Abstract

The interference from Rayleigh and other elastic scattering is a serious problem for multichannel Raman spectroscopy. This problem becomes more serious if the surface subjected to the excitation reflects stray light, particularly in front-face excitation and in systems using rotating samples or applying the flow technique to avoid local heating and accumulation of the photoproduct. In order to eliminate this problem the conventional multichannel Raman spectrometer uses a specially designed triple polychromator equipped with a subtractive double-grating filter stage. However, due to the complicated optical arrangements, including three gratings and at least six mirrors, the throughput of the instrument is usually low. This is a severe disadvantage if one needs to detect extremely weak Raman signals. This is particularly true when time-resolved resonance Raman (TRR) spectroscopy is performed. In order to have an optimum TRR signal, the ideal method is to apply a single spectrograph equipped with a sharp line elimination filter tuned to the wavelength of the exciting line. Several filters such as chevron filters and colloid filters have been designed for this purpose. Unfortunately, most of these filters are only applicable in the visible region. In addition, the requirement of a high degree of collimation also reduces the throughput of the signal. Several commercially available sharp-cut long-wavelength bandpass filters and a recently developed <i>J</i>-aggregate filter applying organic dye molecules are also useful for the elimination of exciting lines in the visible region.

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