Displacement photocurrent

Hong F T 1980 Displacement photocurrents in pigment-containing biomembranes ... 

Okajima T L and Hong F T 1986 Kinetic analysis of displacement photocurrents... 

Another consequence of the presence of the series capacitance, Cp, is the reduction of the source impedance of the membrane in the high-frequency range. Because of this peculiar effect, a short-circuit measurement is harder to achieve in the high frequency range than in a steady state because a lower input impedance is required to achieve a short-circuit condition. Sometimes a commercial picoammeter is used to measure displacement photocurrents under a presumed short-circuit condition. However, such instruments often have an input impedance on the order of 100 kft in the megahertz frequency range. In view of the much reduced source impedance of a reconstituted purple membrane, these measurement conditions are actually closer to an open circuit than to a short-circuit condition. The measurement is often accompanied by a telltale observation that the measured current is the first time derivative of the measured photovoltage (26). A detailed analysis of this problem is presented elsewhere (19, 20). 

The apparent symmetry of proton uptake and release on either side of the purple membrane suggests that proton release at the extracellular side and its reverse reaction may be manifest as a displacement photocurrent (a hypothetical B2 component). This signal is blocked in a Trissl-Montal bacteriorhodopsin film because the Teflon film precludes access of aqueous protons at the extracellular side. If, however, bacteriorhodopsin is reconstituted in a lipid bilayer membrane, this hypothetical component, which represents proton release at the extracellular side, might be observable. A complication arises from the expected polarity of the B2 signal, which should be the same as that of the B2 component (both are of opposite polarity to the B1 component). Therefore, a method must be devised to distinguish the B2 from the B2 component. The following kinetic analysis provides the rationale for such a method. 

Hong, F.T. and Montal, M., Bacteriorhodopsin in model membranes a new component of the displacement photocurrent in the microsecond time scale, Biophys. J., 25, 465, 1979. 

Okajima, XL. and Hong, EX, Kinetic analysis of displacement photocurrents elicited in two types of bacteriorhodopsin model membranes, Biophys. ]., 50,901, 1986. 

Hong, EX, Displacement photocurrents in pigment-containing biomembranes artificial and natural systems, in Bioelectrochemistry Ions, Surfaces, Membranes, ACS Advances in Chemistry Series 188, Blank, M, Ed., American Chemical Society, Washington, DC, 1980, p. 211. 

Fig. 4.2(c) displays the interruption of the bias voltage at time ti nntil time t2, when it is reapplied. The ideal interrupted TOF photocurrent in which displacement cnr-rents are absent is displayed in Fig. 4.2(d) as a result of the application of the bias in Fig. 4.2(c). Fignre 4.2(a) and (b) also define the varions time marks used in the remainder of the extensive article [1]. 

The temporal features of photocurrent transients of virtually all polymers and doped polymers described in the literature show (1) an initial spike of short duration, (2) a plateau of variable duration, and (3) a long tail. The initial spike has been explained as due to trapping at sites with waiting times that are comparable to the transit time (Scher, 1976) and by the thermalization of the carrier packet within the DOS (Bassler, 1993). In the plateau region, the current is independent of time, indicating the displacement of a packet of carriers with a... 

Figure 12-23. Simulated photocurrent transients for different values of For the simulations, <7=0.065 eV, c = 3xl0 , and , =0.25 eV. For clarity the curves have been displaced vertically and horizontally (Ref. [76]).

Here, (0) is the instantaneous photocurrent observed when the light is switched on. It corresponds to the displacement current qjp.j oo) is the steady-state photocurrent, that is, the plateau following the initial decay, jit/to) is the off transient observed when the light is switched off a t = to- Inspection of these equations reveals that the two unknown parameters... 

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