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J. F. McCann

Publications and source records attributed to J. F. McCann.

At least 19 recordsLinked to original sources

Rovibrationally-Resolved Photodissociaton of SH$^+$

Photodissociation cross sections for the SH$^+$ radical are computed from all rovibrational (RV) levels of the ground electronic state X$~^3Σ^-$ for wavelengths from threshold to 500~Å. The five electronic transitions, $2~ ^3Σ^- \leftarrow$ X$~^3Σ^-$, $3~ ^3Σ^- \leftarrow$ X$~^3Σ^-$, $A~ ^3Π\leftarrow$ X$~^3Σ^-$, $2~ ^3Π\leftarrow$ X$~^3Σ^-$, and $3~ ^3Π\leftarrow$ X$~^3Σ^-$, are treated with a fully quantum-mechanical two-state model, {i.e. no non-adiabatic coupling between excited states was included in our work.}. The photodissociation calculations incorporate adiabatic potentials and transition dipole moment functions computed in the multireference configuration interaction approach along with the Davidson correction (MRCI+Q), but adjusted to match available experimental molecular data and asymptotic atomic limits. Local thermodynamic equilibrium (LTE) photodissociation cross sections were computed which assume a Boltzmann distribution of RV levels in the X$~^3Σ^-$ molecular state of the SH$^+$ cation. The LTE cross sections are presented for temperatures in the range 1000-10,000~K. Applications of the current photodissociation cross sections to interstellar gas, photon-dominated regions, and stellar atmospheres are briefly discussed.

astro-ph.EP

Ultracold, radiative charge transfer in hybrid Yb ion - Rb atom traps

Ultracold hybrid ion-atom traps offer the possibility of microscopic manipulation of quantum coherences in the gas using the ion as a probe. However, inelastic processes, particularly charge transfer can be a significant process of ion loss and has been measured experimentally for the Yb$^{+}$ ion immersed in a Rb vapour. We use first-principles quantum chemistry codes to obtain the potential energy curves and dipole moments for the lowest-lying energy states of this complex. Calculations for the radiative decay processes cross sections and rate coefficients are presented for the total decay processes. Comparing the semi-classical Langevin approximation with the quantum approach, we find it provides a very good estimate of the background at higher energies. The results demonstrate that radiative decay mechanisms are important over the energy and temperature region considered. In fact, the Langevin process of ion-atom collisions dominates cold ion-atom collisions. For spin dependent processes \cite{kohl13} the anisotropic magnetic dipole-dipole interaction and the second-order spin-orbit coupling can play important roles, inducing couplingbetween the spin and the orbital motion. They measured the spin-relaxing collision rate to be approximately 5 orders of magnitude higher than the charge-exchange collision rate \cite{kohl13}. Regarding the measured radiative charge transfer collision rate, we find that our calculation is in very good agreement with experiment and with previous calculations. Nonetheless, we find no broad resonances features that might underly a strong isotope effect. In conclusion, we find, in agreement with previous theory that the isotope anomaly observed in experiment remains an open question.

physics.atom-ph

Structure and interactions of ultracold Yb ions and Rb atoms

In order to study ultracold charge-transfer processes in hybrid atom-ion traps, we have mapped out the potential energy curves and molecular parameters for several low lying states of the Rb, Yb$^+$ system. We employ both a multi-reference configuration interaction (MRCI) and a full configuration interaction (FCI) approach. Turning points, crossing points, potential minima and spectroscopic molecular constants are obtained for the lowest five molecular states. Long-range parameters, including the dispersion coefficients are estimated from our {\it ab initio} data. The separated-atom ionization potentials and atomic polarizability of the ytterbium atom ($α_d=128.4$ atomic units) are in good agreement with experiment and previous calculations. We present some dynamical calculations for (adiabatic) scattering lengths for the two lowest (Yb,Rb$^+$) channels that were carried out in our work. However, we find that the pseudo potential approximation is rather limited in validity, and only applies to nK temperatures. The adiabatic scattering lengths for both the triplet and singlet channels indicate that both are large and negative in the FCI approximation.

