Evolution Of Electron Spectrum Research Articles

Photoabsorption of potassium clusters isolated in helium droplets: From discrete electronic transitions to collective resonances

Photoabsorption spectra of neutral potassium clusters in helium nanodroplets are recorded by depletion spectroscopy using a supercontinuum laser source equipped with a variable bandpass filter. The results provide insight into the evolution of electronic spectra from molecular transitions to localized surface plasmon resonances in nanometer-sized clusters. Size selected spectra for potassium clusters ${\mathrm{K}}_{n}$ are acquired from $n=2--110$. Indications for the transition from molecularlike absorption to a collective resonance are already observed below $n=20$. For larger clusters, a splitting of the plasmon mode into two components is observed at around $n=600$. Supported by simulations, this is explained by the presence of nonspherical potassium nanoparticles grown via multicenter aggregation inside the helium droplets. The presented supercontinuum laser-based experimental approach provides a new route for the mass-selective spectroscopic characterization of different materials and material combinations isolated in helium nanodroplets in a size regime ranging from single atoms and molecules to small sub-10-nm particles.

Spatial dependence of high energy electrons and their radiations in pulsar wind nebulae

We investigate the spatial dependence of high energy electrons and their radiations in pulsar wind nebulae (PWNe). By assuming a time-dependent broken power-law injection and spatial dependence of convection velocity, magnetic field strength and diffusion coefficient on the radial distance of an expanding system, we numerically solve the Fokker-Planck transport equation including convection, diffusion, adiabatic loss and radiative loss in spherical coordinates, and investigate the effects of magnetic field, PWN age, maximum energy of electrons, and diffusion coefficient on electron spectra and non-thermal photon emissions. Our results indicate that (1) electron spectra and the corresponding photon spectra are a function of radial distance r of the expanding system; (2) for a given expansion velocity, the increase of the PWN age causes a slower decrease of the convection velocity (V ∝ r−β) and a more rapid decrease of the magnetic field strength (B ∝ r−1+β), but a more rapid increase of the diffusion coefficient (κ ∝ r1−β) because the index β decreases with the PWN age; and (3) the lower energy part of the electron spectra is dominated by convection and adiabatic loss, but the higher energy part is dominated by the competition between synchrotron loss and diffusion, and such a competition is a function of radial distance. Therefore the diffusion effect has an important role in the evolution of electron spectra as well as non-thermal photon spectra in a PWN.

Evolution of electron spectrum in symmetric two-barrier open nanostructure under the influence of strong electromagnetic field

Abstract. A theory of quasi-stationary spectrum of electrons interacting with electromagnetic field of arbitrary intensity in an open two-barrier resonance tunnel structure is developed using exact solution of the complete Schrodinger equation. It is shown that, besides the quasi-stationary electronic states having usual resonance energies and widths, additional satellite quasi-stationary states appear in the spectrum, with their resonance energies located near the resonance energies of every of the quasi-stationary states at the distances being multiples of an energy fixed by the electromagnetic field frequency.

Effet JAHN-TELLER cooperatif et spectre optique de l'ion Cu 2+ dans quelques chromites

Phase diagrams of Cu 1−xM xCr 2O 4 (M 2+ = Ni 2+, Zn 2+, 0,5(Li ++Ga 3+)) are established by X-ray diffraction at high temperature and some cationic distributions deduced from neutron diffraction. The ligand field reflectance spectra of tetrahedrally coordinated Cu 2+ in different oxidic host structures (I41/amd and Fd3m) are analysed at different temperatures (300 – 600 K). The evolution of electronic spectra may be correlated with the disparition of cooperative JAHN-TELLER effect. The quadratic distortion ( c a < 1 ) in mixed nickel-copper chromites (0,5 < x ⩽ 0,8) is mainly due to Ni 2+; but, electronic spectra at high température in the range 4000 – 12000 cm −1 discloses a certain contribution of Cu 2+ ions to JAHN-TELLER distortion.