Saturation spectroscopy detectors

Lasers have largely replaced conventional spectral lamps as light sources for spectroscopic experiments because the great enhancements of spectral power density, monochromaticity, and low divergence all bestow very high sensitivity on laser-based experiments, so that far fewer molecules constitute an acceptable sample. The high power reduces noise problems from detectors, and also makes possible new non-linear processes such as multiphoton excitation and saturation spectroscopy. 

A particularly simple form of saturation spectroscopy is obtained if a sample that is sufficiently dense to completely block a probe beam is used. Only at the line centre does the bleached path induced by a strong counter-propagating pump beam allow the probe beam to emerge from the cell and hit the detector. A simple set-up and a schematic curve are shown in Fig. 9.43 for this type of high-contrast transmission spectroscopy [9.161]. 

In normal saturation spectroscopy the detector essentially "looks straight into the laser and a substantial absorption is needed in order to observe a signal. This means that only strong spectral lines, originating in... 

In normal saturation spectroscopy the detector essentially looks straight into the laser and a substantial absorption is needed in order to observe a signal. This means that only strong spectral lines, originating in well-populated states, can be investigated by this method. In cases where less favourable conditions prevail, the so-called polarization spectroscopy method can instead be used [9.348]. The experimental set-up is shown in Fig. 9.57. 

This may significantly reduce noise problems caused by detector noise or background radiation. Furthermore, the large intensity allows new nonlinear spectroscopic techniques such as saturation spectroscopy (Sect.10.2), or multiphoton processes (Sect.10.5), which open new possibilities of studying molecular transitions not accessible to linear spectroscopy. 

Actually the concentrations of interesting radicals in water radiolysis are about 10 -10 M and it is necessary that they are measured using sensitive methods. The direct absorption spectroscopy is limited by the saturation and the linearity of the detector that must receive a high flux of light to detect a small amount of absorbed light. The emission spectroscopy is more convenient and more sensitive but... 

The properties of the dual-film electrode were characterized by in situ Fourier transform infrared (FTIR) reflection absorption spectroscopy [3]. The FTIR spectrometer used was a Shimadzu FTIR-8100M equipped with a wide-band mercury cadmium teluride (MCT) detector cooled with liquid nitrogen. In situ FTIR measurements were carried out in a spectroelectro-chemical cell in which the dual-film electrode was pushed against an IR transparent silicon window to form a thin layer of solution. A total of 100 interferometric scans was accumulated with the electrode polarized at a given potential. The potential was then shifted to the cathodic side, and a new spectrum with the same number of scans was assembled. The reference electrode used in this experiment was an Ag I AgCl I saturated KCl electrode. The IR spectra are represented as AR/R in the normalized form, where AR=R-R(E ), and R and R(E ) are the reflected intensity measured at a desired potential and a base potential, respectively. 

IR spectroscopy has been applied to characterize the bridging OH groups of zeolites. Several authors reported that the integrated intensity of the OH bands in H-Y zeolites decreased at elevated temperature. For example, the intensity became half when the temperature was raised from room temjjerature to 743K. The authors interpreted this intensity decrease in terms of proton mobility. However, there is an experimental problem for quantitative investigation in IR method. The emission from a sample becomes more intense at higher temperatures and it tends to cause the saturation of the detector. ... 

Sulfided samples were characterized with XRD, BET surface area, NO sorption capacity, ESR and FTIR spectroscopy. The details concerning the characterization procedures as well as certain properties of USY based samples can be found elsewhere (ref. 9, 10). The ammonia adsorption capacity of sulfided and non-sulfided catalysts and supports was measured from the desorption peak obtained during 3the temperature programmed desorption (heating rate 30 K min ). Each sample (0.1 g) after activation or sulfidation was saturated with ammonia (a series of 1 cm NH3 injections) at 375 K until full saturation was achieved. This was monitored as a sharp GC peak detected by thermal conductivity detector. Next, sample was purged 1 hour in purified helium at 375 K to remove the excess of weakly held ammonia and TPD started. 

Also crucial to our experiment were recent advances in the techniques of optical spectroscopy[13]. We combine the pulsed Ps source with a high power narrowband dye laser[23] capable of partially saturating the highly forbidden two photon 1S-2S transition over a sizable volume of space, the use of two-photon techniques[24] that allow us to avoid the considerable first order Doppler width and to excite all the Ps when the laser is tuned to the atomic resonance, and a single atom, resonant ionization detector [25] with a low background counting rate and 40% quantum efficiency. 

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