Light intensity modulation method

Characterization of Electron Diffusion and Back-transfer by Light Intensity Modulated Techniques In Sect. 2.1.2, it was shown that time-resolved methods are required to obtain information on the kinetics and dynamics of a system. 

Prior to describing the possible applications of laser-diode fluorometry, it is important to understand the two methods now used to measure fluorescence lifetimes these being the time-domain (Tl)/4 5 24 and frequency-domain (FD) or phase-modulation methods.(25) In TD fluorometry, the sample is excited by a pulse of light followed by measurement of the time-dependent intensity. In FD fluorometry, the sample is excited with amplitude-modulated light. The lifetime can be found from the phase angle delay and demodulation of the emission relative to the modulated incident light. We do not wish to fuel the debate of TD versus FD methods, but it is clear that phase and modulation measurements can be performed with simple and low cost instrumentation, and can provide excellent accuracy with short data acquisition times. 

In an IMPS experiment the light intensity is modulated to produce an ac photocurrent that is analysed to obtain kinetic information. An alternative approach is to modulate the electrode potential while keeping the illumination intensity constant. This method is called PhotoElectrochemical Impedance Spectroscopy (PEIS). 

If both the light intensity and the electrode potential are modulated at different frequencies, information may, in principle, be obtained at the sum and difference frequencies. This method has been little used the complexities of the a.c. impedance analysis in the light have precluded all but... 

The electronics of an LCD convert the information to be displayed into a series of applied voltages of a given sign, amplitude and pulse width in order to activate the desired pixels at the appropriate time. This results in the modulation of the light intensity at the addressed pixels, thus creating the image or information to be displayed. The main methods by which this can be achieved are described below. 

The exact measurement of the optical path in cells of low thickness is made by interferometry (interference pattern method). The transmittance of the empty cell is measured for an interval between two wavenumbers rq and (in cm ). Figure 10.22 shows that the beam S2 has undergone a double reflection from the internal walls of the cell, thus for a normal incidence, there would be, if 21 = kX, addition of both light intensities (the two beams Sj and S2 are in phase). As a function of the wavelength a modulation of the main beam Sj of a few percent is observed. After calculation, if N represents the number of interference fringes counted between rq and PJ (in cm ), then ... 

IMPS uses modulation of the light intensity to produce an ac photocurrent that is analysed to obtain kinetic information. An alternative approach is to modulate the electrode potential while keeping the illumination intensity constant. This method has been referred to as photoelectrochemical impedance spectroscopy (PEIS), and it has been widely used to study photoelectrochemical reactions at semiconductors [30-35]. In most cases, the impedance response has been fitted using equivalent circuits since this is the usual approach used in electrochemical impedance spectroscopy. The relationship between PEIS and IMPS has been discussed by a number of authors [35, 60, 64]. Vanmaekelbergh et al. [64] have calculated both the IMPS transfer function and the photoelectrochemical impedance from first principles and shown that these methods give the same information about the mechanism and kinetics of recombination. Recombination at CdS and ZnO electrodes has been studied by both methods [62, 77]. Ponomarev and Peter [35] have shown how the equivalent circuit components used to fit impedance data are related to the physical properties of the electrode (e.g. the space charge capacitance) and to the rate constants for photoelectrochemical processes. 

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