MODIFICATION OF A RESEARCH-GRADE FT-IR SPECTROMETER FOR OPTIONAL STEP-SCAN OPERATION

The implementation of step-scanning to a research-grade FT-IR spectrometer (Nicolet system 800) is described. This implementation relies on the complete computer control of the retardation, representing a substantial improvement over results from the previous generation of step-scan spectrometers (IBM IR44) available in our laboratory. Specifically, the instrument represents an improvement in speed, stability, and attainable limit of detection. The most distinctive capability of this instrument is that of high-amplitude phase modulation (tested up to 10 lambda(HeNe) peak to peak) at relatively high phase modulation frequency while maintaining high position certainty. Alternatively. the phase modulation can be turned off completely and the retardation can be maintained within +/- 1 nm for indefinite periods between steps. The step-scan option for this instrument, along with its continuous-scan ''TRS'' (stroboscopic) mode, gives it a unique combination of capabilities for dynamic vibrational spectroscopy. The performance of the instrument in the step-scan mode is demonstrated with photoacoustic spectroscopy (PAS).