MicroChannel plates decay

Figure 2. (a) Reflection TOF mass spectrometer, (b) Depicts the electrostatic potentials. With a judicious selection of potential, the daughter ions arising from metastable decay arrive at the detector prior to the parent ions which have higher kinetic energy. MCP denotes a microchannel plate charged particle detector, (a) Taken with permission from ref. 22 (b) Taken with permission from ref. 19. 

With mode-locked lasers and microchannel plate photomultipliers, the instrument response in terms of pulse width is 30-40 ps so that decay times as short as 10-20 ps can be measured. 

Streak cameras and multianode microchannel plate photomultipliers (MCP-PMs) interfaced to a polychromator also permit multiwavelength fluorescence decay measurements, the spectral response of both being determined by the photocathode composition. 

A time-to-amplitude converter (TAC) system was also employed to measure fluorescence decays without the microscope. Then, the fluorescence decay and the fluorescence lifetime were obtained precisely with the microchannel-plate photomultiplier (MCP-PM) as detection. The time resolution of the lifetime was determined, using a convolution method, to be 10ps. 

A laser system that delivers pulses in the picosecond range with a repetition rate of a few MHz can be considered as an intrinsically modulated source. The harmonic content of the pulse train - which depends on the width of the pulses - extends to several gigahertz. The limitation is due to the detector. For high frequency measurements, it is absolutely necessary to use microchannel plate photomultipliers (that have a much faster response than usual photomultipliers). The highest available frequencies are then about 2 GHz. As for pulse fluorometry, Ti sapphire lasers are most suitable for phase fluorometry, and decay times as short as 10-20 ps can be measured. 

The ions are electrostatically deflected after passing the interaction/detection region, and detected by a microchannel plate. The time difference between the photon and the ion depends on the position of the ion when emitting the photon. An improvement by imaging the fluorescence on a position sensitive detector, yielding a determination of decay position to less than 1 mm and thus a time resolution of 20ns, made it possible to do measurements on ion beams of less than lOOs T However, this method is limited by the maximal count-rate allowed on the channel plates. An alternative is to have a selective deflection, so that only when a photon has been detected the deflection is turned on and the ion detected. 

For fluorescence measurements, by far the most versatile and widely used time-resolved emission technique involves time-correlated single-photon counting [8] in conjunction with mode-locked lasers, a typical mo m apparatus being shown in Figure 15.8. The instrument response time of such an apparatus with microchannel plate detectors is of the order of 70 ps, giving an ultimate capability of measurement of decay times in the region of 7 ps. However, it is the phenomenal sensitivity and accuracy which are the main attractive features of the technique, which is widely used for time-resolved fluorescence decay, time-resolved emission spectra, and time-resolved anisotropy measurements. Below ate described three applkations of such time-resolved measurements on synthetic polymers, derived from recent work by the author s group. 

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