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Although rapid-scan FT-IR has made many important advances in IR spectroscopy possible, it characterized by limitations that can interfere with some types of spectroscopic measurements. The first drawback is the wide modulation frequency range across the spectrum. This results since in the rapid-scan mode, modulation frequency is a function of both mirror speed and the wavelength of the IR light. Thus, difficulties may occur when studying systems involving detectors of specific, narrow bandwidths. Also, the depth profiling of photoacoustic samples can be obscured by this characteristic. A second limitation of the rapid-scan method lies in acquiring scans with sub-millisecond time resolution, since the fastest scanner can collect only about 50 scans per second at reasonable resolution. An alternative step-scan technique has been developed to deal with the above demanding applications. In this system, the moving mirror of the interferometer is "stepped" to precise locations where (using either amplitude or phase modulation) interferogram points are collected by means of lock-in amplifier. After acquisition of each interferogram point, the moving mirror is translated to the next optical retardation position. This system offers numerous advantages: Signal modulation is tunable and the same across the spectrum. This characteristic offers strong advantages in studies involving slow thermal detectors, such as photoacoustic and photothermal beam deflection spectroscopies. For repeatable events, time resolution is limited only by the speed of the acquisition electronics and detectors. Interferograms of short-lived states are collected by triggering the event of interest at each of the moving mirror's optical retardation positions and then following the response of the detector as a function of time. In this way, interferograms with time resolution of 5 microseconds or less can be collected. Finally, the step-scan system is fully compatible with a rapid-scan system, so that the user may easily switch from one mode to the other. Examples of step-scan applications will be presented and compared with corresponding spectra taken using rapid-scan methods.
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