Photoconductivity measurements

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 ... 

Determination of the T2 Mycotoxin Level by the sNPS Photoconductivity Measurement... 

In discussing the use of photoconduction measurements for testing the photo-bleaching process, it is important to recognize that the photoconduction of dyes is due to the formation and migration of electronic charge carriers and not to impurities of photochemical decomposition. However, the following facts should not be overlooked 91>. 

Fig. 1. The relationship of the sample and the various pieces of equipment necessary to automatically measure the current, and the parallel and perpendicular voltages. A standard contact configuration is shown in the main drawing, whereas a van der Pauw configuration is shown in the inset. The lock-in amplifier is used in ac photoconductivity measurements. [From Look and Farmer (1981), copyright by The Institute of Physics.]...

A nonelectronic method of measuring impurity concentrations is that of absorption spectroscopy. From Eq. (36a) it is seen that ani = avnini0, where a i is the absorption constant due to electronic transitions from level i to the conduction band. The total impurity concentration Nt can be related to ni0 by a knowledge of EF. The photon-capture cross section doping experiments or by independently measuring Nt in some sample. This process has been carried out for Cr impurity (Martin, 1979) as well as (EL2) (Martin, 1981) in GaAs. The same considerations hold for photoconductivity measurements, except that t also needs to be known, as seen from Eq. (35). 

Fig, la, b. Typical schemes of the electrophotographic (a) and time of flight (b) methods for photoconductivity measurements in polymers [11]... 

Kellogg s photoconductivity measurements (quoted in ref. 107) showed that the photoconductivity of 0.2 pm AgBr grains increased with increasing degree of hypersensitization, passed through a maximum, then decreased. The sensitivity of the... 

In our experiments with the low-temperature sensitivity of monodisperse silver bromide emulsions having 0.5 ym cubic grains, the sensitivity of the (S+Au)-sensitized emulsion differed from that of the S-sensitized emulsion both with respect to the dependence on the degree of sensitization and the correlation with microwave photoconductivity measurements (16). 

Photoconductance of n-type semiconductor oxides exposed to O9 or NO. Photocatalytic reactions involve the exchange of electrons between an excited semiconductor and at least one adsorbed reactant or surface species. Consequently, photoconductance measurements appear as a discriminating method to investigate these reactions. 

The time-resolved photoconductivity measurements shown in Fig. 15 give further support for a difference in the photoinduced charge transport in the polymerized samples versus the unpolymerized samples. For the incident laser of 100 mW/cm2 and a spot size of 2.5 mm, the decay time of the photoconductivity for the unpolymerized samples is 7.4 sec, whereas the photoconductivity of the polymerized samples does not significantly drop over a 30 sec period. Also, the photoconductivity of the polymerized sample is nearly twice that of the unpolymerized samples even at the peak of the unpolymerized photoconductive response. The unnormalized values for the dark conductivity in both samples is 1.7 x 10-10 S cm-1. The photoconductivity is 5.8 x 10-11 S cm-1 for the unpolymerized sample and 1.1 x 10-10 S cm-1 for the PSLC at an optical intensity of 2 W cm-2. 

A spin nature of the effect of MF on photogeneration of free charge carriers was verified under experimental conditions of the registration of an EPR spectrum detected by photoconductivity measurements (RYDMR-Reaction Yield Detected Magnetic Resonance) of TBPDA(C6o)2- The spectrum (Fig. 4) contains two resonance peaks of negative polarity at 0.189 and 0.312 T, the peak halfwidth is 0.009 T. The second peak has a fine structure. 

A number of photoconductuve resins made by condensation of anthracene, pyrene etc. with formaldehyde, benzaldehyde etc. have been disclosed in the patent literature. Their structure is however ai defined and no exact photoconductivity measurements have been reported in the literature. 

The experimentally most intensively investigated systems are PTS (R= CH2S03C6H4CH3) and TCDU [R=(CH2)4OCONHCeH5], the former with an acetylene-like backbone69 while the latter is best represented by the butatriene structure.70 The experiments performed on these and other PDA crystals involved photoconductivity measurements,71 Raman spectroscopic studies,72 investigation of the core- and valence-electron spectra by UPS and XPS,78 as well as by absorption and reflectivity measurements.74... 

