Circular Dichroism Spectroscopy Using Coherent Laser-Induced Thermal Gratings

A new pulsed four-wave mixing technique for the detection and real-time measurement of circular dichroism (CD) in liquid samples is demonstrated. The technique is based on the formation and detection of transient thermal gratings formed by the interference of two laser beams whose polarizations are controlled and modulated using a photoelastic modulator. Through an internal heterodyne process, coherent thermal gratings interfere to greatly enhance a weak circular dichroism signal. By measuring the ratio of the difference of scattered laser light from two different polarizations, ΔS, to the average amount of scattered light, S ave, one can determine the value of Δε/ε for the compound under investigation. Samples of chiral camphorquinone are used to demonstrate the technique in the ultraviolet region of the spectrum. These studies of camphorquinone, which has a value of Δε/ε ≈ 10-3 at 266 nm, produce values of ΔS/S ave which are orders of magnitude larger than Δε/ε and which approach 2 (the mathematical limit for ΔS/S ave). Possible extensions of the technique for measuring CD in very small sample volumes and monitoring time-dependence are discussed.