Application of doubled-angle phase correction method to time-resolved step-scan FT-IR spectra

To increase the sensitivity with which time-resolved Fourier transform infrared (FT-IR) difference spectra are measured, the detector is often AC-coupled. Thus, the measured interferograms correspond to spectra with both positive and negative intensities. The presence of signed intensities presents problems for the standard Mertz and Forman phase correction methods. The Mertz Signed phase correction method was designed to handle signed intensities, but the smoothing inherent in calculating the phase angles at reduced resolution introduces other errors in AC-coupled spectra produced with this algorithm. These errors are evident as signal remaining along the imaginary axis after phase correction. A new approach to phase correction, the Doubled-Angle method, can directly correct the phases of transient AC-coupled spectra without the need for a DC interferogram [M.S. Hutson, M.S. Braiman, Appl. Spectrosc. 52 (1998) 974]. When this method was applied to the transient AC interferograms measured after photolysis of bacteriorhodopsin, the signal was fully rotated onto the real axis following phase correction. Here, we show that the Doubled-Angle method can be applied to time-resolved difference FT-IR spectra of halorhodopsin, a more demanding biological system due to its intrinsically small differential absorption signals.