Turker Topcu

The time-dependent close-coupling method for atomic and molecular collision processes

We review the development of the time-dependent close-coupling method to study atomic and molecular few body dynamics. Applications include electron and photon collisions with atoms, molecules, and their ions.

Effects of molecular resonances on Rydberg blockade

We study the effect of resonances associated with complex molecular interaction of Rydberg atoms on Rydberg blockade. We show that densely spaced molecular potentials between doubly excited atomic pairs become unavoidably resonant with the optical excitation at short interatomic separations. Such molecular resonances limit the coherent control of individual excitations in Rydberg blockade. As an illustration, we compute the molecular interaction potentials of Rb atoms near the 100s states asymptote to characterize such detrimental molecular resonances and determine the resonant loss rate to molecules and inhomogeneous light shifts. Techniques to avoid the undesired effect of molecular resonances are discussed.

Enhancement of high-order harmonic generation in the presence of noise

We report on our simulations of the generation of high-order harmonics from atoms driven by an intense femtosecond laser field in the presence of noise. We numerically solve the non-perturbative stochastic time-dependent Schrodinger equation and observe how varying noise levels affect the frequency components of the high harmonic spectrum. Our calculations show that when an optimum amount of noise is present in the driving laser field, roughly a factor of 45 net enhancement can be achieved in high-order harmonic yield, especially, around the cut-off region. We observe that, for a relatively weak noise, the enhancement mechanism is sensitive to the carrier-envelope phase. We also investigate the possibility of generating ultra-short intense attosecond pulses by combining the laser field and noise and observe that a roughly four orders of magnitude enhanced isolated attosecond burst can be generated.

Calculation of state selective field ionization of hydrogen atoms in a strong magnetic field

We have performed classical and quantum calculations for a hydrogen atom in a strong magnetic field exposed to a parallel electric field that linearly increases with time. The calculations were performed for the situation where the electron is launched from near the nucleus and for a microcanonical ensemble. For the case of low angular momentum, the classical and quantum calculations are compared. We show that there exist stable classical trajectories at positive energy and that these contribute to the possibility of the atom surviving to strong electric fields. The dependence of the survival probability versus electric field strength can be used to estimate the behaviour of Rydberg anti-hydrogen atoms in the ALPHA and ATRAP experiments.

Dynamic polarizability of Rydberg atoms: Applicability of the near-free-electron approximation, gauge invariance, and the Dirac sea

Ponderomotive energy shifts experienced by Rydberg atoms in optical fields are known to be well approximated by the classical quiver energy of a free electron. We examine such energy shifts quantum mechanically and elucidate how they relate to the ponderomotive shift of a free electron in off-resonant fields. We derive and evaluate corrections to the ponderomotive free electron polarizability in the length and velocity (transverse or Coulomb) gauges, which agree exactly as mandated by the gauge invariance. We also show how the free electron value emerges from the Dirac equation through summation over the Dirac sea states. We find that the free-electron AC Stark shift comes as an expectation value of a term proportional to the square of the vector potential in the velocity gauge. On the other hand, the same dominant contribution can be obtained to first order via a series expansion of the exact energy shift from the second order perturbation theory in the length gauge. Finally, we numerically examine the validity of the free-electron approximation. The correction to the free-electron value becomes smaller with increasing principal quantum number, and it is well below a per cent for 60s states of Rb and Sr away from the resonances.

Intensity landscape and the possibility of magic trapping of alkali-metal Rydberg atoms in infrared optical lattices

Department of Physics, University of Nevada, Reno, NV 89557, USA(Dated: September 27, 2013)Motivated by compelling advances in manipulating cold Rydberg (Ry) atoms in optical traps, weconsider the effect of large extent of Ry electron wave function on trapping potentials. We find thatwhen the Ry orbit lies outside inflection points in laser intensity landscape, the atom can stablyreside in laser intensity maxima. Effectively, the free-electron AC polarizability of Ry electron ismodulated by intensity landscape and can accept both positive and negative values. We apply theseinsights to determining magic wavelengths for Ry-ground-state transitions for alkali atoms trappedin infrared optical lattices. We find magic wavelengths to be around 10 µm, with exact values thatdepend on Ry state quantum numbers.

