Atoms In Electric Fields Research Articles

Recurrence spectroscopy of atoms in electric fields: Failure of classical scaling laws near bifurcations

The photoabsorption spectra of atoms in a static external electric field shows modulations from {ital recurrences}: electron waves that go out from and return to the vicinity of the atomic core. Closed-orbit theory predicts the amplitudes and phases of these modulations in terms of closed classical orbits. A classical scaling law relates the properties of a closed orbit at one energy and field strength to its properties at another energy and field strength at fixed scaled energy {epsilon}=EF{sup {minus}1/2}. The scaling law states that the recurrence strength of orbits along the electric field axis scale as F{sup 1/4}. We show how this law fails near bifurcations when the effective Planck constant {cflx {h_bar}}{equivalent_to}{h_bar}F{sup 1/4} increases with increasing field at fixed {epsilon}. The recurrences of orbits away from the axis scale as F{sup 1/8} in accordance with the classical prediction. These deviations from the classical scaling law are important in interpreting the recurrence spectra of atoms in current experiments. This leads to an extension of the uniform approximation developed by Gao and Delos [Phys. Rev. A {bold 56}, 356 (1997)] to complex momenta. {copyright} {ital 1998} {ital The American Physical Society}

L-mixing dynamics of Rydberg states of hydrogen atom in a static electric field

Both exact analytical formula and numerical calculations are performed to study the l-mixing population decay of the n = 10 and n = 100 high Rydberg states of hydrogen atoms in an electric field. The two time constants of and observed in zero-electron-kinetic-energy (ZEKE) experiments in the common stray field condition, i.e. - can be obtained by 2-exponential fitting of the exact decay curve. The principle of high-resolution ZEKE spctroscopy based on the two time constants is then elucidated. The inverse Born-Oppenheimer approximation will be introduced to treat the ZEKE states of complex systems like many-electron atoms, molecules, clusters etc.

Quantum manifestations of bifurcations of closed orbits in the photoabsorption spectra of atoms in electric fields

The methods developed in the preceding paper (paper II) are used to construct a wave function near a bifurcation of classical orbits of an atomic electron in an electric field. A formula for the recurrence strength near the bifurcation is derived and compared with experimental measurements.

A theoretical study of some negative ions of interest to accelerator mass spectrometry

Accelerator mass spectrometry (AMS) exploits the properties of negative ions to measure the abundances of certain rare radioactive atoms. In cases such as 14C, 26Al and 129I, the complete instability of the negative ion of the abundant isobar makes the mass spectrometry relatively easy. Clearly a systematic understanding of the properties of negative ions is of value to AMS and the development of methods of isobar separation that involve the manipulation of negative ions by atomic collisions, electric fields and lasers. The purpose of the present work is to attempt to understand the stability or instability of some of the negative ions which are important to AMS and to point out that the periodic table of the negative ions has many differences from that of the neutral atoms.

Molecular Dynamics Simulation of Electromigration in Nano-sized Metal Lines

The molecular dynamics simulation of electromigration has been made for nano-sized metal lines. By solving Laplace equations for the electric field using the relaxation method the current direction and the local current density were determined. In this model, a two-dimensional Lennard-Jones single crystal structure in contact with an insulator was first assumed. The periodic boundary conditions of both the current and atomic flows were setup in the direction of the applied voltage. The forces originated from atomic pair potential and electric field were setup to atoms in the molecular dynamics simulation. The simulations showed that the generations of vacancies, their movements and disappearances could be realized at the areas with a large current density. When the current direction was along the closest packing direction of atoms, the diffusion coefficient became largest. Results suggest an existence of thresholds of the voltage and temperature for the metal line lifetime.

Experimental investigation of rubidium atoms above the field-ionization limit using a time-resolved wave-packet approach.

We have performed a time-resolved study of the behavior of Rb atoms in electric fields (0.1--3 kV/cm) at energies close to and above the field-ionization threshold. This is done using a pair of picosecond laser pulses that creates an electronic wave packet and monitors its evolution in terms of the time-autocorrelation function. In the discussion of the results, emphasis is put on the relation with the classical picture of field ionization, viz. the escape of an electron over the saddle point in the potential energy. This picture is found to be capable of qualitatively explaining the observed dependences on excitation energy and light polarization. Furthermore, the observed dynamics as a function of field strength is shown to be in agreement with the general scaling properties of the Hamiltonian of hydrogen, which also accounts for the measured time periods quantitatively. The relation between time- and frequency-domain measurements is shortly addressed, as is the question of why a hydrogenic approximation explains the observed dominant features adequately.

Collisions of Na+ ions with Na Rydberg atoms in electric fields: depopulation cross sections and final-state distributions from 36s and 30p initial states

An experimental study of the depopulation and final-state distribution of Na 36s and 30p Rydberg states under Na+ bombardment in the presence of electric fields up to 50 V cm-1 has been carried out. Incident energies E from 171 to 967 eV span the range from 0.6 to. 1.4 times the mean orbital speed of the excited electron in the initial state. Absolute depopulation cross sections of order 10-8 cm2, believed accurate within 25%, are reported as functions of field and energy. sigma 36s is proportional to E-1, but sigma 30p is nearly independent of energy. The distribution of final states was measured by selective field ionization. At weak fields for which the initial state is well separated from the nearest Stark manifold, final states are broadly distributed across the nearest manifolds. But for stronger fields that shift the initial state close to a Stark manifold the final states are strongly concentrated among the energetically closest manifold sublevels.

