Electron lifetime

Campillo I, Silkin V M, Pitarke J M, Chulkov E V, Rubio A and Echenique P M 2000 First-principles calculations of hot-electron lifetimes in metals Phys. Rev. B 61 13 484-92... 

Figure 1 Non-local layer dependent conductivity for majority electrons for parallel alignment of the cobalt moments. The scattering rate is assumed to be high so that the electron lifetime is relatively short (4.8X10 sec).

Equation (10.6) for the mobility in the two-state model implicitly assumes that the electron lifetime in the quasi-free state is much greater than the velocity relaxation (or autocorrelation) time, so that a stationary drift velocity can occur in the quasi-free state in the presence of an external field. This point was first raised by Schmidt (1977), but no modification of the two-state model was proposed until recently. Mozumder (1993) introduced the quasi-ballistic model to correct for the competition between trapping and velocity randomization in the quasi-free state. 

As the lattice interacts with light only through electrons, both DECP and ISRS should rely on the electron-phonon coupling in the material. Distinction between the two models lies solely in the nature of the electronic transition. In this context, Merlin and coworkers proposed DECP to be a resonant case of ISRS with the excited state having an infinitely long lifetime [26,28]. This original resonant ISRS model failed to explain different initial phases for different coherent phonon modes in the same crystal [21,25]. Recently, the model was modified to include finite electronic lifetime [29] to have more flexibility to reproduce the experimental observations. 

In his article mainly mode-locked tunable dye lasers are discussed. Giant pulse ruby lasers (3 nsec pulse halfwidth) have been successfully used to probe electron densities as a function of time in a rapidly expanding plasma 22). The electron lifetime in the conduction band can be determined with nanosecond semiconductor lasers. By absorption of the laser pulse the electrons in the semiconductor probe are excited into the conduction band, resulting in a definite conductivity. The mean lifetime is obtained by measuring the decrease of conductivity with time 26). 

Because all electron decays for O2-M mixtures in the above-mentioned experimental conditions show pseudo-first-order behavior, each decay curve gives an electron lifetime To, which is related to molecular number densities [O2] and [M] as ... 

Each o-vni is a strong function of monochromatic light energy hv as a threshold hv = Et is approached. This phenomenon will be discussed in the next section. However, for completeness we should relate Eq. (34a) to a common expression involving the absorption coefficient a and electron lifetime x... 

The strong peak shown at 0.88 eV for sample A in Fig. 5 is not consistent with the Lucovsky theory, nor, indeed, with most of the other theories that assume bound-state-to-band excitations. [Some recent calculations show the possibility of a sharp peak in the cross section if certain conditions are met, but it is not clear that these conditions are reasonable (Blow and Inkson, 1980).] One suggestion is that free-hole excitation above 0.88 eV causes a drastically reduced electron lifetime (Masut and Penchina, 1981) [i.e., larger denominator in the first term of Eq. (44)], but this interpretation is questionable because the... 

Reuther A, Iglev FI, Laenen R, Lauberau A (2000) Femtosecond photo-ionization of nucleic acid bases electronic lifetimes and electron yields. Chem Phys Lett 325 360-368 Reynisson J, Steenken S (2002) DFT calculations on the electrophilic reaction with water of the guanine and adenine radical cations. A model for the situation in DNA. Phys Chem Chem Phys... 

Nanocrystalline systems display a number of unusual features that are not fully understood at present. In particular, further work is needed to clarify the relationship between carrier transport, trapping, inter-particle tunnelling and electron-electrolyte interactions in three dimensional nan-oporous systems. The photocurrent response of nanocrystalline electrodes is nonlinear, and the measured properties such as electron lifetime and diffusion coefficient are intensity dependent quantities. Intensity dependent trap occupation may provide an explanation for this behaviour, and methods for distinguishing between trapped and mobile electrons, for example optically, are needed. Most models of electron transport make a priori assumptions that diffusion dominates because the internal electric fields are small. However, field assisted electron transport may also contribute to the measured photocurrent response, and this question needs to be addressed in future work. 

Reuther A, Iglev H, Laenen R, Laubereau A (2000) Femtosecond photo-ionization of nucleic acid bases Electronic lifetimes and electron yields. Chemical Physics Letters 325 360-368. 

The trapped holes which recombine slowly because of their low mobility are called safe hole traps . Their presence increases the electron lifetime and the photoconductivity and seems to account for the features of the photoconductivity not explained by the simple model of Eq. (8.69) (McMahon and Crandall 1989). Safe hole traps are most significant in low defect density material, when their concentration can exceed the defect density. A detailed analysis needs to take into account the full distribution of hole traps as well as the dispersive transport of holes. The role of transitions between the band edges in the recombination process also needs to be determined. 

At elevated temperatures, where the electron lifetime was much shorter than the pulse lengths of a few nanoseconds used, a second mobile species could be observed as a slowly decaying after-pulse conductivity component for large pulses. This was attributed to proton conduction with a proton mobility of 6.4 x 10 cm /Vs in H,0 ice and a somewhat lower value in D2O ice. ° In the case of the proton, the mobility was found to have an apreciable negative activation energy of 0.22 eV. The motion and trapping of protons was tentatively explained in terms of an equilibrium between free protons and a proton complexed with an orientational L-defect. °... 

The important influence of the surface on the electronic properties of Degussa P25 particles was illustrated by the dramatic effect of a layer of isopropanol on the decay kinetics of electrons. This resulted in an increase in the mobile electron lifetime from a few hundred nanoseconds to seconds. In addition, on repetitive pulsing the end of pulse conductivity increased to a value corresponding to a mobility of ca 1 cm /Vs. These effects were attributed to the retardation of surface recombination with holes which are removed from the surface as protons by reaction with the alcohol. 

These studies indicated that the photoproduction rates in solutions of varying composition were approximately proportional to the dissolved organic carbon (DOC) content. Assuming that the lifetime of the solvated electron in air-saturated water is 0.2 ps and that halocarbon concentrations are much lower than that of oxygen (and thus have little effect on the electron lifetime), the photoproduction rate observed in the Greifensee (DOC = 4 mg of C/L) corresponded to estimated near-surface pseudo-first-order photoreduction rate constants of about lOVh for several halocarbons known to be present in natural waters (Table V). These estimates were derived by using previously measured rate constants for reaction of solvated electrons with the halocarbons (48). 

Two-photon photoemission spectroscopy is known for its capability to reveal not only occupied but also unoccupied electronic density of states [10]. In this scheme, one photon excites an electron below the Fermi level to an intermediate state. A second photon then excites the electron from the intermediate state to a final state above the vacuum le vel. The photoelectron yields are strongly enhanced if the excitation photon energy is tuned to the resonance conditions, and the photoelectron spectrum reflects the electron lifetime in the intermediate states as well as their density of states. It is necessary to keep the employed photon energy below the work function of the sample, otherwise one photon photoemission signal becomes excessive and buries the 2PPE signals. 

Figure 2.9 Electron relaxation dynamics for GaAs (100). (a) Compares the hot electron lifetimes as a function of excess energy (above the valence band) of a pristine surface prepared using MBE methods with device-grade GaAs under the same conditions. The higher surface defect density of the device-grade material increases the relaxation rate by a factor of 4 to 5. (b) The electron distribution as a function of excess energy for various time delays between the two-pulse correlation for MBE GaAs. The dotted lines indicate a statistical distribution corresponding to an elevated electronic temperature. The distribution does not correspond to a Fermi-Dirac distribution until approximately 400 fs. The deviation from a statistical distribution is shown in (c) where the size of the error bars on the effective electron temperature quantifies this deviation.

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