Light photon counting

Experiments on the stress-chemllumlnescence (SCL) behavior of TGDDM/DDS epoxy and nylon 66 specimens were performed under various conditions to determine the potential of the technique to measure stress-induced and stress-accelerated aging processes. Representative results of typical SCL experiments with epoxy specimens are shown In Figures 2 and 3 where emitted light (photon counts/s) Is plotted as a function of time and stress. In both... 

The technique of time-correlated single-photon counting (Figure 3.4) is used to measure an excited singlet-state lifetime, x. The sample is irradiated with a very short-duration light pulse ( lns) to ensure any given molecule will only be excited once during the pulse. As soon as... 

Yeh and Keeler 244) extended the method of laser-scattering spectroscopy to probe systems undergoing rapid chemical reactions. They observed the spectral line broadening in light from a singlemode He-Ne laser scattered from multicomponent solutions, as a function of time. The experiment employed a pressure-scanned Fabry-Perrot interferometer and photon counting techniques. 

FIGURE 7.6 Schematic of a laser-induced fluorescence detector. A lamp with focusing optics and an appropriate band-pass filter could be used in place of the laser excitation when tightly collimated light is not required. The emitted fluorescence is detected by a PMT that can be operated in current mode or photon counting mode. Inset shows the mutually perpendicular arrangement of excitation, capillary, and detection optics. 

Instrumentation. The steady-state fluorescence spectra were measured with Perkin-Elmer MPF-44B fluorescence spectrophotometer. The single-photon counting instrument for fluorescence lifetime measurements was assembled in-house from components obtained from EG G ORTEC. A PRA-510B light pulser filled with gas was used as the excitation source. Instrument response function was obtained with DuPont Ludox scatter solution at the excitation wavelength. 

The 5 ns pulses of about 10 electrons released at the anode by a photon absorbed by the photocathode of a PM tube can be used to count photons. In such instruments the intensity of light is displayed as a count per second which varies between about 15 (dark count) and 105. A photon-counting detector system is of course much more complex than the simple PM/ampli-fier used in conventional spectrofluorimeters. Figure 7.27(a) is a block diagram of such a photon counter (b) gives a simple illustration of the important process of pulse selection through a discriminator. The output of... 

Photon counting detection reaches the ultimate limits of sensitivity in light detection at the present time. It is useful for the detection of very weak luminescence of quantum yields below 10-4 some phosphorescence emissions in liquids at ordinary temperatures can be measured in this way (Figure 7.28). 

Figure 7.34 Principle of kinetic single-photon counting. L, pulsed light source S, sample P, photodiode F, interference filter or monochromator D, photomultiplier R, voltage ramp generator (1 to start the ramp, 0 to stop it). The voltage V is fed into a multichannel analyser M. Inset voltage ramp V(t)...

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