Electron flux distributions in photodetachment of H<Superscript>-</Superscript> in parallel electric and magnetic fields

Photo-detached electrons of negative hydrogen ion in parallel electric and magnetic fields show quite complicated classical dynamical behavior, and a sequence of bifurcation and anti-bifurcation occurs. We investigate the effects of bifurcations on the flux distribution of photo-detached electrons by using position and momentum diagrams. Detached-electron flux distributions are calculated based on a uniform semi-classical theory. The flux distributions exhibit patterns with multiple rings. The bright rings correspond to special points in the diagrams. The flux distributions can be controlled by adjusting the magnetic field strength while fixing the electric field.     

Photodetachment of a Negative Hydrogen Molecular Ion near an Interface

Photodetachment of a negative hydrogen molecular ion near an interface is studied by using the two-centre model and the closed orbit theory. The calculation results show that the photodetachment cross section is related to the distance between the two centres in the H₂^- and different molecular ion-interface distances. The comparison between the cross section of H₂^- near an interface with the section of Hˉ shows that at the equilibrium distance of two centres and at low photon energy, the photodetachment cross section of H₂^- is about twice the cross section of Hˉ, which shows that the interference of the two nuclei is very strong; when the distance between the two centres is large, the section of H₂^- is almost the same as the cross section of Hˉ near one interface, which indicates that the interference effect of the two centres anishes.     

Photodetachment of H- in a static electric field

We derive a formula using closed-orbit theory for the photodetachment cross-section of H^- in the presence of a static electric field when there is an arbitrary angle θ_L between the laser polarization direction and the static electric field. This formula generalizes the previous result for laser polarization parallel to the static electric field, the effect of laser polarization direction appears as a factor cos²(θ_L) in the amplitude of the oscillation. A photodetachment cross-section formula valid above and below detachment threshold is proposed.     

Higher-order <Emphasis Type="Bold">?</Emphasis> corrections in the semiclassical quantization of chaotic billiards

In the periodic orbit quantization of physical systems, usually only the leading-order ? contribution to the density of states is considered. Therefore, by construction, the eigenvalues following from semiclassical trace formulae generally agree with the exact quantum ones only to lowest order of ?. In different theoretical work the trace formulae have been extended to higher orders of ?. The problem remains, however, how to actually calculate eigenvalues from the extended trace formulae since, even with ? corrections included, the periodic orbit sums still do not converge in the physical domain. For lowest-order semiclassical trace formulae the convergence problem can be elegantly, and universally, circumvented by application of the technique of harmonic inversion. In this paper we show how, for general scaling chaotic systems, also higher-order ? corrections to the Gutzwiller formula can be included in the harmonic inversion scheme, and demonstrate that corrected semiclassical eigenvalues can be calculated despite the convergence problem. The method is applied to the open three-disk scattering system, as a prototype of a chaotic system. Received 10 September 2001 and Received in final form 3 January 2002     

Recurrence spectra and dynamical simulation of Rydberg hydrogen atom near a metal surface

By using the closed-orbit theory including the effect of Coulomb scattering together with an electrical image potential approach, the recurrence spectra and the dynamical behaviours of the Rydberg hydrogen atom near a metal surface are presented. Theoretical analysis and numerical simulation reveal that the impacts of the image potential contributing to the recurrence spectrum are qualitatively analogous to that of the parallel electrical and magnetic fields on the Rydberg atom. The recurrence spectra are computed for a few selected scaled energies and the results demonstrate that the scaled energy dominates the dynamical properties of system. With the increase of the scaled energy e from small to large, the whole trend of spectral structure is from simple to complex,and then simple.     

Semiclassical calculation of ionization rate for Rydberg hydrogen atoms near a metal surface

The ionization of Rydberg hydrogen atoms near a metal surface at different scaled energies above the classical saddle point energy has been discussed by using the semiclassical method. The results show that the atoms ionize by emitting a train of electron pulses. In order to reveal the chaotic and escape dynamical properties of this system in detail, the sensitive dependence of the ionization rate upon the scaled energy is discussed. As the scaled energy is close to the saddle point energy, the ionization process of the hydrogen atom is nearly the same as the case of hydrogen atom in an electric field. There is only a single pulse of electrons, with an exponentially decaying tail. With the increase of the scaled energy, the ionization rates are similar to the case of the hydrogen atom in parallel electric and magnetic field, a series of electron pulses appear in the ionization process. This is caused by classical chaos, which occurs for the metal surface. Our studies also suggest that the metal surface can play the role of both the electric and the magnetic fields. Our theoretical analysis will be useful for guiding experimental studies of the ionization of atoms near the metal surface.     

