Time-resolved microwave absorption measurements

Summary The luminescence properties of a variety of silicon structures are presented The sizes range from simple organosilicon compounds over nano- and microcrystalline silicon /to bulk silicon. Time-resolved microwave absorption measurements have been used to show the importance of localization of the photogenerated charge carries for an efficient luminescence in systems with reduced effective dimensionality of the Si matrix... 

Fig. 3. Scheme for time-resolved microwave absorption measurements. 

Time-resolved microwave conductivity TRMC) Technique allowing the quantitative and qualitative detection of radiation-induced charge separation by time-re-solved measurement of the changes in microwave absorption resulting from the production and decay of charged and dipolar molecular entities. 

Transient terahertz spectroscopy Time-resolved terahertz (THz) spectroscopy (TRTS) has been used to measure the transient photoconductivity of injected electrons in dye-sensitised titanium oxide with subpicosecond time resolution (Beard et al, 2002 Turner et al, 2002). Terahertz probes cover the far-infrared (10-600 cm or 0.3-20 THz) region of the spectrum and measure frequency-dependent photoconductivity. The sample is excited by an ultrafast optical pulse to initiate electron injection and subsequently probed with a THz pulse. In many THz detection schemes, the time-dependent electric field 6 f) of the THz probe pulse is measured by free-space electro-optic sampling (Beard et al, 2002). Both the amplitude and the phase of the electric field can be determined, from which the complex conductivity of the injected electrons can be obtained. Fitting the complex conductivity allows the determination of carrier concentration and mobility. The time evolution of these quantities can be determined by varying the delay time between the optical pump and THz probe pulses. The advantage of this technique is that it provides detailed information on the dynamics of the injected electrons in the semiconductor and complements the time-resolved fluorescence and transient absorption techniques, which often focus on the dynamics of the adsorbates. A similar technique, time-resolved microwave conductivity, has been used to study injection kinetics in dye-sensitised nanocrystalline thin films (Fessenden and Kamat, 1995). However, its time resolution is limited to longer than 1 ns. 

A laser pulse can be used instead of the van der Graff generator to produce the electron-hole pair, which is a technique known as flash photolysis, time-resolved, microwave conductivity or FP-TRMC. The main disadvantage is that the concentration of electron-hole pairs formed in this way is more difficult to estimate. Usually, it can only be used to measure the product of the quantum yield and the sum of the mobilities (oriented films [15,16]. More recently a variation on this method has been introduced in which the transient absorption spectrum is measured simultaneously. This provides structural information on the charge carriers and (provided assumptions are made) allows their concentration to be determined yielding Sp, [17,18]. 

So far, we have considered only experiments with continuous-wave lasers under steady state conditions. With time-resolved experiments, on the other hand, energy transfer rates and transition probabilities can be obtained. Such measurements were carried out by mechanically chopping the laser beam directed into an external absorption cell together with the microwave radiation. Later, Levy et at reported time-resolved infrared-microwave experiments with an N2O laser Q-switched with a rotating mirror to produce pulses less than 1 /tsec in duration. They observed a transient nutation of the inversion levels of the molecule following the infrared laser pulse. Based on the Bloch equations, the observed phenomena could be explained quantitatively. From the decay envelope of the oscillations a value for the transverse relaxation time T2 was determined. Similar effects were produced by rapidly switching a Stark field which brings the molecules into resonance with the cw microwave radiation. 

The time and wavelength resolved fluorescence dynamics of bianthryl has been investigated by several groups [30, 82, 132, 133, 115, 116]. In addition, this molecule has been studied by picosecond absorption spectroscopy [115], electric field induced fluorescence anisotropy measurements [117] and optically induced dielectric absorption (microwave) measurements [118, 119]. The results are generally in accord with the theoretical model presented in Sections III.A and III.B. One of the challenges of studying the photodynamics of BA is that the LE and CT interconversion is so rapid (i.e., on the time scale of solvation) that it is necessary to employ ultraviolet subpicosecond and even femtosecond fluorescence spectroscopy which has only recently become available [30, 82, 132, 133]. 

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