An improved linear retarder for time-resolved circular dichroism studies

Time-resolved natural and magnetic circular dichroism measurements on the nanosecond to second time scale have been used to investigate many chemical and biological processes. The key element that makes these measurements possible is a simple and inexpensive variable linear retarder (a strain plate) which produces well-characterized, elliptically polarized light throughout a broad spectral range (180-1500 nm). It essentially consists of a fused silica plate of high optical quality appropriately compressed from the edges to produce similar to 1 degrees of retardation. Early designs that consisted of a square plate compressed with wedges have worked satisfactorily in the past, but their use was complicated by inhomogeneity of strain across the probe beam (as large as 6 mm diameter for white light measurements with a flashlamp) and a temperature dependence of the strain. An improved strain plate that consists of a round fused silica plate compressed with springs is presented, and its performance is evaluated with respect to earlier designs. This improved strain plate not only simplifies time-resolved circular dichroism measurements but also improves the quality of the data and the kinetic resolution of intermediates. (C) 1997 American Institute of Physics.