A nonadiabatic semi-classical method for dynamics of atoms in optical lattices

We develop a semi-classical method to simulate the motion of atoms in a dissipative optical lattice. Our method treats the internal states of the atom quantum mechanically, including all nonadiabatic couplings, while position and momentum are treated as classical variables. We test our method in the one-dimensional case. Excellent agreement with fully quantum mechanical simulations is found. Our results are much more accurate than those of earlier semi-classical methods based on the adiabatic approximation.     

Simulations of Sisyphus cooling including multiple excited states

We extend the theory for laser cooling in a near-resonant optical lattice to include multiple excited hyperfine states. Simulations are performed treating the external degrees of freedom of the atom, i.e., position and momentum, classically, while the internal atomic states are treated quantum mechanically, allowing for arbitrary superpositions. Whereas theoretical treatments including only a single excited hyperfine state predict that the temperature should be a function of lattice depth only, except close to resonance, experiments have shown that the minimum temperature achieved depends also on the detuning from resonance of the lattice light. Our results resolve this discrepancy.     

Photodetachment microscopy of

Photodetachment of ions is performed in the presence of a static electric field. A high spatial resolution electron detector is used to map out the ejected electron's wave function. Propagation over half a metre does not alter the stationary structure the electron wave possesses in the direction orthogonal to the detachment motion. Nodes and antinodes of electron current density are observed, that can be interpreted as direct imaging of the internal atomic wave function. Because detachment of a negative ion can be represented accurately in the free-electron approximation, the obtained images can also be viewed as direct experimental representations of the Green function of the uniform acceleration problem. Agreement of the measured distributions with the exact expression of this wave-function appears quite satisfactory. Received: 5 June 1998 / Accepted: 16 October 1998     

H<Superscript>+</Superscript><Emphasis Type="Bold">and He</Emphasis><Superscript>2+</Superscript> impact single and double ionization of lead

H⁺ and He²⁺ impact single and double ionization cross sections of ground state lead atoms have been calculated in the binary encounter approximation. Calculations of direct double ionization cross sections have been performed in the modified double binary encounter model. The accurate expressions of σ_ΔE (cross-section for energy transfer ΔE) and Hartree-Fock velocity distributions for the target electrons have been used throughout the calculations. Contributions to double ionization from Auger effect following ionization of inner shells have been considered in the present work. Our H⁺ impact single and double ionization cross sections are in good agreement with the experimental observations. In calculations of He²⁺ impact cross sections, the present theoretical approach shows limited success in the experimentally investigated region (50–350 keV amu^-1).     

A comparative nearside-farside analysis of the He–N2 + and He–N2 inelastic collisions

A comparative study of the inelastic scattering of ¹⁴N₂ ⁺ and ¹⁴N₂ in collision with ³He atoms is presented. The unrestricted nearside-farside (NF) method proposed by Connor [J. Chem. Phys. 104, 2297 (1995)] is applied to analyse the Close Coupling rotationally state selected angular distributions for four kinetic energies. These four energies illustrate different regimes of the dynamics. The relationships between the structures of the calculated differential cross-sections (DCS) and the different regions of the potential energy surfaces involved which can be extracted from semi classical models are here easily obtained from a simple reading of the (NF) figures. At the higher energy far-off the wells (1000 cm^-1) the shape of the DCS are quite similar for the two systems and their nearside-farside components also, showing that the inelastic process is controlled by the short range repulsive part of the potential which is essentially the same for these two collisions. When the energy is decreased the differences between the two wells associated with the He–N₂ ⁺ and He–N₂ complexes are responsible for the differences between the DCS for the two systems. The farside component associated with the well become more and more prominent for the elastic scattering while inelastic scattering remains controlled by the repulsive core in a large angular interval. The nearside farside analysis gives also a new picture of a resonance which is regarded as an equilibrium between the repulsive and the attractive parts of the potential.     

Doubly uniform semiclassical quantization formula for resonances

The radial Schrödinger equation with an effective potential containing a single well and a single barrier is treated with an improved uniform semiclassical method. The improved quantization formula for complex energies (or resonances) contains a correction term that originates from a uniform treatment of the classically forbidden region near the origin in addition to the more familiar uniform treatment of the barrier region. In the present case the origin has a second-order pole, due to the centrifugal barrier potential term, and/or a Coulomb-type singularity, and these terms dominate the region inside the innermost classical turning point.Numerical results for first-order and third-order approximate complex resonance energies are compared with those of a standard (first- and third-order) barrier-uniform semiclassical method and also with those of ‘exact’ numerical computations.The improved quantization formula provides results in significantly better agreement with the exact results as the angular momentum quantum number l approaches zero.     

Ion extraction from the surface ablated materials in electric fields using an intense femtosecond laser pulse

In order to develop a femtosecond laser ablation (fsLA) ion source for TOF mass spectrometry, we have analyzed time-resolved images of laser-induced fluorescence from Sm⁺ ions produced by fsLA of a solid samarium in electric fields. The polarity and the strength of electric fields had a remarkable effect on the expansion of Sm⁺ ions. Moreover, accelerating electric fields elongated the duration of the ion emission from the samarium surface in fsLA, which degraded time-focusing of the ions. We have found that suppression the continuous ion emission caused by fsLA in electric fields is most important in TOF measurements.     

