arXiv subjects

Phay J. Ho

Publications and source records attributed to Phay J. Ho.

26 records · Page 2Linked to original sources

arXiv2011

Decoherence in Attosecond Photoionization

The creation of superpositions of hole states via single-photon ionization using attosecond extreme-ultraviolet pulses is studied with the time-dependent configuration interaction singles (TDCIS) method. Specifically, the degree of coherence between hole states in atomic xenon is investigated. We find that interchannel coupling not only affects the hole populations, it also enhances the entanglement between the photoelectron and the remaining ion, thereby reducing the coherence within the ion. As a consequence, even if the spectral bandwidth of the ionizing pulse exceeds the energy splittings among the hole states involved, perfectly coherent hole wave packets cannot be formed. For sufficiently large spectral bandwidth, the coherence can only be increased by increasing the mean photon energy.

physics.atom-ph↗

arXiv2010

Computational studies of x-ray scattering from three-dimensionally-aligned asymmetric-top molecules

We theoretically and numerically analyze x-ray scattering from asymmetric-top molecules three-dimensionally aligned using elliptically polarized laser light. A rigid-rotor model is assumed. The principal axes of the polarizability tensor are assumed to coincide with the principal axes of the moment of inertia tensor. Several symmetries in the Hamiltonian are identified and exploited to enhance the efficiency of solving the time-dependent Schrödinger equation for each rotational state initially populated in a thermal ensemble. Using a phase-retrieval algorithm, the feasibility of structure reconstruction from a quasi-adiabatically-aligned sample is illustrated for the organic molecule naphthalene. The spatial resolution achievable strongly depends on the laser parameters, the initial rotational temperature, and the x-ray pulse duration. We demonstrate that for a laser peak intensity of 5 TW/cm^2, a laser pulse duration of 100 ps, a rotational temperature of 10 mK, and an x-ray pulse duration of 1 ps, the molecular structure may be probed at a resolution of 1 Å.

physics.atom-ph↗

arXiv2008

Using strong electromagnetic fields to control x-ray processes

Exploration of a new ultrafast-ultrasmall frontier in atomic and molecular physics has begun. Not only is is possible to control outer-shell electron dynamics with intense ultrafast optical lasers, but now control of inner-shell processes has become possible by combining intense infrared/optical lasers with tunable sources of x-ray radiation. This marriage of strong-field laser and x-ray physics has led to the discovery of methods to control reversibly resonant x-ray absorption in atoms and molecules on ultrafast timescales. Using a strong optical dressing field, resonant x-ray absorption in atoms can be markedly suppressed, yielding an example of electromagnetically induced transparency for x rays. Resonant x-ray absorption can also be controlled in molecules using strong non-resonant, polarized laser fields to align the framework of a molecule, and therefore its unoccupied molecular orbitals to which resonant absorption occurs. At higher laser intensities, ultrafast field ionization produces an irreversible change in x-ray absorption. Finally, the advent of x-ray free electron lasers enables first exploration of non-linear x-ray processes.

physics.atom-ph↗

arXiv2007

Nonsequential double ionization: a comparison of classical simulations and $S$-matrix results

In a fully classical simulation, we investigate the recollision mechanism of non-sequential double ionization (NSDI) and its manifestation in the end-of-pulse electron momentum distributions. We compare two different electron-electron potentials: a soft-core Coulombic potential and a Yukawa potential. We also implement the strong-field approximation (SFA), which is commonly made in quantum-mechanical $S$-matrix calculations, in our classical simulations and study its consequences. We find that, regardless of the form of the e-e potential, the SFA modifies the momentum distributions and recollision dynamics significantly, but more so for the long-range than for the short-range e-e interaction. Surprisingly, our classical results, especially those obtained with the Yukawa potential under the SFA, agree well with the results from the $S$-matrix calculations with a contact e-e interaction. This implies that the recollision dynamics initiated by a quantum tunneling process and a purely classical process do not deviate much from each other. Furthermore, our classical predictions of travel times are consistent with the results from the simple-man model, and the most probable thermalization time is found to be within 0.15 and 0.25 laser periods.

physics.atom-ph↗

arXiv2006

In-plane Theory of Non-Sequential Triple Ionization

We describe first-principles in-plane calculations of non-sequential triple ionization (NSTI) of atoms in a linearly polarized intense laser pulse. In a fully classically correlated description, all three electrons respond dynamically to the nuclear attraction, the pairwise e-e repulsions and the laser force throughout the duration of a 780nm laser pulse. Nonsequential ejection is shown to occur in a multi-electron, possibly multi-cycle and multi-dimensional, rescattering sequence that is coordinated by a number of sharp transverse recollimation impacts.

physics.atom-ph↗

arXiv2005

Classical Effects of Laser Pulse Duration on Strong-field Double Ionization

We use classical electron ensembles and the aligned-electron approximation to examine the effect of laser pulse duration on the dynamics of strong-field double ionization. We cover the range of intensities $10^{14}-10^{16} W/cm^2$ for the laser wavelength 780 nm. The classical scenario suggests that the highest rate of recollision occurs early in the pulse and promotes double ionization production in few-cycle pulses. In addition, the purely classical ensemble calculation predicts an exponentially decreasing recollision rate with each subsequent half cycle. We confirm the exponential behavior by trajectory back-analysis.

physics.atom-ph↗

arXiv2005

Momentum Analysis in Strong-field Double Ionization

We provide a basis for the laser intensity dependence of the momentum distributions of electrons and ions arising from strong-field non-sequential double ionization (NSDI) at intensities in the range $I=1-6.5 \times 10^{14} W/cm^2$. To do this we use a completely classical method introduced previously \cite{ho-etal05}. Our calculated results reproduce the features of experimental observations at different laser intensities and depend on just two distinct categories of electon trajectories.

physics.atom-ph↗

arXiv2005

Non-Sequential Double Ionization is a Completely Classical Photoelectric Effect

We introduce a unified and simplified theory of atomic double ionization. Our results show that at high laser intensities ($I \ge 10^{14}$ watts/cm$^2$) purely classical correlation is strong enough to account for all of the main features observed in experiments to date.

physics.atom-ph↗