Monte Carlo simulation has been applied to study time-resolved Raman and Tyndall photon migration in opaque samples under isotropic and forward scattering conditions. For isotropic scattering, Raman and Tyndall intensities are predicted to decay according to t((1-n)) and t(-n), respectively, where the value of n depends on the ratio of the optical collection aperture to the mean scattering length. The simulation correctly reproduced the analytical results of n = 3/2 and n = 5/2 for a point source in infinite and semi-infinite media, respectively. In addition the model can be used to relate the time at which a Raman photon exits the sample to the mean depth at which it was generated. This could provide a useful tool for depth profiling the chemical composition of turbid systems, and hence be a useful addition to the established array of photon-migration techniques. The model was applied to analyze experimentally observed Raman and Tyndall decay profiles from powdered trans-stilbene. The transport mean free path (l(t)) was calculated to be approximately 400 microm, which was significantly larger than the particle sizes present in the sample (approximately 10-100 microm). This implies that the particles were highly forward scattering, as would be expected for this size range. When highly anisotropic scattering was introduced into the model a much more reasonable scattering length (l(s) approximately 40 microm) was obtained. Finally, a simple analytical model was developed that gives the correct relationship between the Raman and Tyndall decay exponents for isotropic scattering. To the best of our knowledge this work represents the first detailed study of Raman photon migration under time-resolved conditions.

It has been known for many years that it should be possible to discriminate between Raman and uorescence phenomena on the basis of their differing temporal responses. However, it is only relatively recently that optical tech nology has advanced suf ciently 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 signicantly im prove the Raman spectra of highly uorescent solutions. However, the performance of such system s with solid materials has not been rep orted in detail. To partially redress this imbalance, this article describes the ps-time-resolved Raman spectro scopy of solid lms and powders. Excellent temporal resolution and uorescence rejection was obtained with homogeneous lms, but with powders, multiple scattering has the potential to signi cantly blur the time resolution. For example, after incidence of a 1-ps pulse on a powdered sample of trans-stilbene, the Rayleigh signal was spread over 100 ps in time and the Raman signal persisted for m ore than 300 ps. Simple models are presented that predict these temporal responses on the assum ption that photons random ly ''diffuse'' through the powder, scattering at particle boundaries and sometimes reem erging to be detected at a later time. These dynamics imply that uorescence 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 uorescence 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 ndings.