Spectroscopy measures charge separation

The fundamentals of electron transfer from an adsorbed molecule to a solid substrate has been considered by Willig and co-workers [9]. In this study, the photoinduced electron injection from the excited state of [Ru(dcbpy)2(NCS)2] into nanocrystalline TiC>2 was investigated both in high vacuum and in methanol containing 0.3 M LiCl. The central issue addressed in this investigation was to determine the time-scale on which the photoinduced electron injection from the molecular component to the solid substrate occurs. Transient absorption spectroscopy was used to follow this charge separation. The measurement was based on... 

The initial charge separation in PS I and PS II can be followed by what are known as ultrafast optical spectroscopy techniques. Several variations on this method exist, but they can be grouped into pump-probe absorbance difference and transient fluorescence methods (25, 26). In the first instance, the sample is irradiated with a pump pulse to initiate the electron transfer and the absorbance is measured using a probe pulse at a... 

The P +-Q state can be observed by transient absorption spectroscopy because the absorption spectrum of the charge-separated state differs from that of the porphyrin first excited singlet state. Measurement of the transient absorption spectrum of a benzonitrile solution of 8 as a function of time following excitation with a 150 to 200-fs laser pulse at 590 nm allowed determination of the rate constant for decay of P" -Q back to the ground state (step 3 in Figure 3). The value obtained for kj, is 5.3 X 10" s- [61]. 

Fig. 7. (A) Absorbance difference spectra of PS-II particles measured at 0 ps (a), 200 ps (b) and 1 ns (c) after excitation with 35-ps, 532-nm pulses. (B) the solid-dot absorbance-difference spectrum of the PS-II particles was obtained by subtracting the 1-ns spectrum (c) from the 200-ps spectrum (b) in (A) the empty-circie spectrum was obtained by subtracting the 1 -ns spectrum (c) in (A) from a 200-ps difference spectrum of PS-II particle containing dithionite [not shown], (C) kinetics of absorbance changes at 655 nm in the presence of ferricyanide (solid dots) or dithionite (empty circles). See text for discussion. Figure source Nuijs, van Gorkom, Plijter and Duysens (1986) Primary-charge separation and excitation of chlorophyll a in photosystem II particles from spinach as studied by picosecond absorbance-difference spectroscopy. Biochim BiophysActa 848 170,171.

One of the most important experimental techniques to appear in the last few years has been transient grating spectroscopy and this methodology has numerous important applications. The technique has been used to measure the quantum yield for photodissociation of diphenyl disulfide, and compared to photoacoustic and transient absorption spectoscopic methods. Additional studies have used transient grating spectroscopy to monitor the energetics and dynamics of charge separation of an ion pair into free ions. The approach permits direct measurement of the enthalpy change accompanying formation of the free ions... 

This approach was appUed to heterostructured seeded nanorods, both for CdSe-CdS (type I) and for ZnSe-CdS QD/QR core-shell nanocrystals. The former system was studied extensively, using a variety of optical spectroscopy methods the data acquired suggested a charge separation, where the hole is located in the QlSe core [positioned close to one end of the nanorod (NR)j and the electron extends over the CdS shell [84]. This picture is consistent with a small value of the conduction band offset, typically Ac<0.2eV, extracted from the bulk regime. However, a direct measurement of the band offsets and the consequent charge distribution in such nanocrystals is of major interest... 

The mechanism and dynamics of photoinduced charge separation and charge recombination have been investigated in synthetic DNA hairpins possessing donor and acceptor stilbenes (stilbene-4,4 -dicarboxylic acid, bis(3-hydroxypropyl)amide of stilbene-4,4 -dicarboxylic acid, bis(2-hydroxyethyl)stilbene 4,4 -diether) (Figure 11.1) using femtosecond broadband pump-probe spectroscopy, nanosecond transient absorption spectroscopy, and picosecond fluorescence decay measurements [11]. Nanosecond time-resolved spectra of stilbenes attached to DNA are shown in Figure 11.4. 

Because of their large charge separations and dipole moments, mesionic thiadiazoles are suitable for examination by X-ray photoelectron spectroscopy. Measurements performed on (205) and (206) (as well as on phenyl-sydnone) confirm the view that a betaine structure predominates in the... 

Two-Probe AC Impedance Spectroscopy Measurements In principle, the electrochemical processes of redox charge tfansfer and ion diffusion can contribute to the measured DC conductance [20, 31]. Therefore, to better define the intrinsic biofilm conductivity, it was desirable to use AC impedance measurements as an independent probe of conductance between the electrodes [46-48]. This method also serves to separate electronic and ionic conductivity in mixed-conductor system [49]. The applicability of this approach to anode biofilms was verified by evaluating the linearity and stability conditions described in the following section on the details of this experimental procedure. 

In 2003, the initial stages of the heterogeneous photoreduction of quinone species by self-assembled porphyrin ion pairs at the H20-1,2-DCE interface were studied by ultrafast time-resolved spectroscopy and dynamic photo-electrochemical measurements. Photoexcitation of the water-soluble ion pair formed by ZnTPPS and ZnTMPyP + leads to a charge-separated state of the form ZnTPPS -ZnTMPyP + within 40 ps that can inject electrons to acceptors in the organic phase in the microsecond time scale, resulting in photocurrent responses [228]. 

The dication KrN was formed by a charge-stripping process from KrN. The appearance potential for the unimolecular charge-separation process KrN " Kr + N was measured to be 40 1.5eV. The translational energy release, Te = 6.0 0.2 eV, connected with this dissociation process was determined by ion kinetic energy spectroscopy [8]. 

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