Picosecond time-resolved Raman spectroscopy of solids: Capabilities and limitations for fluorescence rejection and the influence of diffuse reflectance

It has been known for many years that it should be possible to discriminate between Raman and fluorescence phenomena on the basis of their differing temporal responses. However, it is only relatively recently that optical technology has advanced sufficiently to achieve the necessary combination of high repetition rate and picosecond laser pulses, coupled with "gateable" multichannel detectors with matched repetition rates and short on-times. Both electronic and optical gating technologies have been shown to significantly improve the Raman spectra of highly fluorescent solutions. However, the performance of such systems with solid materials has not been reported in detail. To partially redress this imbalance, this article describes the ps-time-resolved Raman spectroscopy of solid films and powders. Excellent temporal resolution and fluorescence rejection was obtained with homogeneous films, but with powders, multiple scattering has the potential to significantly blur the time resolution. For example, after incidence of a 1-ps pulse on a powdered sample of traits-stilbene, the Rayleigh signal was spread over 100 ps in time and the Raman signal persisted for more than 300 ps. Simple models are presented that predict these temporal responses on the assumption that photons randomly "diffuse" through the powder, scattering at particle boundaries and sometimes reemerging to be detected at a later time. These dynamics imply that fluorescence rejection with bulk powders might be less effective than with homogeneous solids as the broadened Raman signal would be incompletely captured within the short detector "on" period. The fluorescence would be rejected, but so would the Raman signal (to some extent), giving a poor signal-to-noise ratio. This long-term signal persistence could also complicate the interpretation of pump-probe spectroscopy studies. However, further work is needed to assess the practical implications of these findings.