Asynchronous pulsed-laser-excited Fourier transform Raman spectroscopy and its applications

An asynchronous pulsed-laser-excited Fourier transform Raman spectrophotometer based on a conventional continuous-scan interferometer has been developed. The additional assembly required for pulsed-laserexcited measurements, which consists of a pulsed Nd:YAG laser, a gate circuit, and a low-pass filter, can be attached to any conventional FTRaman spectrophotometer. The principle of the signal-processing of this method is almost the same as that of the asynchronous time-resolved Fourier transform infrared spectroscopy reported previously. This method does not require the synchronization between the Raman excitation and the sampling of the A/D converter. As an application of this method, it is demonstrated that the use of a pulsed laser and a gate circuit can give a significant increase in signal-to-noise ratios over continuous-wave measurements with the same average laser power. It is also shown that when a constant background (for example, thermal radiation from samples at high temperatures) or a long-lived background is present, the use of pulsed excitation and a gate circuit can effectively reduce the background. Moreover, pulsed excitation can be used for recording timeresolved Raman spectra by using an FT-Raman spectrophotometer. The time resolution is governed only by the width of the probe laser pulse. The potentiality of this method is demonstrated.