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Most commercially available FUR instruments operate in the rapid-scan mode. In the rapid-scan mode, the moving mirror of the interferometer is scanned repeatedly at a constant velocity, thus every wavelength present is modulated at its own Fourier frequency. The sum of the sinusoids for all wavelengths in the bandwidth of the instrument is the recorded interferogram. Repeated interferograms are usually coadded before transformation in order to achieve the desired SNR. However, the rapid scan mode is difficult to apply to dynamic systems, especially when the relaxation time of the system is in the range of the commonly used Fourier modulation periods.1 This convolution of the temporal aspects of the experiment with the spectral multiplexing is avoided, and most of the advantages of (rapid-scan) FT interferometry are retained, by proper application of step-scan techniques of data acquisition. In the step-scan mode the moving mirror is stopped at, or vibrated about, each data collection point and data can be collected either in the impulse/response mode at discrete intervals of time following the impulse or in the synchronous modulation mode as in-phase and quadrature components of the signal with respect to the modulation. This modulation can be imposed either on the IR beam, or the sample's absorption, reflection or emission can be modulated by exciting the sample itself. For dynamic IR spectroscopy, the step-scan FUR mode simplifies data collection and allows relatively simple retrieval of the signal phase separate from the instrument phase. Applications illustrating how these advantages are realized in classical time-resolved spectroscopy and in phase-resolved two dimensional IR spectroscopy are illustrated below. The design principles of the step-scan interferometer used for the experiments described in this paper are covered in the paper by Manning, Palmer and Chao, in this volume (see also reference 2). In brief, the instrument is a modified IBM-IR-44; stepping is controlled by the application of a synchronous modulation to the moving mirror position and use of lock-in feedback loops to monitor the resulting modulation of the HeNe laser interference pattern. The mirror position is electronically and viscously damped. Data are collected at intervals corresponding to integral multiples of λ/4HeNe; depending on the desired free spectral range, at stepping paces as slow as desired and up to 10 Hz. The design is adapted from that originally published by Debarre, Boccara and Fournier.3
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