Stimulated Raman scattering as an excitation source for time-resolved excitation-emission fluorescence spectroscopy with fiber-optical sensors

For a fiber-optical sensor system based on time-resolved excitation-emission (TEE) fluorescence spectroscopy, stimulated Raman scattering (SRS) was used to generate simultaneously several excitation wavelengths between 250 and 400 nm. For a frequency-quadrupled Nd:YAG laser and a hydrogen-methane mixture as the Raman medium, an optimum pressure and mixing ratio was identified. The observed pulse energies in the micro-Joule range were sufficient for remote sensing with fiber lengths on the order of 50 m. The TEE probe is comprised of eight independent fiber-optical sensors and was interfaced to an intensified charge-coupled device (CCD) camera with an imaging spectrograph. The system permitted a nanosecond time resolution and parts-per-billion detection limits, for pyrene and fluorescein. The preliminary study of a binary mixture of benzo(a)pyrene and benzo(k)fluoranthene indicates a gain in selectivity for TEE spectra compared to time-resolved emission spectra.