Low-intensity scattering

Raman scattering is normally of such very low intensity that gas phase Raman spectroscopy is one of the more difficult techniques. This is particularly the case for vibration-rotation Raman spectroscopy since scattering involving vibrational transitions is much weaker than that involving rotational transitions, which were described in Sections 5.3.3 and 5.3.5. For this reason we shall consider here only the more easily studied infrared vibration-rotation spectroscopy which must also be investigated in the gas phase (or in a supersonic jet, see Section 9.3.8). 

Since powerful X-ray sources and sophisticated beam shaping have generally become available, point-collimated setups for the study of X-ray scattering have lost their former handicap of low intensity. Today they benefit from their simple and versatile geometry. This section is devoted to an overview of modern apparatus -beginning with the source of X-radiation and ending with the detector and the data acquisition system. 

A further low-intensity peak, known as the Raman band, is also observed in spectra (see Fig. B3.6.1). This is a low-intensity band of scattered radiation whose distance from the excitation band is a measure of the vibrational energy of the H-0 bond in solvent water. At A,ex = 280 nm, the Raman band for water occurs at 311 nm in general, it occurs at l/ (1/X.ex) - (3.6 x 10-4), where A,ex is the excitation wavelength in nm. The intensity and resolution of the Raman band provide a useful empirical check on the performance of the spectrofluorometer. A decrease in the signal-to-noise ratio usually indicates a deterioration of the lamp. 

The Rh(lll) surface was covered with carbon by decomposing 5 x 10 7 torr of either acetylene or ethylene at 1100 K for 10 minutes and subsequent flashing to 1200 K (230. Pre-adsorbed carbon had a very strong inhibiting effect on carbon monoxide chemisorption. This is the same effect it had on the methanation rate (36). The low inelastic scattering intensity indicated relatively small CO coverages while the broad elastic peak and... 

The intrinsically low intensity of Raman scattering strongly influences both the sensitivity and penetration depth of SORS and its variants. Dominant noise components (photon shot noise or thermal/dark count [1]) can be minimised relative to signal by increasing absolute signal levels. In many Raman systems, collection optics, laser power and other relevant parameters are usually maximised for optimum performance of the system current detectors (CCD devices), for example, have detection efficiencies approaching 100%. Typically, acquisition time provides the only straightforward means available... 

Fig. 8.7 Electronic absorption spectra in n-hexane. The upper trace is due to pristine C H, and the lower trace with low intensity peaks is due to CM II18 after 1,230 s photolysis under He. Note the increase of baseline at 400 nm due to light scattering due to the formation of insoluble products...

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