Resolution double monochromator

The probability of Raman scattering is quite small. This normally requires the use of intense laser sources and concentrated samples. A high-resolution double or triple monochromator is used to separate the Raman lines from the intense Raleigh line. 

In Raman measurements [57], the 514-nm line of an Ar+ laser, the 325-nm line of a He-Cd laser, and the 244-nm line of an intracavity frequency-doubled Ar+ laser were employed. The incident laser beam was directed onto the sample surface under the back-scattering geometry, and the samples were kept at room temperature. In the 514-nm excitation, the scattered light was collected and dispersed in a SPEX 1403 double monochromator and detected with a photomultiplier. The laser output power was 300 mW. In the 325- and 244-nm excitations, the scattered light was collected with fused silica optics and was analyzed with a UV-enhanced CCD camera, using a Renishaw micro-Raman system 1000 spectrometer modified for use at 325 and 244 nm, respectively. A laser output of 10 mW was used, which resulted in an incident power at the sample of approximately 1.5 mW. The spectral resolution was approximately 2 cm k That no photoalteration of the samples occurred during the UV laser irradiation was ensured by confirming that the visible Raman spectra were unaltered after the UV Raman measurements. 

The first laser Raman spectra were inherently time-resolved (although no dynamical processes were actually studied) by virtue of the pulsed excitation source (ruby laser) and the simultaneous detection of all Raman frequencies by photographic spectroscopy. The advent of the scanning double monochromator, while a great advance for c.w. spectroscopy, spelled the temporary end of time resolution in Raman spectroscopy. The time-resolved techniques began to be revitalized in 1968 when Bridoux and Delhaye (16) adapted television detectors (analogous to, but faster, more convenient, and more sensitive than, photographic film) to Raman spectroscopy. The advent of the resonance Raman effect provided the sensitivity required to detect the Raman spectra of intrinsically dilute, short-lived chemical species. The development of time-resolved resonance Raman (TR ) techniques (17) in our laboratories and by others (18) has led to the routine TR observation of nanosecond-lived transients (19) and isolated observations of picosecond-timescale events by TR (20-22). A specific example of a TR study will be discussed in a later section. 

Align the incident beam monochromator to deliver a beam passing accurately over the second (specimen) axis. This procedure is exactly as for high resolution double-axis diffraction. 

In the case of the sulphur triimide S(NBu-f)3, the dispersive Raman technique applying a double monochromator and a CCD camera was employed to obtain the information from polarized measurements (solution studies) and also to obtain high-resolution spectra by low-temperature measurements. In the case of the main group metal complex, only FT-Raman studies with long-wavenumber excitation were successful, since visible-light excitation caused strong fluorescence. The FT-Raman spectra of the tetraimidosulphate residue were similar to those obtained from excitation with visible laser lines. 

There is a commercially available instrument for HR-CS AAS in which a flame, a graphite furnace, or a CVG system are used to carry out atomization. The instrument has a double monochromator with a prism premonochromator and a high-resolution echelle monochromator, which allows a wavelength from 189 to 900 nm to be used in a sequential measurement mode.19... 

SE-measurements were done with a Spectral Ellip-someter ES4G from Sopra, with a Xenon high-pressure 1 2174-01 light source from Hamamatsu, a double monochromator with a slit width of 400 xm, a spectral range from 230-930 nm and a spectral resolution of 0.05 nm. The evaluation of the measurements was carried out by Film Wizard 32 with the Cauchy-model. The observation of the relative changes in the refractive index at 589.3 nm was sufficient to monitor the sorption of the analytes. 

We have performed optically heterodyne-detected optical Kerr effect measurement for transparent liquids with ultrashort light pulses. In addition, the depolarized low-frequency light scattering measurement has been performed by means of a double monochromator and a high-resolution Sandercock-type tandem Fabry-Perot interferometer. The frequency response functions obtained from the both data have been directly compared. They agree perfectly for a wide frequency range. This result is the first experimental evidence for the equivalence between the time- and frequency-domain measurements. 

The depolarized low-frequency LS measurement is performed under stationary excitation by means of a double monochromator for a wide frequency range and a Sandercock-type tandem Fabry-Perot interferometer for a high-resolution study. The same samples as above are measured using 1 cm glass cell. The LS is measured under a depolarized condition in a right-angled configuration. 

Raman spectroscopy. — Routine Raman spectra were obtained on a Cary 83 Raman Spectrometer (488 nm). For higher resolution work, and for compounds sensitive to blue light, a Spex 1401 double monochromator, and a detection system that utilized photon counting techniques was used in conjunction with a variety of laser lines (principally 488, 514.5 and 647.1 nm). The spectrometer was coupled to an on-line computer which allowed the data to be collected, stored, corrected for phototube sensitivity, normalized and plotted. Powdered samples were loaded into 1 mm o. d. quartz capillaries in the Drilab, sealed temporarily with a plug of Kel-F grease, and the tube drawn down and sealed in a small flame outside the drybox. 

For experiments demanding a higher AE/E resolution (possibly for experiments on highly ordered superlattices), a double monochromator could be added. The use of two double monochromators (Fig. lOd) would allow to maintain the direction of the beam. 

Such double beam instruments can be controlled more easily in processes by relays or microprocessors. No mechanics for automatic exchange of sample and reference cells have to be included. The energetic efficiency of the light paths is lower. A double monochromator supplies higher quality photometry. The spectral resolution can be increased and the amount of stray light is drastically decreased. The slit in-between the two monochromator parts is essential. A high performance instrument is shown in Fig. 4.3. Such spectrometers are rather expensive but are very useful in the examination of complex photoreactions as well as in the measurement of problematic samples such as turbid solutions, viscous samples, or thin films. 

Fig. 4.3. Double beam set-up with a double monochromator for very precise photometric measurements and the possibility of taking high resolution spectra (Lambda 9, Perkin Elmer, Uberlingen). This instrument covers the wavelength range from the UV until the near infrared...

Stray light can be reduced and higher resolution obtained by using double monochromators. An echelle spectrometer equipped with a predisperser has been used. As the two instruments are operated in tandem, the exit slit of the predisperser is the entrance slit of the monochromator. 

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