physics.atom-ph

Transport, atom blockade and output coupling in a Tonks-Girardeau gas

Recent experiments have demonstrated how quantum-mechanical impurities can be created within strongly correlated quantum gases and used to probe the coherence properties of these systems [S. Palzer, C. Zipkes, C. Sias, and M. Köhl, Phys. Rev. Lett. 103, 150601 (2009).]. Here we present a phenomenological model to simulate such an output coupler for a Tonks-Girardeau gas that shows qualitative agreement with the experimental results for atom transport and output coupling. Our model allows us to explore nonequilibrium transport phenomena in ultracold quantum gases and leads us to predict a regime of atom blockade, where the impurity component becomes localized in the parent cloud despite the presence of gravity. We show that this provides a stable mixed-species quantum gas in the strongly correlated limit.

cond-mat.quant-gas

Dynamic correlation in high-harmonic generation for helium atoms in intense visible lasers

We investigate the effect of dynamic electron correlation on high-harmonic generation in helium atoms using intense visible light (λ=390nm). Two complementary approaches are used which account for correlation in an approximate manner: time-dependent density-functional theory and a single-active-electron model. For intensities I~10^{14} W/cm^2, the theories are in remarkably good agreement for the dynamic polarization and harmonic spectrum. This is attributed to a low-frequency collective mode together with a high-frequency single-electron response due to the nuclear singularity, both of which dominate electron correlation effects. A time-frequency analysis is used to study the timing and emission spectrum of attosecond bursts of light. For short pulses, we find a secondary maximum below the classical cut-off. The imprint of the carrier-envelope-phase, for the time-integrated spectral density appears at frequencies above the high-frequency drop-off, consistent with previous studies in the infrared λ~800nm.

quant-ph

Quantum Chessboards in the Deuterium Molecular Ion

We present a new algorithm for vibrational control in deuterium molecules that is feasible with current experimental technology. A pump mechanism is used to create a coherent superposition of the D2+ vibrations. A short, intense infrared control pulse is applied after a chosen delay time to create selective interferences. A `chessboard' pattern of states can be realized in which a set of even- or odd-numbered vibrational states can be selectively annihilated or enhanced. A technique is proposed for experimental realization and observation of this effect using 5 fs pulses of 790 nm radiation, with intermediate intensity (5e13 W/cm2)

quant-ph

Low-energy excitations of a boson pair in a double-well trap

The states of a boson pair in a one-dimensional double-well potential are investigated. Properties of the ground and lowest excited states of this system are studied, including the two-particle wavefunction, momentum pair distribution and entanglement. The effects of varying both the barrier height and the effective interaction strength are investigated.

quant-ph

Isolated vibrational wavepackets in D2+: Defining superposition conditions and wavepacket distinguishability

Tunnel ionization of room-temperature D$_2$ in an ultrashort (12 femtosecond) near infra-red (800 nm) pump laser pulse excites a vibrational wavepacket in the D2+ ions; a rotational wavepacket is also excited in residual D2 molecules. Both wavepacket types are collapsed a variable time later by an ultrashort probe pulse. We isolate the vibrational wavepacket and quantify its evolution dynamics through theoretical comparison. Requirements for quantum computation (initial coherence and quantum state retrieval) are studied using this well-defined (small number of initial states at room temperature, initial wavepacket spatially localized) single-electron molecular prototype by temporally stretching the pump and probe pulses.

physics.chem-ph

Boson Pairs in a One-dimensional Split Trap

We describe the properties of a pair of ultracold bosonic atoms in a one-dimensional harmonic trapping potential with a tunable zero-ranged barrier at the trap centre. The full characterisation of the ground state is done by calculating the reduced single-particle density, the momentum distribution and the two-particle entanglement. We derive several analytical expressions in the limit of infinite repulsion (Tonks-Girardeau limit) and extend the treatment to finite interparticle interactions by numerical solution. As pair interactions in double wells form a fundamental building block for many-body systems in periodic potentials, our results have implications for a wide range of problems.

quant-ph

Strong-field approximation for Coulomb explosion of H_2^+ by short intense laser pulses

