Quantum counters

Quantum-Yield and Quantum-Counter Standards. The materials recommended as quantum-yield and quantum-counter standards are summarized In Table IV. As can be seen, several of the materials suggested as spectral responslvlty standards have also been suggested as quantum-yield and quantum-counter standards. The materials listed In Table IV cover the wavelength range from 200 to almost 800 nm, giving an excellent choice for the researcher. Quantum counters that operate further Into the red are still needed, especially for the area of... 

Table IV. Quantum Yield and Quantum Counter Standards...

Fluorescence and phosphorescence spectra corrected for the instrumental sensitivity were measured with a spectrometer described previously (()). Corrected excitation spectra were obtained with constant excitation intensity controlled by a rhodamine B quantum counter. For phosphorescence polarization measurements the apparatus was set up in an "In Line" arrangement (j ) and equipped with a Glan-Thomson polarizer and a sheet polarizer (analyser) (10). 

A reference channel (quantum counter or photodiode) has two advantages (i) it compensates for the time fluctuations of the lamp via a ratiometric measurement (ratio of the output signals of the photomultiplier detecting the fluorescence of the sample to the output signal of the reference detector) (ii) it permits correction of excitation spectra (see below). 

Spectrofluorometers equipped with a photodiode instead of a quantum counter provide excitation spectra that should be further corrected because, in addition to the reasons explained above, the wavelength response of the photodiode is not strictly flat over the whole wavelength range available. 

K. Mandel, T. D. L. Pearson, and J. N. Demas, New luminescent quantum counter systems based on a transition metal complex, Inorg. Chem. 20, 786-789 (1981). 

Wavelength dependence of detector response can also be compensated by using a fluorescent screen in front of the photocell or photomultiplier. This screen acts as a quantum counter. A concentrated solution of Rhodamin B in glycerol (3g per litre) or fluorescein in 0.01N NajCO, has been used for this purpose. Quantum counters work on the principle that whatever be the wavelength of radiation incident on the screen, if completely absorbed, the photodetector sees only the wavelength distribution of fluorescence from the dye. It requires that the fluorescence yield of the counter material be independent of wavelength of excitation and therefore that its emission intensity is directly proportional to the incident intensity. 

When quantum counter methods are used instead of photographic recording, the whole of each diffracted beam can be allowed to enter the counter for a definite time, and the intensity is thus measured. 

Quantum counter methods of recording diffraction patterns 117... 

In this laboratory we use an SLM-Aminco 8100, equipped with Glan-Thompson polarizers. The electronics have been updated by the ISS Phoenix system. Measurements of FRET efficiency are performed under photon counting conditions, with the polarizers crossed at the magic angle (54.7°) to remove polarization artifacts. Fluctuation of lamp intensity is corrected using a concentrated rhodamine B solution as a quantum counter. 

Quantum counter A medium emitting with a quantum yield independent of the excitation energy over a defined spectral range (e.g., concentrated rhodamine 6G solutions between 300 and 600 nm). Also used for devices producing an electrical signal proportional to the photon flux absorbed in a medium. 

Luminescence spectra were recorded on a double Czerny-Turner scanning monochromator Model 1902 Fluorolog Spex spectrof1uoro-meter. Variations in the excitation radiation are automatically corrected by a reference detector equipped with a Rhodamine B quantum counter. Emission spectra were recorded with the right angle mode. Further details are available elsewhere (31). 

The quantum yield of H atoms (4> ) is measured as the quantum yield of C3H6(<1>C3H ) i-n limit of zero pressure where the col-lisional deactivation process (eq. 32) is negligible compared to the unimolecular decomposition process (eq. 31). Therefore, under appropriate experimental conditions, <1 torr, C3H6 acts as a quantum counter of the H atoms formed. The kinetics involved in reactions 30-33 are expected to give a linear Stern-Volmer plot of 1/ c3H6 vs ptotal This butene scavenger/unimolecular decomposition method has been successfully tested. 

In other words, the actinometers respond in exactly the same way to the absorption of a photon from the blue or the red edge of the emission profile of the lamp. Because in this case the response of the actinometer is independent of the energy of the photons and only depends on the number of incident photons, such actinometers are also referred to as quantum counters. 

Taylor DG, Demas JN. Light intensity measurements. I large area bolometers with microwatt sensitivities and absolute calibration of the rhodamine B quantum counter. II luminescent quantum counter comparator and evaluation of some luminescent quantum counters. Anal Ghem 1979 51 712-717, 717-722. 

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