Vapor-deposition-induced magnetic anisotropy in amorphous RE-TM alloys (invited) (abstract)

Macroscopic anisotropy may be induced in amorphous materials by either preparation or subsequent annealing under appropriate conditions. Large, macroscopic magnetic anisotropy is observed in amorphous rare earth–transition metal (a-RE-TM) alloys produced by vapor deposition. The vapor deposition process inherently contains characteristic directions about which an anisotropy can occur. These include the substrate, incident atomic beams, and magnetic or electric fields at the surface during growth. These intrinsic directions combine to produce effects such as columnar microstructure, stress and strains, pair ordering in alloys, and nonperpendicular film growth directions. All of these structural effects potentially can produce magnetic anisotropy. The presence of the localized nonspherical f-electron nature of the RE ions causes a large local magnetic anisotropy; a macroscopic structural anisotropy can then cause a coherence in the directions of the local anisotropy axes, thereby producing a large macroscopic magnetic anisotropy. When the induced structural anisotropy is local in nature, it can be subtle and difficult to resolve, even though the resultant magnetic anisotropy itself is easily measured. We have used the magnetic anisotropy directly to monitor the effects of the important parameters of vapor deposition growth (e.g., substrate temperature, alloy composition, state of stress during growth, incident atomic beam angles) in order to understand what fundamental growth process is causing the anisotropy. Both the local anisotropy (as reflected by the high-field magnetic susceptibility) and the macroscopic anisotropy has been analyzed. I will present results which show that the dominant contribution to the macroscopic anisotropy in a-Tb-Fe is an amorphous phase texturing at the surface during the growth, due to minimization of the free energy of the surface. This effect is analogous to well-known effects in growth of crystalline materials. A secondary contribution due to stress plus magnetostriction is also observed.

Analytical estimation of the energies and widths of the Rydberg states of a hydrogen atom in an electric field

Abstract We have obtained analytical formulae permitting one to find with sufficiently high accuracy the positions and widths of highly excited states of the hydrogen atom in a homogeneous electric field. The functional dependence of these values on the quantum numbers of the system and on the field amplitude can be investigated by means of these formulae.

Electric field fluctuations in liquid tellurium alloys — a hint to bond character

Atomic scale electric field fluctuations in liquid tellurium alloys are detected as they induce nuclear spin relaxation rate R Q in noble gas impurity atoms, via quadrupolar interaction. Results for Xe in liquid Ag, Ga, In, Tl, Ge, SnTe alloys are discussed, assuming that bonding in these alloys may be described as a mixture of metallic, covalent and ionic bonding. Using an inhomogeneous model, and obtaining the metallic fraction from electrical conductivity, the Xe R 0 data are used to derive estimates of the ionic bonding fraction in the alloys studied.

Ionization of highly excited helium atoms in an electric field

We present detailed measurements of ionization of highly excited triplet helium atoms in a static electric field. The atoms were prepared in states with energy $E$ close to the saddle-point threshold $E=\ensuremath{-}2{[F(\mathrm{a}.\mathrm{u}.)]}^{\frac{1}{2}}$. The electric field $F$ was sufficiently strong for the states to be characterized by total spin $S$ and absolute value of the magnetic quantum number ${M}_{L}$. For ${M}_{L}=0$ states the experiments measured ionization properties of adiabatic states. In another case, $|{M}_{L}|=2$, they predominantly measured those of diabatic states. In both cases the ionization rate was found to be a highly nonmonotonic function of the field strength. The observations are analyzed in terms of a theory of the helium density of states in an electric field. A companion paper [D. A. Harmin, Phys. Rev. A 30, 2413 (1984)] develops in detail the general theory, which uses quantum defects to parametrize the effect of the core interaction. The agreement between measured and calculated ionization curves is good, indicating that the field ionization of a nonhydrogenic atom can now be understood in a detailed, quantitative, and predictive sense.

Atoms and Molecules in Electric Fields

The scope of this book is perhaps rather narrower than the title might suggest. Except for one chapter of 33 pages concerned with some specific types of molecules, the remainder of the work is really concerned with atoms in electric fields.

Selective ionization of atoms in optical and electric fields

A theoretical analysis is made of the method of selective ionization of atoms based on selective resonance excitation by laser radiation to quantum states near the ionization threshold and subsequent autoionization decay of the excited states under the influence of an external electric field. The autoionization of atoms in static and pulsed electric fields is considered. Calculation formulas are obtained for the probability of autoionization of a hydrogenlike atom and corrections are computed for the influence of the electron shells. Numerical estimates are obtained for the autoionization of the n = 7–12 states of the Rb atom in an electric field.

Valence Structure from X-Ray Diffraction Data: Physical Properties

It is assumed that the one-electron density function for a molecular crystal can be determined from x-ray diffraction data with the use of generalized x-ray scattering factors. The relationship between several physical properties and electron population parameters are given. These include molecular dipole moments and atomic electric field gradients, which can be compared to results from other experiments. The physical properties that follow from electron population analysis can serve as criteria in judging the reliability of x-ray diffraction data for charge density information.

AN INERATIVE PROCEDURE FOR THE SOLUTION OF PERTURBATION EQUATIONS

Iterative procedure for solving Rayleigh- Schroedinger perturbation equations applied to harmonic oscillator and H atom in electric field

Nuclear magnetic shielding at large distances of a hydrogen atom in an electric field

The nuclear magnetic shielding constant of a hydrogen atom in a uniform electric field is calculated at a pointP which is situated at a large distance from the nucleus in the direction of the electric field. The main term of the screening constant is found to be proportional to the first power of the electric field and to the inverse second power of the distance betweenP and the hydrogen nucleus.

Nuclear magnetic shielding of a hydrogen atom in an electric field

The nuclear magnetic screening constant of a hydrogen atom in a uniform electric field is calculated, neglecting any effect of electron spin. The screening is found to be reduced by the electric field for all directions of the applied magnetic field, the reduction being greatest if the two fields are perpendicular.