Single-Electron Response

The output pulse of a detector for a single photoelectron is called the Single-Electron Response or SER . Some typical SER shapes for different detectors are shown in Fig. 6.10. 

---

Single-Electron Response. Output pulse delivered by a detector for a single photoelectron generated at its input. In most detectors the SER is identical with the pulse shape for a single detected photon. 

The signal bandwidth of an analog signal recording technique is limited by the bandwidth of the detector. In other words, the width of the instrument response function, or IRF, cannot be shorter than the width of the single electron response, or SER, of the detector. The SER is the pulse that the detector delivers for a single photoelectron, i.e. for a single deteeted photon. 

As an example. Fig. 1.4 shows the single-electron response measured with a high-speed oscilloscope and the transit-time distribution for a Hamamatsu R3809U MCP PMT measured by TCSPC. 

Of special interest for time-correlated single photon counting are the linear fo-cused dynodes, which give fast single electron response and low transit-time jitter, and the fine mesh and metal channel types, which offer position sensitivity when used with an array of anodes. Moreover, PMTs with fine-mesh and metal channel dynodes can be made extremely small, which results in low transit time, low transit-time jitter, and a fast single-electron response. 

The single-electron response (SER) of a PMT can be measured under weak continuous illumination by a fast oscilloscope. The input impedance of the oscilloscope must be switched to 50 A very fast oscilloscope must be used to obtain a reasonable result, and precautions should be taken to avoid damaging the PMT or the oscilloscope input circuitry (see Sect. 7.6, page 315). 

The zero cross level adjustment minimises the timing jitter induced by amplitude jitter of the detector pulses. The zero cross level is therefore often called walk adjust". In early TCSPC systems the walk adjust had an enormous influenee on the shape of the instrument response function (IRF). In newer, more advaneed systems the influence is smaller. The reason is probably that detectors with shorter single electron response are used and the discriminators in the newer CFDs are faster. Therefore, the effective slope of the zero cross transition is steeper, with a correspondingly smaller influence of the zero eross level. Figure 7.63 shows the IRF for an XP2020UR linear-focused PMT and an H5773-20 photosensor module for different zero cross levels. 

Considering that J 2 I 2 and that the ionization rates at R = Rq have little 9 dependence [35], the dominant difference should be due to the electron and nuclear dynamics in the steps 2 and 3. The observed single molecule responses are obtained by superposing the radiation from all the molecules with random orientation coherently. For linearly polarized laser field, whose direction is defined as x axis, the observed dipole moment is given by... 

From Bohr s postulates, it is possible to derive the energies of the possible stationary states responsible for the radiation that is absorbed or emitted by an atom consisting of a single electron and nucleus. The specification of these states permits one to then compute the frequency of the associated electromagnetic radiation. To begin, one assumes the charge on the nucleus to be Z times the fundamental electronic charge, e, and that Coulomb s law provides the attractive force, F, between the nucleus and electron ... 

It is clear that the decrease of the rate of the electron transfer operated by the temperature makes the oxidation of ferrocene become quasi-reversible for both the electrode materials. Moreover, it is noted that for both types of electrode the faradaic current increases with temperature. For both the electrodes the oxidation process is governed by diffusion, since in both cases the plot of log(/p) vs. 1/T is linear. Furthermore, one should note in particular that, contrary to the naive expectation, for the superconducting electrode one does not observe any abrupt change in the response upon crossing the barrier from superconductor (that should exchange pairs of electrons) to simple conductor (that should exchange single electrons). 

We are now able to understand the response of our solid to an electromagnetic field oscillating at frequency >. For the sake of simplicity, we return to the use of expressions (4.17) and (4.18), related to a solid made of single-electron classical atoms, and to only one resonant frequency coq, related to the band gap. Using these expressions, in Figure 4.1(a) we have displayed the dependencies of si and si on the incident photon energy. 

---