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

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