Step-scan Fourier-transform infrared spectrometer

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Abstract
This article describes the modification of a commercial Fourier-transform infrared (FT-IR) spectrometer for step-scan operation. Step-scan operation decouples the FT-IR spectral multiplexing from time and is therefore applicable to a variety of time-dependent spectroscopic experiments, including, particularly, photoacoustic and photothermal spectroscopy. The step-scan instrument described controls the retardation (moving mirror position) with a feedback loop. The loop uses path difference, or phase, modulation of the reference laser intensity, together with lock-in amplifiers to detect the mirror position. Since the interferogram can be sampled at intervals as small as 1/4 λHeNe, the maximum free-spectral range is 31 600 cm−1. For initial positioning of the mirror, stepping can be as rapid as 100 Hz. The current software will allow data collection at ∼1.6 Hz, although the mirror settling time of ≤20 ms would allow data to be collected at 20–30 Hz stepping frequency with more efficient software. The mirror control phase modulation (PM) can also be used for efficient modulation of the infrared beam by use of a PM amplitude on the order of 1–2 λHeNe. The stability of the retardation, which imposes an upper limit on the SNR of the measured spectra, is ∼±15 nm. Data collection times on the order of 20 min are typical for 8 cm−1 resolution. Examples of dynamic spectroscopic data obtained using the step-scan mode of operation are presented. Future improvements to the instrument are briefly discussed.