The outline of phase-modulation infrared spectroscopy is described. The relationship between the frequency-domain response from phase-modulation experiments and the time-domain response from time-resolved experiments is derived. The microsecond dynamics following photoexcitation in trans-polyacetylene has been studied by phase-modulation infrared spectroscopy. The magnitude and phase (or in-phase and quadrature components) of the photo-induced infrared band at 1367 cm J have been measured at 78 K with modulation frequencies between 600 Hz and 10.5 kHz by using an experimental setup based on a continuousscan Fourier transform infrared spectrophotometer. The magnitude of this band shows an approximate ε-1 dependence for frequencies below 9 kHz and a rapid decrease above 9 kHz. These results are inconsistent with a unimolecular decay, which should show a dependence of the form ε-1.
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