K s , is correspondingly higher than in the doped material. Some relaxation data are shown in Fig. 6.9 and the general similarity to the data in Fig. 6.3 is obvious. The activation energy is nearly 1.5 eV, again consistent with Eq. (6.4). The temperature-dependent equilibrium defect density has been observed by ESR, PDS and by photoconductivity measurements (McMahon and Tsu 1987, Xu et al. 1988). Some ESR data have found the opposite behavior, in which the defect density is reduced after a rapid quench and relaxes to a higher value... 

Transient photoconductivity measurements of the depletion width, as described in Section 9.1.3, show that there is an electron accumulation layer at the interface with SijN4 (Street et al. 1985b). In contrast, an oxide interface (either a native or deposited oxide) has a depletion layer (Aker, Peng, Cai and Fritzsche 1983). The band bending causes similar changes in the conductance of the films as is described for adsorbed molecules in Section 9.2.2. 

Fig. 9.19. Transient photoconductivity measurements of the band bending at the interface of a-Si H and a-SiN H, showing the effects of growth order (Street and Thompson 1984).

Photoionization of other rare-earth ions (Eu, Sm ) has been reported by Pedrini et al. (48). They were able to detect the charge carriers in the conduction by photoconductivity measurements. 

They also observed that the species absorbing at 720 nm which decays on the microsecond time-scale by first-order kinetics is a long-lived SSIP. Evolution of the SSIP to free-radical ions was ruled out, because free-radical ions should decay by second-order kinetics. Later observations by Mataga [159, 160] and by Haselbach [161], on the basis of photoconductivity measurements and transient absorption spectroscopy of the benzophenone-DABCO system, showed that the species absorbing at 710 nm decays not by first-order kinetics but by second-order kinetics this is consistent with the formation of free-radical ions. 

From picosecond transient photoconductivity measurements on PPP films,22 we know that mobile charged states decay within 110 ps. In conventional routes to PPPs, defects like branched chains and large torsion angles of neighboring rings are known to occur. These defects act as shallow or deep traps for positive and negative polarons,38,39 which limit the mobility of charge carriers.40 The synthetic route toward the PPP-type ladder-polymers prevents the described defects and leads to a trap concentration of less than 1 trap per 1000 monomer units,28 whereas substi-... 

Moreover, from the electronic point of view, it has been shown by photoconductance measurements that (i) under vacuum the electrons photo-produced in the support are attracted by the metal because of the alignment of the Fermi levels corresponding to the illuminated state - the electron enrichment of the metal is supported by the fact that the higher the number of platinum particle, the higher the number of photo-electrons trapped and the smaller the photoconductance of the sample (33) - and (ii) under H.., there still exists a reversible spill over of atomic hydrogen, described by a photoconductivity isotherm (a = a + b P 1/2) formally identical to that obtained in the dark (Eq,7) but with "a" being proportional to the radiant flux. 

The electron transfer from a n-type semiconductor support to a metal is quite general and can be observed in milder conditions when high temperature reduction is replaced by suitable illumination at room temperature. It has been detected either under vacuum or in hydrogen by photoconductance measurements (33) and even in oxygen by photosorption determinations (35). ... 

After several reports between 1965 and 1980, no new information has been published on the spectroscopy of donors in GaP. Odd-parity transitions from the ground to excited states associated with the lowest X band for the Si, S and Te donors have been reported in the 55-100 meV ( 440-810cm-1) spectral domain [10,39,196,223]. The spectra are superimposed on the two-phonon spectrum of GaP and the FWHMs of the absorption lines at LHeT are - ()Ai meV. LHeT photoconductivity measurements in the photoionization region of shallow impurities in GaP revealed dips due to electronic transitions accompanied by the emission of LA(X) and LO (r) phonons with energies of 404 and 254 cm 1, respectively, and they have contributed to the understanding of the donor spectra [222]. LHeT transmission spectra of GaP Si samples at LHeT showing Si donor transitions are displayed in Fig. 6.45. 

For shallow multiple acceptors associated with the Fs+VB, the fourfold degeneracy allows one to accommodate a maximum of four holes. Photoconductivity measurements in the very-far IR at LHeT and down to 1.2 K have indeed shown that group-II neutral acceptors and Cu° could bind an extra... 

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