Step-scan Fourier-transform infrared spectrometer
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.
Manning, CJ; Palmer, RA; Chao, JL
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