Electron time-resolved measurements

The IP system seems apparently to be poorly suited to time-resolved measurements when compared with electronic area detectors such as MWPCs which permit a time-... 

In a pulse-radiolysis optical-absorption method, the value of kje, where 8 is a molar absorption coefficient, is measured by the time-resolved measurement of the optical absorption of solvated electrons, and then the kr value is determined by the observed value of kr/s and the value of s known separately. 

In a method using a radioisotope, e.g., as an internal radiation source, the difference in a fluorescence intensity with and without a d.c. electric field is assumed to be due to electron-ion recombination, and the kr value is determined by the time-resolved measurement of the fluorescence with the known number density of ionization, which is separately determined. 

Time-resolved measurements of photogenerated (very intense illumination, up to 0.56 GW/cm ) electron/hole recombination on CD (selenosulphate/NTA bath) CdSe of different crystal sizes has shown that the trapping of electrons, probably in surface states, occurs in ca. 0.5 ps, and a combination of (intensity-dependent) Auger recombination and shallow-trapped recombination occurs in a time frame of ca. 50 ps. A much slower (not measured) decay due to deeply trapped charges also occurred [102]. A different time-resolved photoluminescence study on similar films attributed emission to recombination from localized states [103]. In particular, the large difference in luminescence efficiency and lifetime between samples annealed in air and in vacuum evidenced the surface nature of these states. 

Femtosecond time-resolved measurements formation and relaxation dynamics of the solvated electron in water... 

Flow does the occurrence of two fluorescing states for MK fit into the dynamic picture developed in Section IV The observed temperature dependence of the fluorescence quantum yield of MK in ethanol206 yields direct evidence that in this case, also, EBA < Ev. Recent time-resolved measurements at the Berlin Electron Storage Ring for Synchrotron Radiation (BESSY)207 support this argument The viscosity dependence of the decay of the short-wavelength fluorescence band in ethanol is consistent with an apparent value BA — 0.5Ev. Moreover, the decay is nonexponential, as would be expected for a barrierless relaxation. The lifetime of the TICT state (exponential decay) is 0.65 ns in acetonitrile at room temperature, that is, it is unusually short. 

Another approach of great importance for studies of excited state dynamics is sub-picosecond time resolved spectroscopy. A number of authors have reported femtosecond pump-probe measurements of excited state lifetimes in A, C, T, and G [13-16] and base pair mimics [17]. Schultz et al. have reported time resolved photoelectron spectroscopy and electron-ion coincidence of base pair mimics [18]. these studies can also be compared with similar measurements in solution [19-24], While time resolved measurements provide direct lifetime data, they do have the limitation that the inherent bandwidth reduces the spectral resolution, required for selecting specific electronic states and for selecting single isomers, such as cluster structure and tautomeric form. 

Figure 1 depicts a typical sequence of events started by absorption of an incident photon with an energy near the nuclear excited state energy Eq. The Fe nucleus has an excited state lifetime of 141ns, and excited nuclei have two decay channels. About 10% of them reemit a 14.4kev photon. For recoilless absorption, where no vibrational levels are excited, time-resolved measurements of 14.4kev photons scattered in the forward direction reveal information on hyperfine interactions comparable to conventional Mossbauer spectroscopy (see Mossbauer Spectroscopy). The remaining nuclei expel electrons from the atomic K shell, followed by... 

Time resolved measurements, perhaps the most important application of X-ray synchrotron radiation, are only meaningful with electronic area detectors. 

In the present report we describe new time-resolved measurements of solvation dynamics using the probe molecule coumarin 153 (hereafter referred to as C153) [8]. This well-known solvation probe has been thoroughly characterized in a number of past studies [2,9]. Its main attribute which makes it an excellent solvation probe is that upon electronic excitation its dipole moment changes from a ground state value of -6 D to a value of -IS D in 5, [10]. Furthermore, its S state is well separated fiom other excited singlet states and there is no indication of the occurrence of any excited-state reaction in most solvents [8,9]. 

Dutton et examined the efficiency of rotational relaxation of SO2 by various gases by measuring the terminal translational and rotational temperatures of SO2 seeded in various supersonic molecular beams. They noted that an increase in relaxation efficiency was obtained with increasing temperature in each case. Rotationally cold SO2 was prepared in a supersonic expansion of SO2 seeded in Ar and excited to the first electronically allowed state, A A2, by pulsed laser radiation in the range 300-320 Time-resolved measure-... 

Measurement of r in the absence of the electron donor will therefore yield k, and in its presence, obs will be obtained, from which k i can be evaluated. However, the proviso discussed earlier concerning product identity still applies, and the wise investigator will be concerned with whether it is electron transfer that is actually causing the quenching process. In the past twenty years, time-resolved measurements for fluorescence lifetime determination have become highly developed and they have been much used in electron-transfer research. 

In evaluating the experimental data obtained from the time-resolved measurements we have taken into account artificial time lags of different origins, such as the electron transit time of the photomultiplier. The experimental data presented here was corrected for these time lags. 

Time-resolved measurements of electron transfer times for quantum well photoelectrodes which can be compared with hot electron relaxation times, have not yet been reported. Only some excitation spectra, i.e. photocurrent vs. photon energy for MQWs and single quantum wells (SQWs), have been published so far [2]. In both cases, the photocurrent spectra show distinct structures corresponding to transitions between the hole and electron wells as shown for SQW electrodes in Fig. 9.32. The... 

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