Rotational excitation of CH<Subscript>4</Subscript> by He atoms

Quantum close-coupling and coupled-state approximation scattering calculations for rotational energy transfer of rotationally excited CH₄ due to collisions with He are presented for collision energies between 10⁻⁷ and 3000 cm⁻¹ using the MP4 potential of Calderoni et al. [J. Chem. Phys. 121, 8261 (2004)]. State-to-state cross sections and rate coefficients from selected initial rotational states of CH₄ in symmetries A, E, and F are studied from the ultra-cold to the thermal regime. Comparison of the cross sections with available theoretical results and experimental data show good agreement. Applications to astrophysics and cold laboratory environments are briefly addressed.     

Semiclassical analysis of edge state energies in the integer quantum Hall effect

Analysis of edge-state energies in the integer quantum Hall effect is carried out within the semiclassical approximation. When the system is wide so that each edge can be considered separately, this problem is equivalent to that of a one dimensional harmonic oscillator centered at x = x_c and an infinite wall at x = 0, and appears in numerous physical contexts. The eigenvalues E_n(x_c) for a given quantum number n are solutions of the equation S(E,x_c)=π[n+ γ(E,x_c)] where S is the WKB action and 0 < γ < 1 encodes all the information on the connection procedure at the turning points. A careful implication of the WKB connection formulae results in an excellent approximation to the exact energy eigenvalues. The dependence of γ[E_n(x_c),x_c] ≡γ_n(x_c) on x_c is analyzed between its two extreme values as x_c ↦-∞ far inside the sample and as x_c ↦∞ far outside the sample. The edge-state energiesE_n(x_c) obey an almost exact scaling law of the form and the scaling function f(y) is explicitly elucidated.     

Comparative study of rare gas-H<Subscript>2</Subscript> triatomic complexes

This paper presents a comparative analysis of complexes made of one rare gas (Rg) atom and molecular hydrogen, for five different Rg atoms. In particular, the vibrational band origins have been calculated, as well as the structural properties of the associated wavefunctions. The study is extended to ultra-cold Rg–H₂ collisions, object of a recent experiment aimed at producing ultra-cold H₂ [1]. The molecular systems are studied variationally using a simple, yet effective, isotropic trial wavefunction. A large number of potential energy surfaces available from the literature is considered. A comparative analysis shows that differences of up to two orders of magnitude exist for the zero-energy elastic cross sections of the five complexes. Corrections to the isotropic model have also been considered, showing no significant effect.     

Photo-atomic effect: Temperature dependence of the photodesorption of Na and from polymer surfaces

Alkali metals adsorbed to surface films of the polymer poydimethylsiloxane (PDMS) have been shown to exhibit a unique photodesorption behavior, characterized by a frequency threshold and high efficiency. In this work, the temperature dependence of the photodesorption yields of Na and Na ₂ from PDMS surfaces were measured between room temperature and 183 K. Over most of the temperature range, the yields exhibited an Arrhenius behavior characterized by thermal activation energies of 0.36 eV and 0.34 eV for Na and Na ₂ , respectively. These values are suggestive of a surface diffusion as one of the elementary steps in the photodesorption mechanism. Moreover, the similarity of the two values indicates that the same elementary step applies to the desorption of both Na and Na ₂ . Received 23 April 1999 and Received in final form 15 July 1999     

Photoionization of atoms in parallel electric and magnetic fields: Cross-section calculations using the Lanczos algorithm and the complex coordinate method in a discrete variable representation

A combination of the complex-coordinate method and the Lanczos recursion scheme is implemented in the discrete variable representation (DVR) to obtain total photoionization cross-sections using an iterative procedure. Applications to photoionization of hydrogen atoms in electric fields and sodium atoms in electric and parallel electric and magnetic fields are presented and discussed. Received 15 May 2000 and Received in final form 4 October 2000     

Spontaneous emission of an atom placed near a prolate nanospheroid

The frequency shift and linewidth variation of an atomic oscillator placed next to a prolate dielectric or metal spheroid are found within the framework of the classical approach. Both the frequency shift and linewidth are shown to be substantially dependent on the location of the atom and the form of the nanospheroid and capable of reaching very high values near the surface of the nanospheroid under plasmon (polariton) resonance conditions. The predictions are compared with those found for spherical and cylindrical geometries. The prolate spheroid is treated as a model of a needle tip in apertureless optical scanning microscopy. Effects of sharpness of interaction between the nanospheroid tip and atoms are considered. Received 2 January 2002 and Received in final form 3 April 2002 Published online 28 June 2002