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Charge exchange has been recognized as a primary source of soft X-ray emission in many astrophysical outflow environments, including cometary and planetary exospheres impacted by the solar wind. Some models have been set up by using different data collections of charge-exchange cross-sections. However, multiple-electron transfer has not been included in these models. In this paper, we set up a charge-exchange model with the inclusion of double-electron capture (DEC), and make a detailed investigation of this process on X-ray emissions of highly charged carbon, nitrogen, oxygen, and neon ions by using available experimental cross-sections. We also study the effect of different n- selective cross-sections on soft X-ray emission by using available experimental n -distributions. This work reveals that DEC enhancement on line intensity is linearly proportional to the ratio of ion abundance in the solar wind. It is more obvious for soft X-rays from carbon ions (C4+) in collision with CO2, and the enhancement on line intensity can be up to 53 per cent with typical ion abundances [ Advanced Composition Explorer (ACE)] in the solar wind. The synthetic spectra with parameters from the Ulysses mission for the solar wind reveal velocity dependence, target dependence, as well as the non-negligible contribution from the DEC. [ABSTRACT FROM AUTHOR]

Semiconductor nanocrystals are called artificial atoms because of their atom-like discrete electronic st9ucture resulting from quan- tum confinement. Artificial atoms can also be assembled into at-ti- ficial molecules or solids, thus, extending the toolbox for material design. We address the interaction of artificial atoms with bulk semiconductor surfaces. These interfaces are model systems for understanding the coupling between localized and delocalized electronic structures. In many perceived applications, such as nano- electronics, optoelectronics, and solar energy conversion, interfa- cing semiconductor nanocrystals to bulk materials is a key ingredi- ent. Here, we apply the well established theories of chemisorption and interfacial electron transfer as conceptual frameworks for understanding the adsorption of semiconductor nanocrystals on surfaces, paying particular attention to instances when the nona- diabatic Marcus picture breaks down. We illustrate these issues using recent examples from our laboratory. [ABSTRACT FROM AUTHOR]

The article reports on the photoinduced electron transfer. The photoinduced electron transfer is of interest to research fields such as photocatalysis, surface photochemistry and dye-sensitized solar cells. The process of photoinduced electron transfer is divided into two mechanisms, which is either direct or indirect.

Summarizes experiments using two-photon photoemission spectroscopy to characterize the electronic structure and electron transfer dynamics at molecule-metal interfaces. Experimental techniques in two-photon photoemission; Discussion of image states; Implications for molecule electronic devices.

Solar wind (SW) charge exchange (CX) is an important mechanism responsible for X-ray emissions in comets' coma and many other astrophysical environments. By incorporating the experimentally derived total cross-sections and the tested state-selectivity models into the Kronos program, we generate X-ray spectra in H-like and fully stripped C, N, O ion CX collision with atomic H. Using these data, we revisit the fitting of Chandra X-ray data of Comet C/1999 S4 by considering the typical SW collision energies of 0.8 and 3 keV/u, respectively. Our results are in an excellent agreement with the astrophysical observation. We find that charge exchange between C5+,6+, N6+,7+, O7+,8+ and H significantly contributes to the comet's spectrum ranging from 300 to 800 eV for both slow and fast SW ions, where N7+ abundance is underestimated in previous studies. [ABSTRACT FROM AUTHOR]

The state-resolved capture cross sections for principal n and orbital angular momentum l play an important role in modeling soft X-ray emissions induced by charge exchange for many astrophysical environments. However, the empirical and semiclassical theories used to produce these data of n - and l -resolved state-selective capture have not been well tested. Using the cold target recoil ion momentum spectroscopy apparatus at Fudan University, we perform a series of measurements of Ar8+ ion charge exchange with He in the collision energy range from 1.4 to 20 keV uâˆ'1. We find that electrons are mainly captured in the n = 4 state of Ar7+ ions. This agrees with the prediction of the scaling law for n capture. Moreover, the relative cross sections are reported for 4 s-, 4 p-, 4 d-, and 4 f -resolved state-selective capture. The often used analytical l distributions in the astrophysical literature are evaluated by comparing to the measurements. [ABSTRACT FROM AUTHOR]

Heterogeneous electron transfer (ET) across interfaces is frequently discussed on the basis of Marcus theory taking into account the rearrangements of the solvent along a nuclear coordinate q. The ET process itself occurs via tunnelling through a barrier normal to the interface. The key point is not whether tunnelling occurs, but whether thermally activated solvent fluctuations initiate the tunnelling. Here, we discuss the role of thermally activated tunnelling in heterogeneous ET versus direct ET due to the strong electronic coupling to a metal substrate. As a model system, we investigate the ultrafast dynamics of ET at amorphous ice-metal interfaces (4-6 bilayers D2O/Cu(111) and Ru(001), respectively) by time-resolved two-photon photoelectron spectroscopy. We find that the ET rate is independent of temperature within the first 500 fs after excitation, which demonstrates that for this system interfacial ET occurs in the strong-coupling limit and that thermally assisted tunnelling plays a negligible role. [ABSTRACT FROM AUTHOR]

The band structure and intra- and interband scattering processes of the electrons at the surface of a bismuth bilayer on Bi2Se3 have been experimentally investigated by low-temperature Fourier-transform scanning tunneling spectroscopy. The observed complex quasiparticle interference patterns are compared to a simulation based on the spin-dependent joint density of states approach using the surface-localized spectral function calculated from first principles as the only input. Thereby, the origin of the quasiparticle interferences can be traced back to intraband scattering in the bismuth-bilayer valence band and Bi2Se3 conduction band and to interband scattering between the two-dimensional topological state and the bismuth-bilayer valence band. The investigation reveals that the bilayer band gap, which is predicted to host one-dimensional topological states at the edges of the bilayer, is pushed several hundred meV above the Fermi level. This result is rationalized by an electron transfer from the bilayer to Bi2Se3 which also leads to a two-dimensional electron state in the Bi2Se3 conduction band with a strong Rashba spin splitting, coexisting with the topological state and bilayer valence band. [ABSTRACT FROM AUTHOR]

Multiple exciton generation (MEG) refers to the creation of two or more electron-hole pairs from the absorption of one photon. Although MEG holds great promise, it has proven challenging to implement, and questions remain about the underlying photo-physical dynamics in nanocrystalline as well as molecular media. Using the model system of pentacene/fullerene bilayers and femtosecond nonlinear spectroscopies, we directly observed the multiexciton (ME) state ensuing from singlet fission (a molecular manifestation of MEG) in pentacene. The data suggest that the state exists in coherent superposition with the singlet populated by optical excitation. We also found that multiple electron transfer from the ME state to the fullerene occurs on a subpicosecond time scale, which is one order of magnitude faster than that from the triplet exciton state. [ABSTRACT FROM AUTHOR]

In typical semiconductor solar cells, photons with energies above the semiconductor bandgap generate hot charge carriers that quickly cool before all of their energy can be captured, a process that limits device efficiency. Although fabricating the semiconductor in a nanocrystalline morphology can slow this cooling, the transfer of hot carriers to electron and hole acceptors has not yet been thoroughly demonstrated. We used time-resolved optical second harmonic generation to observe hot-electron transfer from colloidal lead selenide (PbSe) nanocrystals to a titanium dioxide (TiO2) electron acceptor. With appropriate chemical treatment of the nanocrystal surface, this transfer occurred much faster than expected. Moreover, the electric field resulting from sub-50-femtosecond charge separation across the PbSe-TiO2 interface excited coherent vibrations of the TiO2 surface atoms, whose motions could be followed in real time. [ABSTRACT FROM AUTHOR]