The Optical Multichannel Analyzer

Figure 8-1. Layout of the experimental scl-up for field-assisted pump-probe spectroscopy. PD photodiode OMA optical multichannel analyzer.

Optical emision spectra nowadays are simply measured using a fiber optic cable that directs the plasma light to a monochromator, which is coupled to a photodetector. By rotating the prism in the monochromator a wavelength scan of the emitted light can be obtained. Alternatively, an optical multichannel analyzer can be used to record (parts of) an emission spectrum simultaneously, allowing for much faster acquisition. A spectrometer resolution of about 0.1 nm is needed to identify species. 

A schematic diagram of the apparatus used in the energy transfer experiments is shown in Figure 8.22. The particles are produced and levitated in an electrodynamic levitator as described previously. Excitation is provided by the filtered output of either a Xe or Hg-Xe high-pressure arc. The intensity produced at the particle was found to be 10-50 mW/cm2. The fluorescence emitted from each of the levitated particles was monitored at 90° to the exciting beam using //3 optics, dispersed with a j-m monochromator, and detected with an optical multichannel analyzer. The levitator could be... 

The use of optical multichannel analyzers (OMAs) has increased the signal-to-noise ratio in the measurements of weak absorption and luminescence spectra. The basic operational scheme of an OMA is illustrated in Figure 3.24. 

Figure 3.24 The basic operational scheme of an optical multichannel analyzer.

The second-generation FOCS is shown in Figure 1. It consists of a He-Cd laser excitation source (Omnichrome model 139), a polychronator (Instruments SA model HR-320), an optical multichannel analyzer (either PAR-0MA2 or PAR-0MA3), and a coupler interface of the type described by Hirshfeld et al. (8) which couples the excitation light (4ill. 6 nm) into the optical fiber (Quartz Products QSF 1000) and... 

Structural characterization of the surface metal oxide species was obtained by laser Raman spectroscopy under ambient and dehydrated conditions. The laser Raman spectroscope consists of a Spectra Physics Ar" " laser producing 1-100 mW of power measured at the sample. The scattered radiation was focused into a Spex Triplemate spectrometer coupled to a Princeton Applied Research DMA III optical multichannel analyzer. About 100-200 mg of... 

Fig. 2 Transient spectrum of singlet phenylnitrene produced upon LFP of phenyl azide. Spectrum 1 was recorded 2ns after the laser pulse (266nm, 35 ps) at 233 K. Long-wavelength band (2) was recorded with an optical multichannel analyzer at 150 K (with 100 ns window immediately after the laser pulse, 249 nm, 12 ns). The computed positions and oscillator strengths (/, right-hand axis) of the absorption bands are depicted as solid vertical lines. For very small oscillator strength, the value multiplied by 10 is presented (f x 10).

For a same molecular ratio of aqueous NaY solutions (Y = OH, Cl), experimental data underlines specific effects of nascent OH radicals on transient UV and near-IR electronic configurations. Complex investigations of PHET reactions in the polarization CTTS well of aqueous CT and OH ions are in progress. We should wonder whether a change in the size of ionic radius (OH -1.76 A vs Cl" 2.35 A) or in the separation of the energy levels influence early branchings of ultrafast electronic trajectories. A key point of these studies is that the spectroscopic predictions of computed model-dependent analysis are compared to a direct identification of transient spectral bands, using a cooled Optical Multichannel Analyzer... 

With single-photon exposure, excitations may decay either through a variety of processes including chain scission and fluorescence (47). We would therefore expect to observe fluorescence from two-photon excitation as well. To observe the fluorescence, we used a Spectra Physics mode locked dye laser system, operating with Rhodamine 560 dye. This was focused onto the polymer film, and the emitted light collected into a spectrometer with a Princeton Instruments Optical Multichannel Analyzer (OMA) attachment. 

To obtain a quantitative measure of the CL intensity of individual CL emissions, an optical multichannel analyzer was coupled to the optical system of an electron microprobe allowing simultaneous collection of CL spectra and minor element data from a single point (Steele, I.M. Meteoritics. submitted). For CL spectra obtained with a 15 kV focused beam, enstatite from both enstatite chondrites and achondrites showed three distinct peaks (Fig.l) centered at about 742, 664, and 483nm. To allow assignment of these peaks, spectra from synthetic Mn and Cr doped enstatite are shown in Fig. 2 and the emissions from these two samples closely match the two red peaks of meteoritic enstatite neither synthetic sample shows a blue peak. The peak positions of Cr and Mn are not constant for different meteoritic enstatites and are not the same as for the Cr and Mn doped standards. The variation is about 20nm... 

