Photoelectron pulses

In the pulse counting method, each photoelectron pulse arriving at the phototube anode is processed. The pulses are amplified and then used to trigger a pulse generator. The output pulses from the generator are integrated and displayed on a recorder. 

The operation and application of streak cameras in fluorescence lifetime spectroscopy has been reviewed previously (see, e.g., Refs. 91 and 92). Streak cameras are useful in 2-D time-resolved imaging applications such as microscopy or multiwavelength array fluorometry. The operating principle is based on converting an optical pulse into a photoelectron pulse and spatially dispersing the electron image on a phosphor by means of a synchronized deflection voltage across two plates. 

Zarowin (68) has made use of a multiple-sampling technique in the measurement of decay times. This method uses a periodically pulsed- or chopped-excitation source and a continuously operating photomultiplier detector. The fluorescent signal is displayed on an oscilloscope. The response of the photomultiplier tube must be fast enough to resolve individual photoelectron pulses, and the time density of pulses is then proportional to the light intensity. 

The photoelectron pulses from the phototube are detected and processed individually through an adjustable gating system, and subsequently integrated over a predetermined period of time. The output is recorded, or treated by a computer. 

Another important feature of TCSPC is the use of the rising edge of the photoelectron pulse for timing. This allows phototubes with nanosecond pulse widths to provide subnanosecond resolution. This is possible because the rising edge of the single photon pulses are usually steeper than one would expect from the time response of the PMT. Also, the use of a constant fraction discriminator provides improved time resolution by removing the variability due to the amplitude of each pulse. 

Diuing the past decade, the instrumentation for time-resolved fluorescence of proteins has advanced dramatically. The flashlamp light sources have been replaced by hi -repetition-rate (MHz) picosecond dye lasers, which provide both higher excitation intensities and raon rapid data acquisition. The dynode-chain PMTs have been replaced by MCP detectors, which provide much shorter single-photoelectron pulse widths than a dynode chain PMT. In con nnation. the new light sources and detectors provide instniment r ponse functions with half-widths near 100 ps, so that picosecond resolution can now be obtained. 

A successful modification to the technique involves delayed pulsed-field extraction which allows discrimination between zero and near-zero kinetic energy electrons. About 1 ps after the laser pulse has produced photoelectrons, a small voltage pulse is applied. This has the effect of amplifying the differences in fhe velocities of fhe phofoelecfrons and allows easy discrimination befween fhem as a resulf of fhe differenf times of arrival af fhe defector. In fhis way only fhe elections which originally had zero kinetic energy following ionization can be counted to give fhe ZEKE-PE specfmm. 

Pyridine, l-lithio-2-phenyl-l,2-dihydro-, 2, 266 Pyridine, 2-methoxy-IR spectroscopy, 2, 129 photoelectron spectroscopy, 2, 140 pulsed ion gas-phase cyclotron resonance spectroscopy, 2, 157 Pyridine, 3-methoxy-nitration, 2, 191 Pyridine, 4-methoxy-pKa,, 2. 150... 

B. Zero Kinetic Energy Threshold Photoelectron Spectroscopy and Pulsed Field Ionization (ZEKE-PFI)... 

Another important property of PMTs is the pulse height distribution. The amplification of individual photoelectrons by the PMT is a stochastic process that causes variations in the gain of individual photoelectrons. As a result significant jitter in the amplitude of the output pulses is observed, see Fig. 3.6. These pulse height variations can be more than a factor of 10. The lowest pulse heights mainly consist of (thermal) noise, indicated by the dashed line in Fig. 3.6. The pulse height distribution exhibits a peak corresponding to detected photons. The threshold level of the... 

Fig. 3.8. Left schematic illustration of TRPE. The IR pump pulse (hi/1) perturbs the electronic states of the sample. The photon energy of the UV probe pulse (h.1/2) exceeds the work function and monitors changes in occupied and unoccupied states simultaneously. Right experimental setup for TRPE. Pairs of IR and UV pulses are time delayed with respect to each other and are focused onto the sample surface in the UHV chamber. The kinetic energy of photoelectrons is analyzed by an electron time-of-flight spectrometer (e-TOF). From [23]...

For UV and visible radiation, the simplest detector is a photomultiplier tube. The cathode of the tube is coated with a photosensitive material (such as Cs3Sb, K CsSb, or Na2KSb, etc.) which ejects a photoelectron when struck by a photon. This photoelectron is then accelerated towards a series of anodes of successively greater positive potential (called dynodes). At each dynode, the electron impact causes secondary electron emission, which amplifies the original photoelectron by a factor of 106 or 107. The result is a pulse of electricity of duration around 5 ns, giving a current of around 1 mA. This small current is fed into the external electronics and further amplified by an operational amplifier, which produces an output voltage pulse whose height is proportional to the photomultiplier current. 

The photoelectron either recombines with the photohole or is transferred to the electron accetor, MV2+. Thereby the reduced species MV+ is formed. This MV+ radical exhibits an absorption band with a maximum at 630 nm. The increase of the absorption at 630 nm of the MV+ radical was measured as a function of time (in the microsecond range) after a laser pulse at 355 nm which creates high enough concentrations of photoelectrons and photoholes. In the experiment shown in Fig. 10.7a bare Ti02 particles were examined where the photoholes are trapped by the surface OH groups ... 

Gamma rays, being powerful, enter the tube very easily but cause little direct ionization. They do, however, produce emission of photoelectrons from the glass or metal walls of the tube and the internal electrodes. These photoelectrons then produce ionization of the gas with the resulting pulse of current. 

Figure 3.20 shows the effect of the transit time dispersion on the measurement of an ideal light pulse. Since photoelectrons spend some time traveling from the photocathode to the anode (transit time), the photomultiplier signal is delayed in time with respect to the incident pulse. Furthermore, due to the transit time dispersion, the... 

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