idea could be applied. In particular we discuss how the symmetric stretching mode of SO₄ which is IR inactive for a free ion but is weakly IR active in this crystal, could be investigated using different wavelengths of a CO₂ laser. Potentiality of the proposed technique for investigating phase transitions, wherein certain Raman active modes acquire weak IR activity across the transition are mentioned.