Abstract
Since Raman scattering is inherently an inefficient process, a practical obstacle to Raman studies is interference from fluorescence. Troublesome fluorescence 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 fluorescence 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 (<i>S<sub>n</sub>, n</i> > 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 fluorescence is through time discrimination. Unfortunately, the fluorescence 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 fluorescence to Raman signal could frequently be greater than 100:1, the application of an electronic gating system to efficiently discriminate Raman signals from fluorescence 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 fluorescence at the present time. In this report, an effective method using a transient absorption gating technique is demonstrated to suppress interfering fluorescence with a relatively long lifetime. Consequently, a general "fluorescence filter" design based on a picosecond system is proposed.
PDF Article
More Like This
Cited By
You do not have subscription access to this journal. Cited by links are available to subscribers only. You may subscribe either as an Optica member, or as an authorized user of your institution.
Contact your librarian or system administrator
or
Login to access Optica Member Subscription
Add to BibSonomy