We present a simple quantum mechanical model to describe Coulomb explosion of H$_2^+$ by short, intense, infrared laser pulses. The model is based on the length gauge version of the molecular strong-field approximation and is valid for pulses shorter than 50 fs where the process of dissociation prior to ionization is negligible. The results are compared with recent experimental results for the proton energy spectrum [I. Ben-Itzhak et al., Phys. Rev. Lett. 95, 073002 (2005), B. D. Esry et al., Phys. Rev. Lett. 97, 013003 (2006)]. The predictions of the model reproduce the profile of the spectrum although the peak energy is slightly lower than the observations. For comparison, we also present results obtained by two different tunneling models for this process.

physics.atom-ph

Collective excitations of trapped Bose condensates in the energy and time domains

A time-dependent method for calculating the collective excitation frequencies and densities of a trapped, inhomogeneous Bose-Einstein condensate with circulation is presented. The results are compared with time-independent solutions of the Bogoliubov-deGennes equations. The method is based on time-dependent linear-response theory combined with spectral analysis of moments of the excitation modes of interest. The technique is straightforward to apply, is extremely efficient in our implementation with parallel FFT methods, and produces highly accurate results. The method is suitable for general trap geometries, condensate flows and condensates permeated with vortex structures.

cond-mat.stat-mech

Pair production and electron capture in relativistic heavy-ion collisions

Results are presented for simulations of electron-positron pair production in relativistic heavy-ion collisions leading to electron capture and positron ejection. We apply a two-center relativistic continuum distorted-wave model to represent the electron/positron dynamics during the collision process. The results are compared with experimental cross-section data for ${\rm La}^{57+}$ and ${\rm Au}^{79+}$ impact on Gold, Silver and Copper targets. The theory is in very good agreement with experiment, verifying the result that the process increases in importance with both collision energy and target atomic number, and improves upon previous simulations of this process.

physics.atom-ph

Vortex shedding and drag in dilute Bose-Einstein condensates

Above a critical velocity, the dominant mechanism of energy transfer between a moving object and a dilute Bose-Einstein condensate is vortex formation. In this paper, we discuss the critical velocity for vortex formation and the link between vortex shedding and drag in both homogeneous and inhomogeneous condensates. We find that at supersonic velocities sound radiation also contributes significantly to the drag force.

cond-mat

Dissipation and Vortex Creation in Bose-Einstein Condensed Gases

We solve the Gross-Pitaevskii equation to study energy transfer from an oscillating `object' to a trapped Bose-Einstein condensate. Two regimes are found: for object velocities below a critical value, energy is transferred by excitation of phonons at the motion extrema; while above the critical velocity, energy transfer is via vortex formation. The second regime corresponds to significantly enhanced heating, in agreement with a recent experiment.

cond-mat.stat-mech

Vortex line and ring dynamics in trapped Bose-Einstein condensates

Vortex dynamics in inhomogeneous Bose-Einstein condensates are studied numerically in two and three dimensions. We simulate the precession of a single vortex around the center of a trapped condensate, and use the Magnus force to estimate the precession frequency. Vortex ring dynamics in a spherical trap are also simulated, and we discover that a ring undergoes oscillatory motion around a circle of maximum energy. The position of this locus is calculated as a function of the number of condensed atoms. In the presence of dissipation, the amplitude of the oscillation will increase, eventually resulting in self-annihilation of the ring.

cond-mat.stat-mech

Vortex structures in dilute quantum fluids

Vortex structures in dilute quantum fluids are studied using the Gross-Pitaevskii equation. The velocity and momentum of multiply quantized vortex rings are determined and their core structures analysed. For flow past a spherical object, we study the encircling and pinned ring solutions, and determine their excitation energies as a function of the flow velocity for both penetrable and impenetrable objects. The ring and laminar flow solutions converge at a critical velocity, which decreases with increasing object size. We also study the vortex solutions associated with flow past a surface bump which indicate that surface roughness also reduces the critical velocity. This effect may have important implications for the threshold of dissipation in superfluids and superconductors.

cond-mat

Pressure drag in linear and nonlinear quantum fluids

We study the flow of a weakly-interacting Bose-Einstein condensate around an obstacle by numerical solution of the Gross-Pitaevskii equation. We observe vortex emission and the formation of bow waves leading to pressure drag. We compare the drag law with that of an ideal Bose gas, and show that interactions reduce the drag force. This reduction can be explained in terms of a `collisional screening' of the obstacle.

cond-mat.stat-mech