Photo-Excitation Current

The transient change of photo-excitation current is shown in Fig. 33, where light from Xe lamp is irradiated for 17 ms onto the iron electrode passivated at various potentials for Ih in pH 6.5 borate solution. The current initially reveals a sharp peak and then decays with time. The steady photo-current which is evaluated from the end current of the 17 ms irradiation is almost zero at the potentials lower than 1.1 V vs. RHE and the appreciable current is observable at the higher potentials. The steady photo-current is assumed to be the transfer of the holes from the va- 

The similar photo-excited current was measured for the film first oxidized at 2.03 V to form a passive oxide 5 mn thick. The photo-current of the oxide film was measured as a function of decreasing potentials (Fig. 34). Due to the thicker oxide film, the photo current is larger than that shown in Fig. 33. The steady pho- 

In the potentials lower than 1.1 V for the forward (positive) potential step and 0.6 V for the backward (negative) potential step, although the hole generated by photon migrates to the ox-ide/solution interface, the transfer of the holes does not occur probably because the position of energy level of the adsorption 

State is occupied by electrons and the electron transfer from the H2O/O2 redox level to the state is inhibited. In that case, the holes excited by photon are accumulated at the oxide surface region. The current decay in Figs. 34 and 35 will be the accumulation process. In the higher potential, the surface state will change to partially occupied state and can accept the electron from the H2O/O2 redox, resulting in observation of the steady photo-current (Fig. 35c). 

Action spectrum of the photo excited current, which was measured by lock-in techniqne with chopped light at 830 Hz by monochromated light, is shown in Fig. 36 in which the photo-current normalized by IW incidence power is plotted by loga- 

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From the intercept the Sg is estimated to be 2.6 eV. Searson et al. replotted the absorption coefficient estimated from the data in Fig. 24 in (ahvf = (Av -sae) to evaluate the band gap energy of 1.75 eV for the indirect transition. Such band gap energy has been evaluated from the photo-excited cd measured as a function of photon energy under an assumption that the cd was proportional to the absorption coefficient. The absorption edge was estimated from the photo-excited cd to be a range from 2 to 3 eV. " The photo-excited current will be discussed in the following section. 

Figure 33. Response of photo-excited current to 17 ms illumination for the iron electrode covered hy the passive oxide in pH 6.5 borate solution. The passive oxide was formed by step-wise increase of potential and the photo-current was measured after l.Sks oxidation at the respective potentials. Reprint from K. Azumi, T. Ohtsu-ka and N. Sato, Analysis of Transient Photocurrent in Passivated Iron Electrode in Neutral Borate Solution , Nippon Kinzoku Gakai-shi (Bulletin of JIM), 53 (1989) 479, Copyright 1973 with permission from Japan Inst. Metals.

Figure 36. Spectra of photo-excited current per normalized power of incidence light. The passive film was formed at 1.60 V in pH 8.4 borate solution for 1 h. The photo-excited current was measured at the same potential. Reprint from K. Azumi, T. Ohtsuka and N. Sato, Spectroscopic Photoresponse of the Passive Film Formed on Iron , J. Electrochem. Soc., 133 (1986) 1326, Copyright 1986 with permission from The Electrochemical Soc.

In the framework of DECP, the first pump pulse establishes a new potential surface, on which the nuclei start to move toward the new equilibrium. The nuclei gain momentum and reach the classical turning points of their motion at t = nT and t = (n + l/2)T. The second pump pulse then shifts the equilibrium position, either away from (Fig. 3.10b) or to the current position of the nuclei (Fig. 3.10c). The latter leads to a halt of the nuclear motion. Because photo-excitation of additional electrons can only shift the equilibrium position further in the same direction, the vibrations can only be stopped at their maximum displacement [32]. 

Pig. 10-18. (a) PolarizatioD curves of anodic dissolution and (b) Mott-Schottky plots of an n-type semiconductor electrode of molybdenum selenide in the dark and in a photo-excited state in an acidic solution C = electrode capacity (iph) = anodic dissolution current immediately after photoexdtation (dashed curve) ipb = anodic dissolution current in a photostationary state (solid curve) luph) = flat band potential in a photostationary state. [From McEv( -Etman-Memming, 1985.]... 

As shown in Eqn. 10-46, the difference in the polarized potential at constant anodic current, between the photoexcited n-type and the dark p-type anodes of the same semiconductor, represents the inverse overvoltage iip sc for the generation and transport of photo-excited holes. 

Compton etal. (1990a) examined the mediated reduction of r-butyl bromide ( BuBr) by the photochemically excited radical anion of tetrachlorobenzo-quinone (TCBQ) in acetonitrile solution using a channel electrode. Under dark conditions, the reduction of TCBQ proceeded via a simple reversible one-electron transfer process in the presence of BuBr. On photo-excitation of the radical anion of TCBQ, the limiting current associated with its formation was enhanced suggestive of the EC mechanism (88). 

LTG GaAs. If > S the electrode spacing, then a significant amount of current will be generated by the photo-excitation. That is... 

The band gap energy has been discussed from the photo-current action spectrum. The band gap energy estimated from the photo-cmrent spectra is abont 2 eV for the assumption for the indirect photo excitation process. We can illustrate a model of the band diagram of n-type semiconductive passive oxide for the photo-induced process in Fig. 38. The indirect transition may pos-... 

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