The Design of an Effective “Fluorescence Filter” for Raman Spectroscopy

Since Raman scattering is inherently an inefficient process, a practical obstacle to Raman studies is interference from fluorescence. Troublesome can arise either from impurities or from the intrinsic relaxation process resulting from an electronic absorption of the sample in resonance Raman spectroscopy. To avoid the interference it is sometimes possible to choose different excitation wavelengths. In the case of resonance Raman spectroscopy, the sample can be excited to a highly electronically excited state (Sn, n > 2) in the condensed phase. Thus, Raman signals can be obtained on the high-frequency side with respect to the fluorescence. In some special cases addition of an external quenching agent, (e.g., iodide ion) to the sample is helpful, but this method usually cannot effectively eliminate the fluorescence. Furthermore, a high concentration of added iodide ion may result in a significant distortion of the molecular structure investigated. For molecules with relatively long-lived emission (>100 ns) a practical method to suppress is through time discrimination. Unfortunately, the processes which obscure Raman spectra frequently have lifetimes in the nanosecond range, while the electronic gating system is limited to several nanoseconds. Knowing that the ratio of to Raman signal could frequently be greater than 100:1, the application of an electronic gating system to efficiently discriminate Raman signals from interference is not possible. Several other techniques using nonlinear processes such as coherent anti-Stokes Raman spectroscopy (CARS), Raman-induced Kerr effect spectroscopy, and inverse Raman spectroscopy, as well as recently developed near-infrared-red FT Raman and CCD detection of diode-laser-excited Raman scattering, have also been applied in an attempt to overcome fluorescence. Although it seems that CARS may offer a high potential for eliminating fluorescence, the nonresonant background solvent susceptibility usually results in complications in the interpretation of CARS spectra. Thus, there is still no general and effective method to overcome interfering at the present time. In this report, an effective method using a transient absorption gating technique is demonstrated to suppress interfering with a relatively long lifetime. Consequently, a general fluorescence filter design based on a picosecond system is proposed.