Photoconductivity ionization

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. 

According to precise photoconductivity measurements, the ionization onset, which is usually taken as the ionization potential, is ca. 1.5 eV lower in liquid alkanes than in the gas phase [38]. The ionization potentials in liquid and gas phases (7/ and Ig, respectively) are related by the equation ... 

Photoconductivity in zinc oxide, on the other hand, appears to be influenced by the surface through a different effect. Absorption of light effectively excites the electrons trapped in surface levels into the conduction band. This chapter will be primarily devoted to a consideration of this concept, proposed by Melnick (11), that photoconducting electrons are produced through the ionization of surface levels, specifically the adsorbed oxygen levels on zinc oxide. The decay of the photoconductivity. 

Photoconductive response (the rate of creation, or the rise, of the photocurrent, and the rate of decay of the photocurrent) appears to be divided into fast and slow responses. The fast responses, with time constants for rise and decay of the order of a second or less, have been adequately interpreted by Mollwo, et al. (53-55), Weiss (56), and Heiland (47,57) as bulk processes. These authors have concluded that the fast response processes are associated with the double ionization of interstitial zinc, and have proposed that the photon excites electrons from the valence band, and that the hole immediately recombines with the electron from an interstitial Zn+, producing double-ionized zinc ions. 

Most of the modern theories of the photoconductivity sensitization consider that local electron levels play the decisive role in filling up the energy deficit The photogeneration of the charge carriers from these local levels is an essential part of the energy transfer model. Regeneration of the ionized sensitizer molecule due to the use of the carriers on the local levels takes place in the electron transfer model. The existence of the local levels have now been proved for practically all sensitized photoconductors. The nature of these levels has to be established in any particular material. A photosensitivity of up to 1400 nm may be obtained for the known polymer semiconductors. There are a lot of sensitization models for different types of photoconductors and these will be examined in the corresponding sections. 

Postcolumn photochemical reactions are another approach to the detection problem. High-intensity UV light, generally provided by a Hg or Zn lamp, photolyzes the HPLC effluent, which passes through a Teflon (47) or quartz tube. The photolysis reaction determines the nature of the subsequent detection. If the compound has a UV chromophore, such as an aromatic ring, and an ionizable heteroatom, such as chlorine, then the products of the reaction can be detected conductometrically. Busch et al. (48) have examined more than 40 environmental pollutants for applicability to detection with photolysis and conductance detection. Haeberer and Scott (49) found the photoconductivity approach superior to precolumn derivatization for the determination of nitrosoamines in water and waste water. The primary limitation of this detection approach results from the inability to use mobile phases that contain ionic modifiers, that is, buffers and... 

We can find the potential at which a free electron appears by measuring the threshold of external photoemission E h. However, the ionization is not always accompanied by electron emission. We can consider the ionization event to have occurred if the electron is transferred to the conductivity band. The corresponding ionization potential 7C equals the energy needed to transfer an electron to the bottom of the conductivity band. It is found experimentally by measuring the threshold of photoconduction current. In crystalline insulators /c can be found from the limit to which the energy series for the Wannier-Mott179 exciton converges. 

The photoconductive properties of PVK were discovered in 1957 (Hoegl et al., 1957) and it is now accepted that the mechanism of photoconduction in PVK and TNF PVK is as summarized in Fig. 39. PVK absorbs uv light (360 nm) forming an exciton state which ionizes in an electric field (Kato et al.,... 

The photoinduced ionization of benzophenon in acetonitrile has also been reported to proceed via triplet-triplet annihilation at very low laser pulse intensities [272], The biexcitonic ionization in this system has been studied by applying the transient photoconductivity technique and described with the conventional (Markovian) rate equations, with the time-independent rate constants [273], Such equations can be represented as follows... 

The polymer exhibits very low dark currents (o60° = 3 x 10 19 ohm-1 cm-1). The photocurrent was reported to be proportional to the applied voltage and light intensity but its magnitude was far inferior to that of PVK. The poor photoconductivity is attributed to the high concentration of exdmer forming sites acting as exciton traps and also to poor transport characteristics. The carrier transport is expected to be slower since the ionization potential of the polymer is higher (7.88 eV) than that of PVK (7.43 eV). 

The situation improved considerably when the active layer was formed by a network of two (donor and acceptor) interpenetrating polymers or small molecules [34,119,121-123,35], For example Buckminsterfullerene C6o mixed with MEH-PPV is very effective in dissociating the exciton created by the incident light. C6o acts as an acceptor and the polymer, as a donor. The transfer of electron from MEH-PPV to fullerene occurs because fullerene has a larger electron affinity. The hole is left at the MEH-PPV because it has small ionization potential. The exciton dissociation results in quenching of the PL by factor up to 104 and in increasing the photoconductivity considerably. The transfer rate... 

Figure 2-2. HF and CIS predictions of the photoconduction (fundamental gap), photoemission (ionization), and optical absorption (excitation) energies of polyethylene [50, 55]...

The extension of TDDFT and Tamm-Dancoff TDDFT to crystalline polymers is straightforward within the formalisms of Section 2.2.2. Figure 2-7 summarizes the results of TDDFT calculations of the photoconduction, photoemission, and optical absorption thresholds (energy gap, ionization energy, and excitation energy) of polyethylene as a function of basis set [50], The Slater-Vosko-Wilk-Nusair functional [116, 117] is used, but the following conclusion is unaltered... 

Yasunaga et al. (1979) studied photoconductivity of single crystals of PbPc doped with O. The photocurrent increased sharply with increasing O. The results were explained by the ionization of a charge-transfer state created by the dissociation of a triplet exciton. The exciton dissociation was suggested to occur at crystal sites occupied by O. 

Impurity photoconductivity (extrinsic photoconductivity) is a type of absorption measurement where the detector is the sample itself. Classical photoconductivity occurs when the absorption of an electron or of a hole takes place between a discrete state and a continuum, where it can contribute to the electrical conductivity. When the final state of a discrete transition is separated from the continuum by an energy comparable to k T at the measurement temperature, the electron or the hole in this state can be thermally ionized in the continuum and give rise to photoconductivity at the energy of the discrete transition. This two-step process, which is temperature-dependent, is known as photo-thermal ionization spectroscopy (PTIS) and is discussed in more detail later in the section on extrinsic photoconductors. 

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