Analysis of noise for rapid-scan and step-scan methods of FT-IR difference spectroscopy

Quantitative vibrational difference spectroscopy of small signals requires techniques that minimize spectral noise. As simple modulation of the sample followed by signal demodulation is often problematic due to interference with the FT-IR Fourier frequencies, alternate methods are necessary. Using vibrational Stark spectroscopy as an example, the DC method involves alternating complete interferogram scans in which a DC electric field is turned on with scans in which the field is turned off. A new synchronized AC method is similar, but the field is switched at every interferogram sampling point. In the step-scan method, a sinusoidal electric field is used and the detector signal is demodulated with a lock-in amplifier at each mirror position. Noise levels for these methods are derived analytically in this paper and compared with experimental values, yielding results that are broadly applicable to FT-IR difference spectroscopy. It was found that the DC method performed the best, although specific experimental factors may favor other methods.