Laser optical multichannel analyzer

Lewis J W, Warner J, Einterz C M and Kliger D S 1987 Noise reduction in laser photolysis studies of photolabile samples using an optical multichannel analyzer Rev. Sol. Instrum. 58 945-9... 

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).

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. 

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 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. 

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. 

In summary, we have combined state of the art optical multichannel analyzer techniques with well established low repetition rate picosecond laser technology to construct an instrument capable of measuring transient spectra with unprecedented reliability. It is, in its present form, a powerful tool for the investigation of ultrafast processes in biological, chemical, and physical systems. We foresee straightforward extension of the technique to the use of fourth harmonic excitation (at 265 nm) and also a future capability to study gaseous as well as condensed phase samples over a more extended spectral range. 

A quantitative study of the RISC quantum yield was carried out with 55 [37]. This study involved UV irradiation of 55 in benzene or cyclohexane solution to produce the 7, state and subsequent photolysis by a second laser tuned to the T-T absorption band. The second pulse was accompanied by depletion (bleaching) of the T-T absorption and 5, — S0 fluorescence [the fluorescence was detected and quantified by an optical multichannel analyzer (OMA)]. The quantum yield of RISC, d>RISC, was calculated using Aberchrome 540, a reversible fulgide, as a two-laser actinometer. The values for benzene solvents, respectively. This compares with 0.19 found for 55 in ethanol solvent [36]. 

Fig. 18. Schematic of apparatus used to measure fluorescence kinetics with a streak camera. The Nd glass laser emits a train of one hundred 1.06 pm pulses separated by 6 ns. A single pulse in the earlier portion of the train is selected by a Pockels cell and crossed polarizers (Pi and P2). The high voltage pulse ( 5 ns) at the Pockels cell is supplied by a laser triggered spark gap and a charged line. The single pulse ( 8 ps, 109 W) can be amplified. The second harmonic is generated from a phase matched KDP crystal. Beam splitters provide two side beams beam (1) triggers the streak camera beam (2) arriving at the streak camera at an earlier time acts as a calibrating pulse. The main 0.53 pm beam excites the sample for fluorescence measurement. The fluorescence collected with f/1.25 optics is focused into the 30 pm slit of the streak camera. The streak produced at the phosphorescent screen is recorded by an optical multichannel analyzer. (After ref. 67.)...

The molecular structures of the surface vanadium oxide species on the different supports were examined with Raman spectroscopy. The Raman spectrometer system possessed a Spectra-Physics Ar+ laser (model 2020-05) tuned to the exciting line at 514.5 nm. The radiation intensity at the samples was varied from 10 to 70 mW. The scattered radiation was passed through a Spex Triplemate spectrometer (Model 1877) coupled to a Princeton Applied Research OMA III optical multichannel analyzer (Model 1463) with an intensified photo diode array cooled to 233 K. Slit widths ranged from 60 to 550 m. The overall resolution was better than 2 cm l. For the in situ Raman spectra of dehydrated samples, a pressed wafer was placed into a stationary sample holder that was installed in an in situ cell. Spectra were recorded in flowing oxygen at room temperature after the samples were dehydrated in flowing oxygen at 573 K. 

Figure 4. Time-resolved fluorescence of 8-hydroxypyrene-1,3,6-trisulfonate in water-ethanol mixture. The samples were excited by a 6-psec laser pulse (352 nm) and the emission was recorded by Hammamatsu C939 streak camera combined with optical multichannel analyzer (PAR 1205 D) (A) the emission of the undissociated state, measured in pure water at the spectral range 400-470 nm (B) the emission of the undissociated state (400-470 nm) measured in 50% vol/vol ethanol—water mixture (C) fluorescence rise time (540 nm) of the dissociated excited form, 45% vol/vol ethanol—water mixture.

The measurements were performed with an usual setup for siuface-enhanced Raman spectroscopy The Raman spectra were measured with a Spex 1406 spectrometer, the samples were illmninated with a Spectroscopy Instruments argon ion laser (A = 514 nm, 30 mW) and the spectra were detected by a Princeton Instruments optical multichannel analyzer imder computer control. All experiments were performed in an electrochemical cell containing an inert platinum working electrode mechanically... 

Applying lasers as the excitation source and either a scanning monochromator coimected to a boxcar integrator or, better, an optical multichannel analyzer for the experimental setup has given rise to the development of the laser-induced fluorescence technique which can be used for diagnostic pmposes in many contexts [37]. Pulsed UV lasers like nitrogen, frequency-tripled Nd YAG or excimer lasers serve as the light source. 

The absence of a, phase-matching condition means that it is also possible to use a fixed frequency pump laser with a broadband probe laser. This gives a complete Raman spectrum over the bandwidth of the probe laser (- 1000 cm l) which can be analysed using a spectrometer placed after the blocking polarizer, followed by an optical multichannel analyzer. 

In order to detect the intensity change of one mode in the presence of many others, the laser output has to be dispersed by a monochromator or an interferometer. The absorbing molecules may have many absorption lines within the broadband gain profile of a multimode dye laser. Those laser modes that overlap with absorption lines are attenuated or are even completely quenched. This results in spectral holes in the output spectrum of the laser and allows the sensitive simultaneous recording of the whoie absorption spectrum within the laser bandwidth, if the laser output is photographically recorded behind a spectrograph or if an optical multichannel analyzer (Vol. 1, Sect. 4.5) is used. 

Often a broadband laser (for example, a pulsed dye laser without etalons), or a multiline laser (for example, a CO2 or CO laser without grating) may simultaneously cover several absorption lines of different molecules. In such cases the reflected beam is sent to a polychromator with a diode array or an optical multichannel analyzer (OMA, Vol. 1, Sect. 4.5). If a fraction of the laser power Po(< ) is imaged onto the upper part of the OMA detector and the transmitted power onto the lower part (insert in Fig. 10.17), electronic difference and ratio recording allows the simultaneous determination of the concentrations V/ of all absorbing species. A retroreflector arrangement is feasible for measurements at low altitudes above ground, where buildings or chimneys can support the construction. Examples are measurements of fluorine concentrations in an aluminum plant [1452], or the detection of different constituents in the chimney emission of power plants, such as NOjc and SOj components [1453]. Often, ammonia is added to the exhaust of power stations in order to reduce the amount of NO emission. In such cases, the optimum... 

Room temperature fluorescence spectra were recorded under 632.8 nm exciting light (He-Ne laser, 12 mW.cm 2) using an optical multichannel analyzer (OMA II, Princeton Instruments) in the experimental set-up described in (5). The variations of the intensity of the total fluorescence atA>660 nm were recorded simultaneously using an EMI photomultiplier. 

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