Interferometry continuous scan

Figure 1.3 Modes of the mirror scanning in interferometry (a) continuous-scan mode and (b) step-scan mode.

Continuous-scan interferometry is the conventional mode in which the mirror is moved with a constant velocity, modulating IR radiation at the Fourier frequencies [2a]. The major advantage of this type of interferometry is its capability... 

However, for the same data collection time, interferometry is more sensitive to multiplicative noise (i.e., noise proportional to the signal) than continuous-scan interferometry [591]. To eliminate the multiplicative and 1// noise, phase modulation (at 400 Hz) of IR radiation in conjunction with LIA demodulation is used [591]. Since the LIA and some IR detectors need the IR signal to be modulated at a single carrier frequency, a mechanical chopper, phase modulation (when at each position the fixed mirror is dithered at a fixed frequency), or modulation of absorption of the sample is used to produce a carrier frequency. In this case, the TR measurement is referred to as a synchronous multiple-modulation experiment. Multiple modulation is unnecessary if the so-called dc coupled detector which does not require a varying signal is used. 

Time-resolved Spectroscopy Finally in this section, the advent of step-scan interferometers has opened up exciting opportunities to study fast, reversible surface processes. Details on step-scan interferometry may be foxmd elsewhere [144] briefly in conventional mode (see in previous text), the mirror moves essentially continuously, with intensity measurements taken at regular intervals (Fig. 12). In step-scan mode, the mirror is paused at each position (retardation), allowing the exploitation of the time-resolved spectroscopy option ofthe spectrometer. Once the mirror has settled at a particular position, a reference point can be taken, after which a reaction can be triggered, that is via a light pulse or potential step, and the intensity measured at regular intervals (Fig. 13). 

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