P F O'Mahony

(e,2e) triple differential cross sections for the simultaneous ionization and excitation of helium

The authors present absolute triple differential cross sections (TDCS) measurements for ionization of helium leaving the ion in both n=1 and n=2 final states, obtained under asymmetric geometry at an incident energy approximately 5.5 keV and ejected electron energies of 5, 10 and 75 eV. The kinematics are chosen to correspond either to a constant ejection energy, or to a constant energy transfer to the target. Angular distributions are measured at both constant ejection angle ( theta a mode) and at constant scattering angle ( theta b mode). In the theta a mode experiments, the momentum transfer dependence of the n=2 triple differential generalized oscillator strength is investigated for the first time. In both modes, the n=2 angular distributions show several new features which are not present for the n=1 ones, and which tend to vanish as the ejected energy is increased. They are attributed to final state interactions between the ejected electron and the excited ion. Comparison with first-order theoretical models shows the inadequacy of a Coulomb wave representation of the ejected electron, while in the R-matrix formalism it is found that a five-state multichannel calculation qualitatively describes the shape (but not the amplitude) of the TDCS measured in the theta b mode. Comparison is also made with the photoionization in the dipolar limit where the momentum transfer approaches zero. When integrated over the ejection direction, the double differential generalized oscillator strength ratio for ionization to the n=1 and n=2 states is found to agree with an earlier first Born close coupling prediction.

From quantum trajectories to classical orbits

Abstract The evolution of open quantum systems can be “unraveled” into individual “trajectories” in a variety of ways. In the mesoscopic regime, quantum jump (QJ) trajectories approach a diffusive limit similar to quantum state diffusion (QSD). In the classical limit, both unravelings show the rise of classical orbits for both regular and chaotic systems.

Quantum-state diffusion model and the driven damped nonlinear oscillator

We consider a driven damped anharmonic oscillator which classically leads to a bistable steady state and to hysteresis. The quantum counterpart for this system has an exact analytical solution in the steady state which does not display any bistability or hysteresis. We use quantum state diffusion theory to describe this system and to provide a new perspective on the lack of hysteresis in the quantum regime so as to study in detail the quantum to classical transition. The analysis is also relevant to measurements of a single periodically driven electron in a Penning trap where hysteresis has been observed.

CONTINUOUS STOCHASTIC SCHRODINGER EQUATIONS AND LOCALIZATION

The set of continuous norm-preserving stochastic Schrodinger equations associated with the Lindblad master equation is introduced. This set is used to describe the localization properties of the state vector toward eigenstates of the environment operator. Particular focus is placed on determining the stochastic equation which exhibits the highest rate of localization for wide open systems. An equation having such a property is proposed in the case of a single non-Hermitian environment operator. This result is relevant to numerical simulations of quantum trajectories where localization properties are used to reduce the number of basis states needed to represent the system state, and thereby increase the speed of calculation.

Quantisation of new periodic orbits for the hydrogen atom in a magnetic field

A new set of periodic orbits is found in the irregular region for the hydrogen atom in a magnetic field. The quantisation of these orbits is discussed and new level spacings near the zero-field threshold are predicted.

(e, 2e) reactions in helium: calculations with correlated wavefunctions

Triple differential cross sections have been calculated for a range of incident energies and momentum transfers. The necessity of using correlated initial- and final-state wavefunctions for helium and of going beyond the first order is demonstrated. Excellent agreement has been obtained with recent absolute experimental measurements in helium.

Ionization and excitation of the excited hydrogen atom in strong circularly polarized laser fields

In the recent work of Herath et al. [T. Herath, L. Yan, S. K. Lee, and W. Li, Phys. Rev. Lett. 109, 043004 (2012)PRLTAO0031-900710.1103/PhysRevLett.109.043004] the first experimental observation of a dependence of strong-field ionization rate on the sign of the magnetic quantum number m [of the initial bound state (n,l,m)] was reported. The experiment with nearly circularly polarized light could not distinguish which sign of m favors faster ionization. We perform ab initio calculations for the hydrogen atom initially in one of the four bound substates with the principal quantum number n=2, and irradiated by a short circularly polarized laser pulse of 800nm. In the intensity range of 1012-1013W/cm2 excited bound states play a very important role, but also up to some 1015W/cm2 they cannot be neglected in a full description of the laser-atom interaction. We explore the region that with increasing intensity switches from multiphoton to over-the-barrier ionization and we find, unlike in tunneling-type theories, that the ratio of ionization rates for electrons initially counter-rotating and corotating (with respect to the laser field) may be higher or lower than 1.

Double scattering in energy-sharing (e, 2e) reactions

The double-scattering mechanism, introduced by Thomas (1927) within the context of ion-atom collisions, is used to interpret observed peaks in the triple differential cross section (TDCS), for high-energy (e, 2e) collisions in atoms, where the two electrons emerge from the collision with equal energies (energy-sharing reactions). As is well known the double-scattering mechanism is contained in the second Born amplitude. Quantitative comparisons are made with experiment by evaluating the scattering amplitude perturbatively, retaining terms up to second order in a Born series. Two different geometries are studied in detail, in which the TDCS for the outgoing electrons has recently been measured. They are (a) the perpendicular plane and (b) the coplanar symmetric geometry. The authors calculate the TDCS at a range of incident energies for both hydrogen and helium targets. The peaks due to double scattering are clearly observed and the theoretical TDCS compares favourably with recent experimental measurements.

Distribution of Fano parameters in a mesoscopic system with broken time-reversal symmetry

We report on numerical calculations of the distribution of complex Fano parameters in a mesoscopic system consisting of a waveguide attached to a disordered cavity. An external magnetic field in the cavity breaks the time-reversal symmetry. The real and imaginary parts of the parameters as well as the widths are obtained by fitting the Beutler-Fano form to the calculated resonance data. The distributions are compared with the predictions from random matrix theory which have been modified to exclude resonances below a certain cut-off width.

The R-matrix method in second Born calculations of electron impact ionisation of atoms: case study for helium

The authors present a general approach to the calculation of the triple differential cross section, for light atoms, applicable at intermediate to high incident electron energies, in the coplanar asymmetric geometry. The structure of the target is taken into account explicitly by calculating the wavefunction of the initial bound state and of the final continuum state of the slow electron moving in the field of the residual ion. These wavefunctions are calculated ab initio using the R-matrix method. Since the first Born approximation is inadequate to explain the experimental data, it is necessary to include higher-order effects. This was done by calculating the second Born amplitude, using these correlated wavefunctions. The authors present results for a helium target, at incident electron energies E0=256 and 600 eV, for a range of scattering angles. They obtained very good agreement with the recent absolute experimental measurements in helium.