Electronics, photon-counting

Figure C1.5.12.(A) Fluorescence decay of a single molecule of cresyl violet on an indium tin oxide (ITO) surface measured by time-correlated single photon counting. The solid line is tire fitted decay, a single exponential of 480 5 ps convolved witli tire instmment response function of 160 ps fwiim. The decay, which is considerably faster tlian tire natural fluorescence lifetime of cresyl violet, is due to electron transfer from tire excited cresyl violet (D ) to tire conduction band or energetically accessible surface electronic states of ITO. (B) Distribution of lifetimes for 40 different single molecules showing a broad distribution of electron transfer rates. Reprinted witli pennission from Lu andXie [1381. Copyright 1997 American Chemical Society.

Figure 5 Schematic layout of a high-sensitivity PL system incorporating a laser and photon-counting electronics.

Figure 4.6 shows an apparatus for the fluorescence depolarization measurement. The linearly polarized excitation pulse from a mode-locked Ti-Sapphire laser illuminated a polymer brush sample through a microscope objective. The fluorescence from a specimen was collected by the same objective and input to a polarizing beam splitter to detect 7 and I by photomultipliers (PMTs). The photon signal from the PMT was fed to a time-correlated single photon counting electronics to obtain the time profiles of 7 and I simultaneously. The experimental data of the fluorescence anisotropy was fitted to a double exponential function. 

In time-gated photon counting, comparatively high photon count rates can be employed count rates as high as 10 MHz are often used. TG has the advantage of virtually no dead-time of the detection electronics ( 1 ns), whereas the dead-time of the TCSPC electronics is usually on the order of 125-350 ns. This causes loss of detected photons, and a reduced actual photon economy of TCSPC at high count rates. 

A high-speed sensor for the assay of dimethyl sulfide in the marine troposphere based on its CL reaction with F2 was recently reported [18]. Sample air and F2 in He were introduced at opposite ends of a reaction cell with a window at one end. The production of vibrationally excited HF and electronically excited fluorohydrocarbon (FHC) produced CL emission in the wavelength range 450-650 nm, which was monitored via photon counting. Dimethyl sulfide could be determined in the 0-1200 pptv (parts per trillion by volume) concentration range, with a 4-pptv detection limit. 

Raman spectra were recorded with a computer-controlled Spex 1401 double monochromator equipped with a cooled photomultiplier and photon counting electronics. Excitation was provided by Spectra Physics Kr+ and Ar+ cw lasers. 

The recording of sequential photon pulses for measurements of low levels of electromagnetic radiation as well as the recording of emission decays. The pulses are recorded from electron emission events from some photosensitive layer in conjunction with a photomultiplier system. See also Time-Correlated Single Photon Counting Fluorescence... 

The time-resolved techniques that are usually used for FLIM are based on electronic-basis detection methods such as the time-correlated single photon counting or streak camera. Therefore, the time resolution of the FLIM system has been limited by several tens of picoseconds. However, fluorescence microscopy has the potential to provide much more information if we can observe the fluorescence dynamics in a microscopic region with higher time resolution. Given this background, we developed two types of ultrafast time-resolved fluorescence microscopes, i.e., the femtosecond fluorescence up-conversion microscope and the... 

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... 

The low level comparator will reject pulses of low intensity which are generated by thermoionic emission in the dynodes. It is obvious that an electron emitted by dynode 5 for instance will produce a much smaller avalanche at the anode than an electron emitted by the photocathode but of course thermoionic emission from the photocathode itself would still appear as a genuine pulse, and for this reason PM tubes used in photon counting applications must be cooled down. 

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