The nature of the Raman methods makes it possible, using an optical multichannel analyzer and, for CARS, a broad-band probe laser, to obtain multispecies, multilevel information from a single laser pulse. This is not in general possible for LIF using a single laser, since here the laser wavelength must be tuned to the absorption line of a particular species. 

The second promising method is the use of the spectrum of a diatomic or larger molecule. As discussed in Section II-C, if one can describe accurately the population distribution for the molecule under excitation conditions, then the temperature can be extracted from the measured spectrum. The difficulty lies in capturing the spectrum in a sufficiently short time period. This can be accomplished through the use of a multiple detector array, or Optical Multichannel Analyzer such as is manufactured by Princeton Applied Research Co. (30). 

The CARS system used to measure temperature and species concentrations in the combustor zone is composed of a single-mode ruby-laser oscillator-amplifier with a repetition rate of 1 Hz and a ruby-pumped, near-infrared broad-band dye laser. The two laser beams are combined collinearly and focused first into a cell containing a nonresonant reference gas and then into the sample volume (approximately 30-u diam. x 2 cm) in the combustion region. The anti-Stokes beams produced in the sample and reference volumes are directed to spatially separated foci on the entrance slit of a spectrometer and detected by separate photomultiplier tubes. An optional means of detection is provided for the sample signal in the form of an optical multichannel analyzer (OMA), which makes it possible to obtain single-pulse CARS spectra. 

Because the EG G PARC optical-multichannel analyzer (OMA) has been specifically designed to operate with OIDs, it will be frequently referred to in the following discussion, where various computer manipulations are described and assessed. 

This report will discuss the results of a study in which an optical multichannel analyzer (OMA) was coupled to standard spectrometers to record both the UV/VIS absorption and fluorescence emission spectra of complex mixtures of PAH s separated by HPLC techniques "on-the-fly" (i.e., one second spectral scans of the HPLC effluent stream) and stored on a floppy disc for subsequent retrieval and data analysis. The system described has the capability of storing 250 (500 point) spectra and can readily be used to increase the effectiveness of HPLC analysis by allowing both quantitative and qualitative data to be obtained. 

Optical Multichannel Analyzer System (OMA 2). The OMA 2 system consisted of a PARC Model 1215 console, two PARC Model 1254 SIT detectors, two PARC Model 1216 detector controllers, and a PARC Model 1217 flexible disc drive. The SIT detector is controlled by the 1216 detector controller, which provides both power and scanning voltages and processes the signal for transmission to the OMA 2 console. The OMA 2 console performs all necessary control functions, data acquisitions, data processing and storage of spectra. The system can store 250 (500 points) spectral curves when equipped with the Model 1217 flexible disc drive. 

C. The Basic Elements of the Experimental Setup. The basic elements of TRRR experiments are a photolysis source a laser probe source (whose scattered radiation by the photolabile sample contains the vibrational spectra of the photodecomposed sample and its transients) a dispersing instrument (e.g., a spectrometer) and an optical multichannel analyzer (OMA) system used as a detector. 

Various possible time resolved techniques are discussed which enable one to measure the vibrational spectra (and what they entail of structural information) of the distinct transient intermediates formed in different photochemical decomposition schemes and at different times (in the sec-picosec range). The techniques make use of 1) the difference in the time development behavior of the different intermediates, 2) the difference in the absorption maxima and thus the difference in the resonance Raman enhancements for the different intermediates, and 3) the laser power. The techniques use one or two lasers for the photolytic and probe sources as well as an optical multichannel analyzer as a detector. Some of the results are shown for the intermediates in the photosynthetic cycle of bacteriorhodopsin. 

Nd glass laser. The two crucial features of this apparatus are an optical configuration designed specifically to optimize the spectrometric range and accuracy, and an advanced two dimensional optical multichannel analyzer system which acquires and processes two full spectral data tracks for each laser shot. In the following sections we present details of the system s design followed by examples of its high accuracy and wide utility in scientific applications. 

To acquire this information, the two displaced continuum beams are imaged with a cylindrical and a spherical lens onto different positions along the length of the entrance slit of a low dispersion spectrograph (Instruments SA, model UFS-200). The two resulting parallel dispersed spectra are fully separated from each other at the focal plane, where they are detected by the model 1254 SIT detector head of an EG + G Princeton Applied Research Corporation optical multichannel analyzer system. In conjunction with a model 1216 detector controller and model 1215 console, this detector is programmed with a two dimensional... 

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