Laser rejection filters

Figure 3. Transient absorption spectrum of a 2 x 10 M solution i in butyronitrile at 100 ps following a 0.3 mJ, 0.5 ps, 600 nm laser flash. Filters that reject stray excitation light cut out the 580-620 nm wavelength region, while the sharp cutoff at 440 nm is due to the intense absorption of the porphyrin Soret band at 419 nm.

Figure 1.7. Generic Raman spectrometer showing main components laser, collection optics, wavelength analyzer, detector, computer. Many variations of geometry and components are in common icse. Laser rejection filter is often called a notch hiter.

Figure 8.4. Raman spectrum of liquid benzene from a single spectrograph with and without a holographic laser rejection filter between the sample and the entrance slit. Intensity scales differ greatly between the two spectra.

The single grating instrument depicted in Figure 8.3 is very common in optical spectroscopy (1), but its application to Raman is relatively recent because of inadequate stray light rejection. The introduction of effective laser rejection filters (Section 8.2.5) removed this limitation and single spectrographs became attractive. They are simpler and more efficient than double or triple systems and usually much more compact. Variations of the classical Czerny-Turner... 

Figure 8.14. Spectrum of solid sulfur obtained with a single spectrograph and holographic laser rejection filter (upper) or a triple spectrograph (lower).

As noted in Section 8.2.4, and Table 8.4, single spectrographs are efficient in terms of collection and transmission but rely on a laser rejection filter to decrease stray light from about 10 to 10 or less. In the vast majority... 

A third type of absorption filter useful for laser rejection is based on a semiconductor with a band gap slighdy lower in energy than the laser photons (18). Scattered laser photons are strongly absorbed by the semiconductor, while... 

Figure 9.13. FT-Raman spectra of a mildly fluorescent, impure sample of ortho dinitrobenzene before (A) and after (B) correction for instrumental response. Modulation at A is caused by the laser rejection filter. (Adapted from Reference 4, p. 104.)...

Major technological and scientific innovation in the past 10 to 15 years has significantly broadened the applicability of Raman spectroscopy, particularly in chemical analysis. Fourier transform (FT)-Raman, charge-coupled device (CCD) detectors, compact spectrographs, effective laser rejection filters, near-infrared lasers, and small computers have contributed to a revolution in Raman instrumentation and made routine analytical applications possible. An increase in instrumental sensitivity by factors as large as 10, plus decreases in both interferences and noise resulted from this revolution. The number of vendors of Raman spectrometers increased from 3 to 12 over a 10-year period, and integrated commercial spectrometers led to turnkey operation and robust reliability. 

Fig. 4.5 Basic diagram of a FT-Raman spectrometer. S, sample NF, notch filter for rejecting non-lasing radiation from laser RF, Rayleigh filter for rejecting radiation at laser frequency Ap, aperture wheel A, analyser I, interferometer.

The analysis was performed with a new state-of-the-art UV-VIS Raman spectrometer (Horiba-Jobin Yvon Labram-HR) equipped with a notch filter to reject the Rayleigh scattering, a single monochromator stage, and a CCD detector cooled to 140 K. The excitation source is 325 nm and 545 nm from an He-Cd laser (Kimmon) and the scattered photons are directed and focused onto a single monochromator (LabRam HR Jobin Yvon). A UV-sensitive LN CCD detector (Spex) is used to collect the signal. The laser power employed on the sample is only 0.2 mW, with a collection time of 40 s. 

In the collinear arrangement the anti-Stokes wave at cua = 2oo — 002 (cua > 0 is detected through filters that reject both incident laser beams as... 

The laser is then fed into the multipass system with the cesium cell in the center. The fluorescence at 1.36/xm is measured in an orthogonal direction. This is passed through a circular polarization analyzer rotating at (Of and then an interference filter to reject oven light and into a Ge photodiode. 

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