Optical multichannel detection

Thus we are challenged by the problem of measuring a small signal against the background of one much stronger. The problem is usually solved by one of two means (a) lock-in-amplifier detection and (b) a boxcar type of detection (to some extent we can include double-input optical multichannel detection in this category). 

Widder, E. A., Latz, M. I., and Case, J. F. (1983). Marine bioluminescence spectra measured with an optical multichannel detection system. Biol. Bull. 165 791-810. 

Unfortunately (alas, not uncommonly), the significance of the optical multichannel detector has as yet escaped most potential users, who erroneously consider it a mere curiosity still emerging from its embryonic stage. This manuscript is another attempt to demonstrate the maturity and viability of the optical-multichannel detection approach. No longer is the technique a novelty toyed with by a handful of curious instrumentalists, but rather a readily available scientific tool whose performance characteristics and spectrometric applicability are relatively well understood. 

A tunable pulsed laser Raman spectrometer for time resolved Raman studies of radiation-chemical processes is described. This apparatus utilizes the state of art optical multichannel detection and a-nalysis techniques for data acquisition and electron pulse radiolysis for initiating the reactions. By using this technique the resonance Raman spectra of intermediates with absorption spectra in the 248-900 nm region, and mean lifetimes > 30 ns can be examined. This apparatus can be used to time resolve the vibrational spectral o-verlap between transients absorbing in the same region, and to follow their decay kinetics by monitoring the well resolved Raman peaks. For kinetic measurements at millisecond time scale, the Raman technique is preferable over optical absorption method where low frequency noise is quite bothersome. A time resolved Raman study of the pulse radiolytic oxidation of aqueous tetrafluoro-hydroquinone and p-methoxyphenol is briefly discussed. 

In the time resolved Raman measurements on radiation-chemical systems, optical multichannel detection offers some distinct advantages over the photon counting techniques. The intense Cerenkov pulse associated with the electron pulse is intense enough to saturate a photomultiplier tube (PMT). In an optical multichannel detector, the Cerenkov pulse can be effectively gated off by turning the detector on within a few nanoseconds after the electron pulse is over. Apart from this, such spectra are free from the variation in electron or laser pulse intensity unlike the spectra obtained by single channel devices. 

A few reports have appeared on combining the streak camera temporal dispersion with polychromators and three dimensional optical multichannel detection. This approach yields three dimensional fluorescence data for each laser pulse. With the present technological limitations of three dimensional detectors and streak cameras, however, data of this type suffer from low wavelength resolution. As detector and streak camera technology improve, this technique may become the method of choice for time and wavelength resolved emission spectra. 

The paper is organized in the following way. First, a brie introduction to fluorescence upconversion and other non-linear processes is given. This is followed by a description of an apparatus for making up converted fluorescence measurements, and experimental examples for this instrument. The following text describes the advantages of optical multichannel detections for... 

The fluorescence spectrum of this molecule is broad and structureless in the wavelength region of Figure 7. With PDA optical multichannel detection we observe, as expected, no signal at negative delay time with respect to the arrival time of the 355 nm pulse at the sample, and at 200 psec after the pulse arrives, the broad structureless spectrum of coumarin 520 is observed. The experimental parameters were as follows ... 

Another example of the use of optical multichannel detection is the picosecond spectroscopic study of acridine, s-tetrazine, and rhodamine B by Barbara et. al.(26) In this type of study, a sample containing the compound of interest is placed in the path of an A,6-ps FWHM laser pulse. The laser pulse is focused onto the sample cell, and the emitted light is collected and directed, by an assembly of lenses, into a streak camera which is capable of time-resolving the emission. Processing and analysis of the streak camera data are accomplished by means of an assembly consisting of a two-dimensional photodiode array... 

Using a mode-locked Nd + YAG laser system to generate picosecond sample excitation pulses and picosecond probing continuum pulses in their double beam spectrometer, Spalink et. al. (30) were able to measure difference absorption spectra of irradiated samples of 11-cis-rhodopsin and 9-cis-rhodopsin at selected times after excitation by means of a PAR OMA-2 optical multichannel detection system. The difference absorption spectral data were obtained over the entire spectral range from 410 nm to 650 nm at one time with an OMCD as opposed to the... 

Natural ROA offers the interesting prospect of measuring optical activity in pure rotational transitions of gas phase chiral molecules. Although such observations have not yet been reported, the detailed theory of rotational ROA in chiral symmetric tops has been published lS7, and the experiment should be feasible using existing technology such as optical multichannel detection. It is also possible that one of the coherent Raman techniques discussed below could be advantageous. 

Within the context of the development of coupled techniques the combination of gas chromatography and MI (GC-MI) utilizing fluorescence spectroscopy for detection and identification is interesting. Until recently this development was hampered by the long scan times required to obtain high-resolution fluorescence spectra of deposited species. Optical multichannel detection can now be invoked to tackle this problem. 

KF Wall, RK Chang. Iodine vapor notch filter with optical multichannel detection of low-frequency-shift inelastic scattering from surface-enhanced Raman-scattering active electrodes. Optics Lett 11 493-495, 1986. 

Por IR-Raman experiments, a mid-IR pump pulse from an OPA and a visible Raman probe pulse are used. The Raman probe is generated either by frequency doubling a solid-state laser which pumps the OPA [16], or by a two-colour OPA [39]. Transient anti-Stokes emission is detected with a monocliromator and photomultiplier [39], or a spectrograph and optical multichannel analyser [40]. 

Alfredson, T. A. and Sheehan, T., Recent developments in multichannel, photodiode-array, optical LC detection, /. Chromatogr. Sci., 24, 473, 1986. 

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 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. 

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... 

The spectral region of interest is then detected and processed by means of an optical multichannel analyzer (EG G-PARC model 1215, OMA). The OMA detector SIT (silicon intensified target), EG G-PARC model 1254 has been used. It is operated by the detector controller, EG G-PARC, model 1216, which performs the signal digitization as well. The acquired spectra are displayed in real time on a TV display and on the OMA console. The data storage and processing are also performed by that console which has a 28K of 16 bit core memory and a floppy disk for permanent storage. 

Fig. 4 below schematically depicts the optical arrangement developed by Wetzel and co-workers which was designed to incorporate multichannel detection into the CD experiment. 

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