<title>Differential Absorption At High Modulation Frequencies Using A Fourier Transform Infrared Spectrometer</title>

Abstract

A general method for the measurement of differential absorption spectra using a Fourier transform infrared spectrometer will be described. The method is based on the rapid modulation of the absorption strength of the sample at a frequency that is significantly higher than the Fourier modulation frequencies. Phase sensitive detection of the detector output at the differential modulation frequency yields an interferogram which represents only the differential spectrum, and this can be processed by the fast Fourier algorithm in the usual way, although care must be taken to secure the proper phase correction for the spectrum. If the polaroization of the infrared beam is modulated between alternate states of circular or linear polarization, the resulting differential spectrum represents circular or linear dichroism, respectively. The differential Fourier intensities could also be generated by directly modulating the sample using some external periodic perturbation such as an electric or magnetic field or intense laser radiation. The method lends itself to the study of kinetic phenomena, where transient species could be detected on the microsecond time scale. An example will be given for this method applied to the measurement of vibrational circular dichroism where the differential signal is over four orders of magnitude smaller than the overall absorption strength.