arXiv subjects

M. L. Du

Publications and source records attributed to M. L. Du.

12 recordsLinked to original sources

arXiv2014

Photodetachment near an attractive force center

This article studies the photodetachment of a single electron anion near an attractive center. Both the differential and total photodetachment cross section are analysed. We obtain the electron flux crossing through a spherical detector centered at the force center using the semiclassical approximation. The closed-orbit theory gives the total cross section which contains a smooth background and an oscillatory part. Concrete calculations and discussions are carried out for two types of wave source: the $s$- and $p_z$-wave source. Photodetachment processes for three conditions are compared: an anion near an attractive center, near a repulsive center and in a homogeneous electric field.

physics.atom-ph↗

arXiv2013

Photodetachment near a repulsive center

We study the total photodetachment cross section of H$^-$ near a repulsive center. An analytical formula for the photodetachment cross section is obtained using the standard closed-orbit theory and extending it to the energy range below the zero-field threshold. The formula is found to be accurate by comparing with an exact quantum calculation. A comparison with the photodetachment cross section in an effective homogeneous electric field is made, and we discuss the similarities and differences of the two systems.

physics.atom-ph↗

arXiv2012

Quantum over-barrier reflection effects manifested in the photodetachment cross sections

Photodetachment of H$^-$ near a potential barrier is studied. A new formula is presented for the cross sections induced by a barrier. The new formula describes two quantum effects near barrier tops. For energies near and above barrier tops, the quantum over-barrier reflection effects are included and the induced oscillations in the cross sections are still prominent; for energies near and below barrier tops, the quantum tunneling across barriers is taken into account and consequently the oscillations are weakened. For energies far away from the barrier tops, the new formula agrees with the standard closed-orbit theory. We demonstrate that a potential barrier of various width and location can be realized by placing a negative ion near a metal surface and applying an electric field pointing to the surface. The width and location of the barrier can be systematically changed by varying the electric field strength and the distance between the negative ion and the surface. Results are also presented for estimating the sizes and locations of aforementioned quantum effects in the cross sections.

physics.atom-ph↗

arXiv2011

Circular Rydberg states of atomic hydrogen in an arbitrary magnetic field

We report a theoretical scheme using a B-spline basis set to improve the poor computational accuracy of circular Rydberg states of hydrogen atoms in the intermediate magnetic field. This scheme can produce high accuracy energy levels and valid for an arbitrary magnetic field. Energy levels of hydrogen are presented for circular Rydberg states with azimuthal quantum numbers $|m|$ = 10 - 70 as a function of magnetic field strengths ranging from zero to 2.35 $\times$ 10$^9$ T. The variation of spatial distributions of electron probability densities with magnetic field strengths is discussed and competition between Coulomb and magnetic interactions is illustrated.

physics.atom-ph↗

arXiv2011

Escape of quantum particles from an open cavity

A negative ion irradiated by a laser provides a coherent source of electrons propagating out from the location of the negative ion. We study the total escape rates of the electrons when the negative ion is placed inside an open cavity in the shape of a wedge. We show the wedge induces significant oscillations in the total escape rates because of quantum interference effects. In particular, we show, for a wedge with an opening angle of π/N, where N is an arbitrary positive integer, there are (2N - 1) induced oscillations in the rates. As a demonstration, the case for a wedge with an opening angle π/5 is calculated and analyzed in detail.

quant-ph↗

arXiv2010

Spatial oscillations in the spontaneous emission rate of an atom inside a metallic wedge

A method of images is applied to study the spontaneous emission of an atom inside a metallic wedge with an opening angle of $π/N$, where N is an arbitrary positive integer. We show the method of images gives a rate formula consistent with that from Quantum Electrodynamics. Using the method of images, we show the correspondence between the oscillations in the spontaneous emission rate and the closed-orbits of emitted photon going away and returning to the atom inside the wedge. The closed-orbits can be readily constructed using the method of images and they are also extracted from the spontaneous emission rate.

