Charge-carrier pairs

Plasma is essentially an ionized gas that consists of a mixture of interacting positive ions, electrons, neutral atoms or molecules in the ground state or any higher state of any form of excitation, as well as photons. Because charge-carrier pairs are generated by ionization, the plasma as a whole remains electrically neutral.2... 

The quantum efficiency, 4> for the generation of useful charge carrier pairs (i.e. those that can potentially be used in a valence control process) per photon absorbed, was found to be 0.27. This is high compared to values reported for similar processes in the literature and is a direct consequence of the charge transfer mechanism in operation. Under the conditions employed in the experiment of Fig. 5, both the particle and the Ce4+ cation are... 

It can be seen from Eq. 9.7 that the resolution is better for the detector with the smaller average energy needed for the creation of a charge carrier pair (and smaller Fano factor). Thus, the energy resolution of a semiconductor detector (w 3 eV, F < 0.1) should be expected to be much better than the resolution of a gas-filled counter (w = 30 eV, F 0.2), and indeed it is (see Chaps. 12 and 13). 

If 10 charge carrier pairs have formed in the detector and C = 100 pF, then AV = 0.16 mV. In practice the effective voltage drop A is somewhat less than the calculated AV. 

We initially restrict ourselves to the simplest process the linear intrinsic photogeneration of charge-carrier pairs. Their production rate is proportional to the absorbed intensity of photons of the excitation light in the crystal, and requires neither excitonic processes at the crystal surface nor at the contacts, nor does it involve biexcitonic processes. 

Fig. 8.16 Scheme of the individual steps in the process of intrinsic photogeneration of charge-carrier pairs, Mp - M, in a molecular C7stal. The charge carriers are polarons (p). a Sq = neutral ground state S], S2, S3 are singlet excitons. Rate constants k/ i for autoionisation, kn for radiationless and kr for radiative intramolecular recombination (fluorescence), b bound charge-carrier pairs... 

In the final step, these bound charge-carrier pairs dissociate by thermal activation. The necessary thermal activation energy for this is = -Ucp = Ecp - E and is reduced by an applied electric field which is superposed onto the Coulomb potential, that is by the Stark effect (see Fig. 8.17). 

Here, n is the concentration of the charge carriers, [S] is the concenhation of singlet excitons, I the intensity of the UV radiation, Uj the constant for photoionisation of the singlet excitons, and k2 is the rate constant for charge-carrier production by two-photon absorption. AU three processes are proportional to 1. For the separation into individual processes, the spectral dependence of the quadratic photogeneration of charge-carrier pairs was determined and analysed [31]. 

Irradiation of liquids by light of quantum energies greater than the ionization energy, hv > Iiiq, generates photoconductivity. Ionization of molecules or atoms in the liquid leads to the generation of charge carrier pairs in the irradiated volume. 

Here the resistivities of the individual layers have to be summed up. The same two cases for k d as discussed in 4.1.1.1 are important. For k d 1, the whole volume is quasi uniformly illuminated and a stationary concentration of charge carrier pairs is produced at low applied voltage given by Equation 12. At sufficiently high voltages... 

In this section we shall discuss the increase in conductivity produced when a nonpolar liquid is subjected to a constant intensity of low LET radiation. This condition implies that the distribution of ionizing events throughout the measurement volume is homogeneous. Irradiation of a nonpolar dielectric liquid leads to an increase of the electrical conductivity due to the generation of free charge carrier pairs. For the following considerations, it is assumed that the intrinsic conductivity is much smaller than the conductivity produced by radiation. In the steady state with no electric field applied, the rate of generation is equal to the rate of recombination, i.e.. 

Gfi is the yield of free charge carrier pairs per 100 eV of absorbed energy. If the dimensions of the measurement cell and the materials used in the construction do not lead to a noticeable attenuation of the radiation then a homogeneous distribution of positive and negative charge carriers in the measurement volume will be obtained and the radiation-induced conductivity, 0 0/ at low applied electric field strength is given as... 

Volume recombination leads to a loss of charge carrier pairs from the irradiated volume. A collection efficiency, /, is defined as the ratio of the measured current to the saturation current which would be measured at a sufficiently high electric field strength where no volume recombination takes place (Boag, 1956). Since in liquids the quantum yield for charge carrier production is a function of the applied electric... 

An attempt to calculate the effect of an applied electric field on the free ion yield was made by Jaffe (1913), who treated the problem of ionization of a gas by a-particles. He assumed that the charge carrier pairs produced were initially in a dense column around the track of the particle. The density of positive and negative charge carriers at a distance r from the track is assumed to be given as... 

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