THE EFFECT OF APODIZATION AND FINITE RESOLUTION ON FOURIER-TRANSFORM INFRARED AND RAMAN-SPECTRA

The interferograms measured on Fourier transform infrared (FTIR) and R aman (FT-Raman) spectrometers are routinely apodised before the Fourie r transform is carried out. The effects of finite resolution and diffe rent apodisation functions on synthetic bands of known Lorentz-Gauss c haracter have been investigated. (The apodisation functions chosen wer e those commonly available on commercial FTIR and FT-Raman spectromete rs). The effects are dramatic, the ratio of resolution/(true bandwidth ) required to obtain less than 10% distortion (as judged by the peak h eight and width) in the band varied from 1 for boxcar to less than 0.2 for Norton-Beer (strong). Increasing Gaussian character of the band r esults in initially (i.e. at high resolution) the band being less alte red by resolution and apodisation than a pure Lorentzian band. As the resolution is degraded, the band is more strongly affected. These resu lts are explained in terms of the effect of truncation and distortion of the Fourier transform of the bandshape. A comparison of calculated and experimental results gave good agreement for the 1216 cm(-1) band of chloroform in deuterochloroform. The experimental results also show ed an increasing degree of Gaussian character as the resolution became comparable with the full width at half height of the band. (C) 1997 E lsevier Science B.V.