physics.atom-ph↗

arXiv2010

Oscillations in the total photodetachment cross sections of a triatomic anion

The total photodetachment cross section of a linear triatomic anion is derived for arbitrary laser polarization direction. The cross section is shown to be strongly oscillatory when the laser polarization direction is parallel to the axis of the system; the oscillation amplitude decreases and vanishes as the angle between the laser polarization and the anion axis increases and becomes perpendicular to the axis. The average cross section over the orientations of the triatomic system is also obtained. The cross section of the triatomic anion is compared with the cross section of a two-center system. We find there are two oscillation frequencies in the triatomic anion in contrast to only one oscillation frequency in the two-center case. Closed-orbit theory is used to explain the oscillations.

physics.atm-clus↗

arXiv2009

What is the difference in the p-wave and s-wave photodetachment in an electric field?

By applying closed-orbit theory to an existing model, a simple formula is derived for the modulation function of s-wave photo-detachment in the presence of a static electric field. We then compare the s-wave modulation function with the p-wave modulation function. We show the maximums (minimums) in the s-wave modulation function correspond to the minimums (maximums) in the p-wave modulation function because of a phase difference of $π$ in their oscillations. The oscillation amplitude in the p-wave modulation function can be larger than, smaller than or equal to the oscillation amplitude of s-wave modulation function when the angle between the laser polarization direction and the static electric field direction is tuned to be smaller than, larger than or equal to a special angle which is approximately $54.74^\circ$.

physics.atom-ph↗

arXiv2009

Modifying the photodetachment near a metal surface by a weak electric field

We show the photodetachment cross sections of H near a metal surface can be modified using a weak static electric field. The modification is possible because the oscillatory part of the cross section near a metal surface is directly connected with the transit-time and the action of the detached-electron closed-orbit which can be changed systematically by varying the static electric field strength. Photodetachment cross sections for various photon energies and electric field values are calculated and displayed.

physics.atom-ph↗

arXiv2007

Escape Rates of the Hénon-Heiles System

A particle in the Hénon-Heiles potential can escape when its energy is above the threshold value $E_{th}={1/6}$. We report a theoretical study on the the escape rates near threshold. We derived an analytic formula for the escape rate as a function of energy by exploring the property of chaos. We also simulated the escaping process by following the motions of a large number of particles. Two algorithms are employed to solve the equations of motion. One is the Runge-Kutta-Fehlberg method, and another is a recently proposed fourth order symplectic method. Our simulations show the escape of H$\mathrm{\acute{e}}$non-Heiles system follows exponential laws. We extracted the escape rates from the time dependence of particle numbers in the H$\mathrm{\acute{e}}$non-Heiles potential. The extracted escape rates agree with the analytic result.

nlin.CD↗

arXiv2006

Oscillation structures in the spontaneous emission rate of an atom in a medium with refractive index n between mirrors: a solvable model

We study the multi-periodic oscillations in the spontaneous emission rate of an atom in a medium with refractive index n sandwiched between two parallel mirrors. The oscillations are not obvious in the analytical formula for the rate derived based on Fermi's golden rule but can be extracted using Fourier transforms by varying the system scale while holding the configuration. The oscillations are interpreted as interferences and correspond to various closed-orbits of the emitted photon going away from and returning to the atom. This system provides a rare example that the oscillations can be explicitly derived by following the emitted wave until it returns to the emitting atom. We demonstrate the summation over a large number of closed-orbits converges to the rate formula of golden rule.

quant-ph↗

arXiv2001

Enhancement of bichromatic high-harmonic generation with a high-frequency field

Using a high-frequency field superposed to a linearly polarized bichromatic laser field composed by a wave with frequency $ω$ and a wave with frequency $2ω$, we show it is possible to enhance the intensity of a group of high harmonics in orders of magnitude. These harmonics have frequencies about 30% higher than the monochromatic-cutoff frequency, and, within the three-step-model framework, correspond to a set of electron trajectories for which tunneling ionization is strongly suppressed. Particular features in the observed enhancement suggest that the high-frequency field provides an additional mechanism for the electron to reach the continuum. This interpretation is supported by a time-frequency analysis of the harmonic yield. The additional high frequency field permits the control of this group of harmonics leaving all other sets of harmonics practically unchanged, which is an advantage over schemes involving only bichromatic fields.

physics.atom-ph↗