DIRECT PHASE CORRECTION OF DIFFERENTIAL FT-IR SPECTRA

Step-scan transient Fourier transform infrared (FT-IR) difference spec tra are often measured in an ac-coupled configuration. The resulting d ifferential intensity spectra contain both positive and negative bands . This condition poses problems for direct phase correction by the sta ndard Mertz and Forman methods. Restricting the calculated phase angle to the range [-pi/2, pi/2] was previously shown to fix some of these problems, but we show that the use of a reduced-resolution phase spect rum can produce other artifacts. The effect of reduced resolution is a nalyzed for a simulated noise-free spectrum and for a measured transie nt spectrum of a real photochemical system, bacteriorhodopsin. Examina tion of these results reveals that the Mertz and Mertz Signed methods can produce spectral bands of reduced magnitude and unusual band shape , with considerable amounts of intensity remaining along the imaginary axis after phase correction. However, these errors can be eliminated by self-convolution of the measured interferogram, which doubles all p hase angles, prior to smoothing. This procedure removes the potential discontinuities in the phase angle due to sign changes in the differen tial spectrum. With bacteriorhodopsin, this doubled-angle method for d irect phase correction is able to produce a transient spectrum which c losely matches that produced by using a separately measured de interfe rogram to calculate the phase angle.