Free electron model

About: Free electron model is a research topic. Over the lifetime, 4678 publications have been published within this topic receiving 103535 citations.

On the formation and decay of a molecular ultracold plasma

Abstract: Double-resonant photoexcitation of nitric oxide in a molecular beam creates a dense ensemble of 50f(2) Rydberg states, which evolves to form a plasma of free electrons trapped in the potential well of an NO+ space charge. The plasma travels at the velocity of the molecular beam and, on passing through a grounded grid, yields an electron time-of-flight signal that gauges the plasma size and quantity of trapped electrons. This plasma expands at a rate that fits with an electron temperature as low as 5 K, colder than typically observed for atomic ultracold plasmas. The recombination of molecular NO+ cations with electrons forms neutral molecules excited by more than twice the energy of the NO chemical bond, and the question arises whether neutral fragmentation plays a role in shaping the redistribution of energy and particle density that directs the short-time evolution from Rydberg gas to plasma. To explore this question, we adapt a coupled rate-equations model established for atomic ultracold plasmas to describe the energy-grained avalanche of electron–Rydberg and electron–ion collisions in our system. Adding channels of Rydberg predissociation and two-body, electron–cation dissociative recombination to the atomic formalism, we investigate the kinetics by which this relaxation distributes particle density and energy over Rydberg states, free electrons and neutral fragments. The results of this investigation point to conditions under which such processes can effect the steady-state temperature of plasma electrons.

Extinction-free electron diffraction refinement of bonding in SrTiO3.

Abstract: Accurate low-order Fourier coefficients of the crystal potential of SrTiO 3 are measured by quantitative convergent-beam electron diffraction. The accuracy in the corresponding derived X-ray structure factors is about 0.1% for the strong low-order reflections (sin θ/λ < 0.3 A -1 ). This accuracy is better than for conventional X-ray diffraction and equivalent to the accuracy of the X-ray Pendellosung method. Combination of these structure factors with high-order X-ray diffraction measurements allows accurate bonding information to be obtained from a multipole model fitted to the experimental data. It is shown that Ti-O has a covalent component and that the Sr-O bond is mainly ionic. The role of Ti 3d electrons in Ti-O bonding is also discussed.

Ultrafast scanning electron microscope applied for studying the interaction between free electrons and optical near-fields of periodic nanostructures

Abstract: In this paper we describe an ultrafast scanning electron microscope setup developed for the research of inelastic scattering of electrons at optical near-fields of periodic dielectric nanostructures. Electron emission from the Schottky cathode is controlled by ultraviolet femtosecond laser pulses. The electron pulse duration at the interaction site is characterized via cross-correlation of the electrons with an infrared laser pulse that excites a synchronous periodic near-field on the surface of a silicon nanostructure. The lower limit of 410 fs is found in the regime of a single electron per pulse. The role of pulse broadening due to Coulomb interaction in multielectron pulses is investigated. The setup is used to demonstrate an increase of the interaction distance between the electrons and the optical near-fields by introducing a pulse-front-tilt to the infrared laser beam. Further we show the dependence of the final electron spectra on the resonance condition between the phase velocity of the optical near-field and the electron propagation velocity. The resonance is controlled by adjusting the initial electron energy/velocity and by introducing a linear chirp to the structure period allowing to increase the final electron energy gain up to a demonstrated 3.8 keV.

The microelectronic structure of platinum particles investigated by NMR

Abstract: The properties of195Pt nuclear magnetic resonance of small particles have been studied over a range of particle sizes from 33 to 200 A using pulsed NMR techniques. This work was initiated to study size-dependent phenomena and to elucidate their physical origin. An anomalous linewidth, more than an order of magnitude larger than that of bulk platinum for the smallest size sample, was discovered. This was found to be inhomogeneous broadening and to have a size dependence (d)−1, whered is the mean particle diameter of the sample. Within the temperature range of 1.7–77 K, no temperature dependence was observed. As a consequence, the broadening was attributed to an intraparticle Knight shift distribution resulting from electron spin density oscillations associated with the metal surface. Spin-echo envelopes from the small platinum particles were found to decay nonexponentially, indicating the presence of a nuclear spin diffusion process in the magnetic field gradients associated with the Knight shift distribution. The diffusion process was measured by the Hahn (90°–180°) pulse technique and the Carr-Purcell-Meibocm-Gill techniques and analyzed using a spin diffusion constant of platinum computed with the moment method of Redfield and Yu. The computed spin diffusion constant wasD z=4×10−12 cm2/sec and from this an rms magnetic field gradient was determined. Upon analyzing the size-dependent spin-echo results, the field gradient was found to be characteristic of a surface region of thickness 1.5 ± 0.5 lattice constants independent of the size of the particles. In contrast to these anomalous properties of small platinum particles, the peak position of the resonance lines and the spin-spin and spin-lattice relaxation times were found to be identical to the values for bulk material. A simple model ascribing free electron behavior tos conduction electrons was applied to study quantitatively the effects of electron spin density oscillation on NMR properties. Applying an infinite-barrier boundary condition to the electrons, we computed numerically the electron charge and spin density distributions for platinum particles of diameter up to 300 A. The computations revealed Friedel oscillations of these densities near the particle surface. From the computed electron spin density of thes electrons, the spatial distribution of the contact Knight shift was obtained, which could account for all the observed NMR properties of small platinum particles—the resonance position, line shape, linewidth, relaxation times, rms field gradient, and the thickness of the surface region.

Spin Wave Resonance and Exchange Parameters in fcc Fe-Ni Alloys

Abstract: Spin wave resonance for a series of fcc Fe-Ni alloys has been measured in order to study the exchange stiffness constant D . In general the resonance field vs the square of the spin wave mode number ( n ) curve is linear for high values of n , whereas some amount of deviation from linearity occurs for low values of n . This is considered to be due to the inhomogeneous demagnetizing field of the sample. We can determine the value of D from the linear part of the curve, provided we have a sufficient number of observed modes. As a supplementary means, we have also made low temperature magnetization measurements from which the value of D was derived. Consistency between these two kinds of measurements is ascertained. The composition dependence of D is not quite coincident with that derived from the neutron small angle scattering experiments by Hatherly et al. The data are discussed both from the standpoint of localized electron model and collective electron model.