Gate detection techniques

Direct observation of phosphorescence from conjugated polymers has been achieved by the application of gated detection techniques. In these techniques the detection window of an intensified CCD is delayed with respect to the excitation laser pulse. Therefore the detector is blocked during the intense prompt fluorescence caused by the conjugated polymer and able to detect the delayed emission that is usually orders of magnitude lower than prompt fluorescence. Spectrally resolved detection allows for the observation of the shape and energetic position of the delayed emission. By varying the width of the detection window... 

In conclusion, GD-OE S is a very versatile analytical technique which is still in a state of rapid technical development. In particular, the introduction of rf sources for non-conductive materials has opened up new areas of application. Further development of more advanced techniques, e. g. pulsed glow discharge operation combined with time-gated detection [4.217], is likely to improve the analytical capabilities of GD-OE S in the near future. 

In Fig. 3.5A a comparison between time-gated detection and TCSPC is shown. The time-gated detection system was based on four 2 ns wide gates. The first gate opened about 0.5 ns after the peak of the excitation pulse from a pulsed diode laser. The TCSPC trace was recorded using 1024 channels of 34.5 ps width. The specimen consisted of a piece of fluorescent plastic with a lifetime of about 3.8 ns. In order to compare the results, approximately 1700-1800 counts were recorded in both experiments. The lifetimes obtained with TG and TCSPC amounted to 3.85 0.2 ns and 3.80 0.2 ns respectively, see Fig. 3.5B. Both techniques yield comparable lifetime estimations and statistical errors. 

A system with four parallel TCSPC channels can be used up to 40 MHz detector count rate. When this count rate is compared to the count rates of other time-resolved detection techniques, the high efficiency of TCSPC must be taken into account. Consider a gated image intensifier that is operated at a gate width of 100 to 200 ps, i.e. at a time resolution equivalent to a mediocre TCSPC system. The short gate width then results in an efficiency of 0.05 to 0.1. A four-channel TCSPC system operated at 40 MHz has an efficiency of 50%. The 40 MHz detector count rate of the TCSPC system therefore corresponds to an input count rate of 200 to 400 MHz in the image intensifier. 

Raman spectra can be recorded in times as short as several nanoseconds using pulsed laser excitation/gated array detection techniques. This capability is important for the identification of transient species during fast chemical reactions or for characterizing structural failure modes in thin films exposed to heat or intense electromagnetic fields. An existing system... 

Another coherent technique that can be combined with gated detection is levelcrossing spectroscopy (Sect. 7.1). If the upper atomic levels are excited by a pulsed... 

Boxcar averagers and their use in signal recovery are described in (39). PTI (Photon Technology International) manu ctures a series of fluorescence lifetime spectrometers (models C-70 to C-73) that use a stroboscopic detection technique (i.e. a gated PMT, as described in the previous paragraph) and either nanosecond fiashlamp or nitrogen/ e laser excitation. 

The PFPD has much lower LOD and better sensitivity than FPD technology. The MDL of sulfur and phosphorous is in the 10" to lO- " g/sec area (see Aviv Amirav et al. on www.tau.ac.il/chemistry/amirav/pfpd.shtml). In addition, the PFPD can simultaneously detect several element combinations using only one photomultiplier. Element identification can be achieved through the dual-gate-response ratio method. This detection technique is capable of identifying sulfur, phosphorous, nitrogen, arsenic, and many other elements with no interference from hydrocarbons. Thus, the PFPD would be very suitable for TIC detection. 

Another time-domain technique is the collection of photons in a fixed number (typically 2 to 8) of discrete time intervals using gated detection. For the simple case of a mono-exponential decay, provided the pulse duration r,the emitted fluorescent intensity can be written as... 

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