Fluorescence suppression in time-resolved Raman spectra

A pulsed Raman spectrometer is described, based on mode-locked Ar + laser excitation and gated detection using time-amplitude conversion techniques. The equipment has been used to suppress unwanted fluorescence relative to Raman signal, and improvements by a factor of 6 for Rhodamine 6G ( tau =3.9 ns) and acridine orange ( tau =9.5 ns) have been obtained with negligible change in signal-to-noise ratio. These figures are better than those from previous attempts because electronics jitter in timing has been reduced so that the limit is set by the transit time spread in the photomultiplier tube. Large random fluctuations in fluorescence are also significantly reduced, which is of great importance when accurate Raman data are required in fluorescing solutions.