Real photon

In the radiative process, a real photon is emitted by the donor molecule and the same is absorbed by the acceptor molecule. The effectiveness of this process depends on, among other factors, the degree of overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. 

The absolute values of the photoabsorption, photoionization, and photodissociation cross sections are key quantities in investigating not only the interaction of photons with molecules but also the interaction of any high-energy charged particle with matter. The methods to measure these, the real-photon and virtual-photon methods, are described and compared with each other. An overview is presented of photoabsorption cross sections and photoionization quantum yields for normal alkanes, C H2 + 2 n = 1 ), as a function of the incident photon energy in the vacuum ultraviolet range and of the number of carbon atoms in the alkane molecule. Some future problems are also given. 

Double Ionization-Chamber Method (Real-Photon Method)... 

Brion and co-workers [14,22-25] have extensively used and applied such an experimental approach by using fast electrons as the virtual-photon source. Brion et al. [14,25] pointed out some expected characteristics of the use of synchrotron radiation in comparison with virtual photons in studies of the photoionization and photoexcitation of molecules, and clarified some necessary assumptions to virtual photons instead of real photons. It should be noted here, as described below, that these two methods, real- and virtual-... 

In most cases, except those in earlier comparative studies between the real-photon method and the dipole-simulation method, the absolute cross-section values obtained by both methods agree with each other [27]. Comparison of obtained cross-section values between the two methods were discussed in detail [27, 2, and references therein] and summarized in conclusion [5]. It should be noted, at least briefly, that it is essentially difficult to accurately obtain the absolute values of photoabsorption cross sections (u) in the dipole-simulation experiments, and it is necessary to use indirect ways in obtaining those values as the application of the TKR sum rule, Eq. (3), to the relative values of the cross sections obtained partly with theoretical assumptions. Moreover, in some cases, in relatively earlier dipole-simulation experiments, particularly of corrosive molecules upon their electron optics with poorer energy resolutions, serious discrepancy from the real-photon experiments was clearly pointed out in the obtained absolute values of photoabsorption cross sections [5,20,25-28]. 

As for the absolute values of photoionization quantum yields ( ],), a situation of the dipole-simulation experiments in comparison with the real-photon experiments is much more serious and controversial because their absolute scales are determined by the assumption that the photoionization quantum yields should be unity around 20 eV photon energy in addition to the above-mentioned difficulty in obtaining absolute cross-section values in the dipole-simulation experiments. 

In case of simple diatomic molecules such as H2 and N2, sharp peaks are observed with large photoabsorption cross sections, especially in the lower energy range. In real-photon experiments, if the peak shape is narrower in energy than the bandpass of the incident... 

The real-photon method is essentially more direct and easier compared to the dipole-simulation method in obtaining absolute values of photoabsorption cross sections (o ), photoionization cross sections and photoionization quantum yields (t],). In the real-photon method, however, there is a practical need to use the big and dedicated facilities of synchrotron radiation where, in many cases, one should change the beam lines equipped with different types of monochromators depending on used photon-wavelengths—and to develop some specific new experimental techniques in the range from the vacuum ultraviolet radiation to soft X-ray. 

Investigating the Structure of the Nucleon with Real Photons By F. Wissmann 2003. 68 figs., VIII, 142 pages... 

Real photons carry energy virtual photons do not. Virtual photons (or other virtual particles) exist within the framework of the uncertainty principle, for lifetimes At below the uncertainty principle limit AEAf [Pg.230]

At this stage, it is convenient to distinguish between the photon creation and destruction events occurring at each center. The first case to be considered is when absorption and emission of the real photon occurs at different sites. In the second situation, scattering of a real photon takes place at the same... 

The sequence of events depicted by Fig. 5(al, for example, is as follows center A first absorbs a real photon of wave vector k and polarization e, and thereby undergoes a virtual transition to. n intermediate excited state r> a virtual photon of wave vector x, polarization e, and frequency — c x ... 

From Eqs. (5.4) and (5.5) it follows that each appearance of is associated with the creation or annihilation of a photon. It is thus readily apparent that the first non-zero contribution from Eq. (5.7) is the fourth-order term, corresponding to four separate photon creation and annihilation events these comprise the two annihilations of real photons from the incident light, and the creation and annihilation of the virtual photon which couples the two molecules. 

In fact, the photon creation and annihilation events at each molecule appear simultaneous, as far as real experimental measurements with finite time resolution are concerned. However, the time-energy uncertainty relation does permit short-lived states that are not properly energy-conserving. This helps explain why it is necessary to include diagrams corresponding to time sequences in which a virtual photon is created before either real photon arrives. It nonetheless transpires that such apparently unphysical cases produce the smallest contributions to the matrix element. 

Working in a similar way, it is found that there are 24 contributions arising from diagrams of the distributive type represented in Fig. 8(b), where both real photons are absorbed at center A, and a virtual photon conveys the energy mismatch to B the final 24 distributive contributions arises from the mirror-image case where both real photons are absorbed at B and the virtual photon propagates to A. The addition of all 96 matrix element contributions then produces the complete fourth-order result for M -j. 

This expression highlights an important distinction between EELS and optical experiment, namely that the virtual photon field associated with the fast electron is longitudinal ( 11 ) whereas the real photon field is transverse (E q) [3.23]. This difference does not affect the main point of the photon analogy namely that electron energy loss and optical experiments measure the same quantity e(a>) in most (and practically all the) cases. It does mean that the polarisation of the electric field acting on the atomic system is defined differently in both cases, a fact of obvious importance for anisotropic materials. 

The carrier multiplication (CM) process generated as a result of a single photon absorption in a spherical quantum dot (QD) is explained as due to multiple,virtual band-to-band electron-photon quantum transitions. Only the electron-photon interaction is used as a perturbation without the participation of the Coulomb electron-electron interaction. The creation of an odd number of electron-hole (e-h) pairs in our model is characterized by the Lorentzian-type peaks, whereas the creation of an even number of e-h pairs is accompanied by the creation of one real photon in the frame of combinational Raman scattering process. Its absorption band is smooth and forms an absorption background without peak structure. It can explain the existence of a threshold on the frequency dependence of the carrier multiplication efficiency in the region corresponding to the creation of two e-h pairs. 

Hamiltonian, which takes part in the first step of the perturbation theory. On this way we will need to discuss along with the creation of an odd number of e-h pairs, also the Raman scattering processes with the creation of an even number of e-h pairs and simultaneously of one real photon. The pure electron-photon interaction mechanism requires the introduction of the virtual and final states of two types one of them is the pure e-h states, when their number is n= 1,3,5,. Another type is the combined electron-hole-photon states, when an even number of e-h pairs =2,4,6,... is accompanied by the creation of one virtual or real photon. 

This flow of electricity can hardly be described as a quantum jump. More realistically the vibrations of the two affected states (emitter and acceptor) are seen to interact and generate a beat (wave packet) that moves to the state of lower energy. The virtual photon that links two equilibrium states turn into a real photon that carries the excess energy, either into or away from the system. 

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