Lifetimes of Hot Electrons

Time-resolved two-photon experiments revealed that the lifetime of an electron in the 2ji -derived level is indeed smaller than 5 fs, in agreement with the lifetimes of hot electrons in metals in this energy range [61 ]. It is believed that this marks the lower limit for the timescale of chemical reactions. 

At the end of the section on time-resolved measurements, we want to mention the work on lifetimes of hot electrons. These data are collected at energies at which no specific surface state exists and are, therefore, attributed to the lifetimes of hot electrons in the continuum of bulk states. Because 2PPE is intrinsically surface sensitive, some contribution from the surface is inherent to such data. An example is shown in Figure 6.17 for Ag(lOO) films of various thicknesses on an MgO(lOO) substrate [104]. Figure 6.17a shows 2-PPE spectra and the panel (b) presents the measured lifetimes. The lifetimes decrease approximately with ( — as... 

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... 

The bleaching recovery of the pristine Au colloids consists of a fast (r = 2.5 ps) and a slow process (t > 50 ps) [71]. The slower component of the recovery had a lifetime of 170 ps. These fast and slow recoveries correspond to the relaxation of hot electrons via electron-phonon coupling and phonon-phonon relaxation of the lattice, respectively. Dumping thermal energy into the solvent causes the dielectric of the surrounding medium to change, which in turn, influences the plasmon resonance frequency of the metal nanoclusters. 

Dissociation of axial ligands has been followed by picosecond spectroscopy for a number of metalloporphyrins. For the well-known photodissociation of O2 and CO from hemoglobin and myoglobin the photoproducts appear very early < 10 psec. Dissociation of basic axial ligands such as pyridine and piperadine occurs within the lifetime of the excited state for Ni(II), Co(III) as well as for Fe(II) porphyrins. Whether the ejected species is "hot" with energy from the electronic deactivation of the porphyrin is not known, but the dissociation process does not appear to be dependent upon the wavelength of the excitation pulse (30,32). 

Within the last 25 years of X-ray spectroscopy on fusion devices, the theory of He-like ions has been developed to an impressive precision. The spectra can be modeled with deviations not more than 10% on all lines. For the modeling, only parameters with physical meaning and no additional approximation factors are required. Even the small effects due to recombination of H-like atoms, which contribute only a few percent to the line intensity, can be used to explain consistently the recombination processes and hence the charge state distribution in a hot plasma. The measurements on fusion devices such as tokamaks or stellarators allow the comparison to the standard diagnostics for the same parameters. As these diagnostics are based on different physical processes, they provide sensitive tests for the atomic physics used for the synthetic spectra. They also allow distinguishing between different theoretical approaches to predict the spectra of other elements within the iso-electronic series. The modeling of the X-ray spectra of astronomical objects or solar flares, which are now frequently explored by X-ray satellite missions, is now more reliable. In these experiments, the statistical quality of the spectra is limited due to the finite observation time or the lifetime of... 

Growth via radical intermediates should lead to the homopolymer of phenylmethylsilane in the presence of unreacted dialkyldichlorosilane, unless an extensive transfer process operates. Radicals can be considered as intermediates that rapidly participate in the second electron transfer that leads to the corresponding anions. High local temperatures (>2000 K) might enable the second electron transfer to proceed despite the short lifetime (<1 (xs) of hot spots (15). Anions can react with both monomeric dichlorides in an SN2-type reaction that yields a copolymer (Scheme I). 

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.

Irradiation of adsorbate-covered surfaces with higher energy photons (typically up to 6.4 eV) with lower intensities opens the possibility of direct valence excitation. Since the lifetimes of electronic excitations at metal surfaces are much shorter than those for nuclear motion, photochemical reactions appear rather improbable. Surprisingly, however, the cross sections determined for photodesorption were found to be comparable to those found for reactions with free molecules, mainly because the short lifetime of the excited state is compensated by a much larger cross section for absorption of the light [32,62-64]. This process takes place in the near-surface region of the metal (within about 10 nm), where relaxation of the photoexcited electrons leads to rapid establishment of a transient energy distribution. As depicted in Fig. 4.11, these hot electrons may scatter at the surface or are resonantly attached to an empty level of the adsorbate. 

Plasmon mediated electron transfer involving direct injection of the hot electrons from plasmonic-metal nanoparticles to close-lying semiconductors has been demonstrated in devices where Au nanoparticles were anchored on a Ti02 nanowires scaffold. Such hot electrons exhibit a lifetime 1 to 2 orders of magnitude longer than those excited within the nanowires themselves. Advancement in this direction is of fundamental significance because it makes possible photochemical reactions at the semiconductor surface against electron/hole recombination. ... 

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