1.2 Continuous-Scan Interferometry
In a continuous-scan FT-IR specterometer, the moving mirror moves continuously at a constant velocity, v(cm/s), and the optical path difference at time t(s) is given by δ=2vt(cm). The interferogram data points are digitized at the zero crossings of a helium neon (He-Ne) laser signal on the fly, as shown in Figure 1.3. The use of laser signal ensure that I(δ) is measured at precisely equal intervals of mirror positions and provides an internal wavelength calibration of every scan. Because of the continuous movement of the mirror, the interferogram I(δ) becomes an explicit function of time. The Fourier frequency (fF) of IR light at wavenumber σ is given by:
(1-4)
Continuous-scan is a preferred choice for routine static or relatively slow kinetic measurements that require time resolution not faster than 20ms. In this mode, the graphite bearing of the research interferometers rides on a cushion of air. This allows precise control of the mirror velocities from 0.0079 to 4.11 cm/s. The slow velocities are chosen for thermal detectors; such as a triglycine sulfate (TGS) or a photoacoustic (PA) detector. The fast velocities are chosen for fast quantum detectors, such as a mercury cadmium telluride (MCT) or an indium antimonide (InSb) detector, for routine or kinetic measurements requiring time resolution that is not faster than 20ms.
For dynamic and time-dependent processes occurring at rates faster than 20ms time scales, continuous-scan is no longer useful, because the temporal Fourier frequencies become convolved with the time-dependence of the processes. These difficulties associated with continuous-scan can be overcome with step-scan interferometery. Step-scan provides advantages for spectroscopic measurements of dynamic experiments where the signal is phase-, time-, or space-dependent.