Hot Electron Model

In V-I DC measurements at constant temperature, non-linear effects appear even at powers as low as 10-14W they become more evident when temperature is lowered (see Section 9.6.3 and Fig. 9.8) [19]. Such non-linearities can be interpreted by the so-called hot-electron model (HEM). 

Much theoretical work went into trying to understand these results more completely. Newns constructed a simple ID model that incorporated the idea of electrons hopping in and out of the tt anti-bonding orbital of NO as the means by which hot electron-hole pairs could transfer energy to the NO molecule.26 Using reasonable assumption, he was able to quantitatively reproduce the experimentally-observed surface temperature and incidence energy dependence. 

Use of high energy radiation to create hot spots. Attempts have been made to initiate explosives by ionizing radiation such a-particles, high speed electrons, y-rays, Pions etc (Refs 8 9). No initiations were observed. Cerny Kaufman (Ref 9) take this absence of initiation to indicate failure of the hot spot model. However a crude preliminary calculation, based on the Friedman model (Ref 15), suggests that the dimensions of the Pion heated regions for Lead Azide (Fig 2 of Ref 9) and for RDX (Fig 3 of Ref 9) are smaller than the critical hot spot dimension at the corresponding temperatures... 

The theoretical model developed to explain these experiments is based on inelastic tunneling of electrons from the tip into the 2ir adsorbate resonance that induces vibrational excitation in a manner similar to that of the DIMET model (Figure 3.44(b)). Of course, in this case, the chemistry is induced by specific and variable energy hot electrons rather than a thermal distribution at Te. Another significant difference is that STM induced currents are low so that vibrational excitation rates are smaller than vibrational de-excitation rates via e-h pair damping. Therefore, coherent vibrational ladder climbing dominates over incoherent ladder climbing,... 

One of the main validations of the nanoplasma model was a measurement of electron kinetic energy distributions by Shao et al [7]. They observed a bimodal distribution, with warm electrons emitted along the polarisation axis, and hot electrons with an isotropic distribution. The nanoplasma model predicted a bimodal distribution, with hot electrons being produced at the resonance, and warm ones created at the beginning of the interaction. 

From the steady state fluorescence spectrum of indole in water a fluorescence quantum yield of about 0.09 is determined. Since the cation appears in less than 80 fs a branching of the excited state population has to occur immediately after photo excitation. We propose the model shown in Fig. 3a). A fraction of 45 % experiences photoionization, whereas the rest of the population relaxes to a fluorescing state, which can not ionize any more. A charge transfer to solvent state (CITS), that was also introduced by other authors [4,7], is created within 80 fs. The presolvated electrons, also known as wet or hot electrons, form solvated electrons with a time constant of 350 fs. Afterwards the solvated electrons show no recombination within the next 160 ps contrary to solvated electrons in pure water as is shown in Fig. 3b). 

The experimental results were analyzed using an integrated approach. To obtain the temporal evolution of the temperature and the density profiles of the bulk plasma, the experimental hot-electron temperature was used as an initial condition for the 1D-FP code [26]. The number of hot electrons in the distribution function were adjusted according to the assumed laser absorption. The FP code is coupled to the 1-D radiation hydrodynamic simulation ILESTA [27]. The electron (or ion) heating rate from hot electrons is first calculated by the Fokker-Planck transport model and is then added to the energy equation for the electrons (or ions) in ILESTA-1D. Results were then used to drive an atomic kinetics package [28] to obtain the temporal evolution of the Ka lines from partially ionized Cl ions. 

A hot fluid model would be highly desirable for applications in astrophysics. As we have already mentioned, the formation of RES in the primordial plasma could be an important source of large-scale nonuniformities in density and temperature, which seeded the formation of galaxies and clusters of galaxies [4], In particular, it is conjectured that in the early universe matter was present in the form of a mixture of electrons, positrons and photons in thermal equilibrium at a temperature above me2. It is evident that the propagation of relativistic EM waves in such peculiar environment should be addressed in the framework of a hot-plasma model. 

Based on the above discussion, the physics of the ion acceleration process can be theoretically modeled under the following assumptions, leading to the formulation of a relatively simple system of equations which can be investigated analytically and numerically. First of all, let us restrict our analysis to a one-dimensional geometry. The electron population can be described as a two-temperature Boltzmann distribution, where the subscripts c and h refer to the cold and hot electron components, respectively,... 

The models described above are definitely stationary, while the electron cloud formation is not. It is assumed that the electron cloud does not evolve during the acceleration process and is not affected by the ions flowing through it. The latter condition requires that the number of the ions that are accelerated is much smaller than the number of hot electrons, Ni -C Ne. 

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