Gated detection

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. 

Fig. 3. Mass spectra of 1014 atoms of "Tc and of the stable isotopes of Mo. (a) Laser off, (b) laser on for "Tc and (c) with laser beams and gated detection [12a]...

The key to the good resolution of ZEKE-PFI is in its discrimination against electrons with >1 cm-1 kinetic energy. This is due to the delayed extraction and time-gated detection. A bubble of 1 cm"1 electrons expands to a radius of 1.6 cm during the 2.0 ps delay. Such kinetic electrons either miss the detector or arrive... 

Time-gated detection offers the possibility to suppress background signals correlated with the excitation pulse. Direct and multiple scattered excitation light as well as Raman scattering reaches the detector at t 0, and can be effectively suppressed by opening the first gate a few hundred picoseconds after t = 0. 

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. 

The upgrade of an existing TIRF setup to TIRF-FLIM is much more serious, both with respect to costs and with respect to investments in software and high-frequency or nanosecond gating detection systems and control software. 

Phosphorescence spectra (uncorrected, front face) were recorded on a Perkin-Elmer LS-5 fluorescence spectrometer using a pulsed excitation source ( 10 ps) and gated detection. The instrument was controlled by a P-E 3600 data station. The samples were typically excited at 313 nm using the instrument s monochromator and an additional interference filter. Excitation and emission bandpasses were 2 nm. Typically the emission spectra were recorded using a 50 ps delay following excitation and a 20 ps gate. The samples were contained in cells made of 3x7 mm2 Suprasil tubing, under a continuous stream of N2, 02 or 02/N2 mixtures of known composition. 

The other intermediates in the Krebs cycle (isocitrate, a-ketoglutarate (KG), succinate, fumarate, L-malate and oxaloacetate) also cause large differences in fluorescence intensity on addition to [Eu(Tc)], which can be imaged with the help of a microwell plate (Fig. 16a). As KG and succinate cannot effectively coordinate with Eu3+, no significant fluorescence enhancement was expected, nor indeed observed. By choosing different lag times for time-gated detections, different intermediates can be seen in different time windows. 

Fig. 28 Schematic view of the Zeiss Plate Vision instrument which is state-of-the art for ultra-high throughput screening (uHTS) for drug discovery. The instrument resembles a 96-well parallel microscope the light of a excitation source (Xe-lamp or pulsed laser) is expanded to illuminate a microtiter plate. The excitation is structured into 96 channels by a mini-lens array (MLA) and focused into the well with a detection volume of < 100 nL. All 96 channels are read simultaneously by a gated, intensifed CCD. With this fast detector and the pulsed laser excitation, the instrument can be used to carry out miniaturized, 96 parallel lifetime measurements in microtiter plate format with nanosecond time resolution or time-gated detection [190]...

In TERS microscopy and spectroscopy, the tip enhancement due to the SPP resonance plays the most essential role both for signal sensitivity and spatial resolution. However, the tip-enhancement effect is not the only one affecting Raman spectra. There coexist other interaction mechanisms between a metal tip and sample molecules, chemical interactions similar to SERS [120-122], and, in addition, mechanical interactions (see Sect. 5.4.1). The latter two interactions show up only when sample molecules are in a close vicinity of a tip. In the TERS system using a ccaitact mode AFM, an experimentally observed TERS spectrum is a complex combination of the contributions of these three interactions, which makes it difficult to interpret experimental TERS spectra. Therefore, elucidation and discrimination of the tip-sample interactions are of scientific and practical importance. This can be realized by measuring a tip-sample distance dependence of TERS, since those three interaction mechanisms have different dependencies on the tip-sample distance. The active control of the distance between the tip and sample is a unique feamre only possible in TERS not in SERS. Two system configurations, time-gated detection and timegated illumination, are described below. 

TERS System Using Tapping Mode AFM with Time-Gated Detection... 

Fig. 16.14 (a) Setup for TERS with time-gated detection and its (b) schematic of the dual-gating photon-counting scheme (Reprinted from [66], Copyright 2007, with permission from American Institute of Physics), (c) Setup for TERS with time-gated illumination and its (d) schematic of the gating relative to the tip oscillation (Reprinted from [67], Copyright 2009, with permission from American Physical Society)... 

FLIM measurements were carried out using a four-channel time-gated detection system [9,11, 34]. The experimental system is shown in Figure 31.2. A mode-locked... 

Spatially resolved, time-gated spectra. A number of experiments require spatially resolved, time-gated detection. 

Figure 4. Time-resolved fluorescence decay curves of two differently concentrated R-6G solutions. The initial sharp peak is due to the excitation pulse. The time window during which fluorescence counts are accumulated in time-gate detection is also shown. (Adopted from [30].)...

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... 

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