The search for oscillations in the near-threshold photo-double ionization cross section of helium

The photo-double ionization cross section for the helium atom is calculated in the near-threshold region by direct solution of the time-dependent Schrodinger equation. Full close-coupling results for the 1s21S ground state are found to be in excellent agreement with experimental measurements. The calculations confirm the validity of the Wannier power law from 0.1 eV to about 1.7 eV excess energy and find no oscillations in the threshold cross section beyond numerical uncertainty. Further time-dependent calculations are made in a simpler s-wave counterlinear model for both the 1s21S ground and 1s2s 1S excited states. Although numerical uncertainties are significantly reduced in the helium model calculations, again no oscillations in the threshold cross sections are found beyond the remaining numerical uncertainty.

Multiphoton population transfer between rovibrational states of HF: adiabatic rapid passage in a diatomic molecule

Efficient population transfer by adiabatically chirping through a multiphoton resonance in microwave driven and kicked Rydberg atoms has recently been reported both experimentally and theoretically. Here we report on our simulations in which we have exploited this mechanism to vibrationally excite a diatomic molecule up to |ν = 4, J from the ground state by chirping through a four-photon resonance condition. This is an efficient means of population transfer, which is an alternative to the ladder climbing scheme requiring chirping through a sequence of states in the correct order. We discuss and compare one-dimensional quantum and classical models where there is no rotational degree of freedom. This comparison suggests that for the lowest laser intensity we consider, the process is classically forbidden and the transition occurs through tunnelling. We show that for larger peak intensities, the transfer can be looked upon as a classical transition in phase space, similar to that observed in the atomic case. We extend our simulations to fully three-dimensional quantum calculations and investigate the effect of coupling between different rotational pathways. We finally discuss the effect of thermal averaging over the initial J-states using a temperature for which the first few rotational levels inside the ν = 0 manifold are populated.

Spectrum of quasistable states in a strong microwave field

When atoms are exposed to intense laser or microwave pulses, ∼10% of the atoms are found in Rydberg states subsequent to the pulse, even if it is far more intense than required for static field ionization. The optical spectra of the surviving Li atoms in the presence of a 38-GHz microwave field suggest how atoms survive an intense pulse. The spectra exhibit a periodic train of peaks 38 GHz apart. One peak is just below the limit, and with a 90-V/cm field amplitude the train extends from 300 GHz above the limit to 3000 GHz below it. The spectra and quantum-mechanical calculations imply that the atoms survive in quasistable states in which the Rydberg electron is in a weakly bound orbit infrequently returning to the ionic core during the intense pulse.

Multiphoton population transfer in a kicked Rydberg atom: adiabatic rapid passage by separatrix crossing

Following an experimental observation, a recent simulation has shown that efficient population transfer can be achieved through adiabatic chirping of a microwave pulse through a 10-photon resonance connecting two Rydberg states with n = 72, l = 1 and n ~ 82. These simulations have revealed that this population transfer is essentially a classical transition caused by separatrix crossing in the classical phase space. Here, we present the results of our fully three-dimensional quantum and classical simulations of coherent multiphoton population transfer in a kicked Li atom in a Rydberg state. We were able to achieve ~76% population transfer from the 40p to 46p state in Li through a 6-photon resonance and contrast our results with those when the transition is driven by microwaves. We further discuss the case when the atom starts out from a Stark state in conjunction with the l-distribution of the transferred population. We use a one-dimensional classical model to investigate the classical processes taking place in the phase space and find that the same separatrix crossing mechanism observed in microwave transitions is also responsible for the transition when the atom is kicked.