Unapodized spectrum

Fig. 19, an unapodized spectrum [response function (sin nx)/nx = sinc(x)] is shown in trace (b). For such a spectrum there will be sidelobes and negative absorption if the natural linewidths are narrower than the full width of the sine-shaped response function. These are seen in Fig. 19, where the linewidth is three points and the response function width eight points. Here the phrase instrument response function may have a slightly different definition, but the meaning is clear. For such a response function, the direct deconvolution methods fall short. 

Figures 26 and 27 show the results of using deconvolution to remove the sidelobes present in a Fourier transform spectrum. These two spectral sections are taken from the v2 absorption band of ammonia at about 848 and 828 cm-1, respectively. The unapodized spectrum exhibits a resolution of 0.125 cm-1, and is shown in trace (a). The spacing of the lines in Fig. 26 is such that the sidelobes when added together partially cancel, minimizing their effect. Sidelobes and the apparent negative absorption between the lines are both still present. The spacing of the lines in Fig. 27 is such that the sidelobes add constructively, accentuating their effect and producing...

Fig. 27 Same as Fig. 26 except v 828 cm-1. Sidelobes add constructively in the unapodized spectrum and produce very large sidelobes. The strongest feature is two unresolved lines. 

Figure 5.12. Unapodized spectrum of a pure rotational transition of CO measured at a nominal resolution of 0.00125 cm on a Bruker 120 HR. (Reproduced by permission of Bruker Optics.)...

Figure 25. FTIR emission spectra at two times following the IRMPE of 70njTorr PhNCO. Unapodized FWHM 3.18 cm 1, Nyquist wavenumber 3950.7 cm the spectrum is corrected for the instrument response function and the maxima of ot spectra have been scaled to unity.

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