It is shown that the two-step excitation scheme typically used to create an ultracold Rydberg gas can be described with effective two-level rate equation, greatly reducing complexity of optical Bloch equations. This allows us solve many-body problem interacting cold atoms a Monte Carlo technique. Our results reproduce blockade effect. However, we demonstrate Autler-Townes double peak structure in scheme, which occurs for moderate pulse lengths as experiment, give rise antiblockade observable...

We develop a theoretical approach for the dynamics of Rydberg excitations in ultracold gases,with realistically large number atoms. rely on reduction single-atom Bloch equations to rate equations, which is possible under various experimentally relevant conditions. Here, we explicitly refer two-step excitation scheme. discuss conditions our valid by comparing results with solution exact quantum master equation two interacting Concerning emergence an blockade gas, are qualitative agreement...

We present long-time simulations of expanding ultracold neutral plasmas, including a full treatment the strongly coupled ion dynamics. Thereby, relaxation laser-cooled plasma is studied, taking into account elastic as well inelastic collisions. It demonstrated that, depending on initial conditions, ionic component may exhibit short-range order or even superimposed long-range resulting in concentric shells. In contrast to plasmas confined traps, shell structures build up from center cloud...

A kinetic approach for the evolution of ultracold neutral plasmas including interionic correlations and treatment ionization/excitation recombination/deexcitation by rate equations is described in detail. To assess reliability approximations inherent model, we have developed a hybrid molecular dynamics method. Comparison results reveals that model describes atomic ionic observables plasma surprisingly well, confirming our earlier findings concerning role ion-ion [Phys. Rev. 68, 010703...

The behavior of the ionization cross section atoms is known classically in limits threshold energy and at high energies. These two are used to construct a simple analytical formula for that depends upon parameters: magnitude maximum its position energy. parametrization has been tested electron- positron-impact as well proton- antiproton-impact ionization. It reproduces all cases shape offers unified treatment by bare projectiles irrespectively their charge mass.

Based on simple rate equations for the Rydberg excitation process, we are able to model microscopically dynamics of in ensembles a large number ultracold atoms, which is beyond capabilities fully ab initio approaches. Our results distribution atom numbers good agreement with recent experimental data, confirming quenching caused by Rydberg–Rydberg interactions.

Recent experiments have demonstrated the spontaneous evolution of a gas ultracold Rydberg atoms into an expanding plasma, as well reverse process plasma recombination highly excited atomic states. Treating on basis kinetic equations, while ionization/excitation and are incorporated using rate we investigated theoretically Rydberg-to-plasma transition. Including influence spatial correlations dynamics in approximate way find that ionic change results only quantitatively but not qualitatively.

We investigate the strongly correlated ion dynamics and degree of coupling achievable in evolution freely expanding ultracold neutral plasmas. demonstrate that ionic Coulomb parameter Gamma(i) increases considerably later stages expansion, reaching coupled regime despite well known initial drop to order unity due disorder-induced heating. Furthermore, we formulate a suitable measure correlation show calculated from temperature density reflects system if it is sufficiently close quasisteady...

For the first time absolute cross sections for electron impact ionization of hydrogen-like ions B4+, C5+, N6+ and O7+ at energies from below threshold up to about 6 keV have been measured using crossed-beams technique. All are in very good agreement with available distorted-wave exchange calculations semiempirical Lotz formula. The behaviour classically scaled along hydrogen isoelectronic sequence is discussed. A recently developed scaling technique allows us predict all non-relativistic on an scale.

We present a simple half-collision model that allows the approximate calculation of absolute cross sections for multiple ionization by breaking this process down into primary event followed half-scattering in which additional electrons are ionized. As critical test feasability approach, we consider double two-electron systems, describe terms single ``primary'' electron impact slow ``secondary'' electron. For triple lithium, decomposes three-electron breakup two inner electron-electron...

The triple-photoionization cross section of atomic Li was measured near its threshold. Our results can be described by a threshold power law up to $\ensuremath{\approx}5 \mathrm{eV}$ with an exponent 2.05(25) which is consistent previous experiment and theoretical predictions. At higher energies, we find that the well for double ionization. present analysis based on notion sufficient excess energy, triple photoionization reasonably ionization inner electrons followed shakeoff outer electron.

We present a simple half-collision model that allows the approximate calculation of absolute cross sections for triple ionization as well cross-section ratios ${\ensuremath{\sigma}}^{3+}/{\ensuremath{\sigma}}^{2+}$ lithium by photon impact. The decomposes three-electron breakup process into double two inner electrons followed electron-electron half-scattering receding at residual ``spectator'' $2s$ electron. find surprisingly good agreement with recent experimental data.

Recent experiments with ultracold neutral plasmas show an intrinsic heating effect based on the development of spatial correlations. We investigate whether this can be reversed, so that imposing strong correlations could in fact lead to cooling ions. find is indeed possible. It requires, however, a very precise preparation initial state. Quantum mechanical zero-point motion sets lower limit for ion cooling.

Recently, a universal shape function for cross sections of ionization by charged particle impact has been established. Here, we show how an analogous multiple photons can be constructed along the same lines. It is argued that this provides excellent parametrization photoionization in cases where all electrons are ionized. In more complicated situations, still useful, although its applicability may limited.

While ion heating by elastic electron-ion collisions may be neglected for a description of the evolution freely expanding ultracold neutral plasmas, situation is different in scenarios where ions are laser-cooled during system evolution.We show that plasmas influence ionic temperature, decreasing degree correlation obtainable such systems.However, taking into account increases temperature much less than what would estimated based on static plasma clouds neglecting expansion.The latter leads...

The role of shake-off for double ionization atoms by a single photon with finite energy has become the subject debate. In this letter, we attempt to clarify meaning at low energies comparing different formulations appearing in literature and suggesting working definition. Moreover, elaborate on foundation justification mixed quantum-classical ansatz calculation single-photon ionization.

We discuss the dynamics of recombination an expanding ultracold plasma into highly excited Rydberg states, with emphasis on influence possible strong coupling between charges and nonequilibrium character electronic component. While former does not significantly affect in current experimental scenarios, latter is shown to have a considerable system dynamics. derive correction factors quantifying deviation collision rates from their respective equilibrium values. The experimentally observed...

We investigate the single-photon double ionization of molecular hydrogen theoretically over a wide range photon energies. Our numerical approach is based on half-collision picture multiple and employs mixed quantum-classical method that splits process into shake-off knockout part. demonstrate this approach, which has been applied to photoionization helium before, can be successfully extended case target systems with two separate nuclei. The treatment given here allows for both...

It is shown that the cross section for multiple breakup of a system into charged fragments near threshold energy $\ensuremath{\epsilon}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ follows power law modified by logarithmic correction terms if possesses degenerate normal mode frequencies about fixed point equilibrium configuration. For more than two identical particles, e.g., multielectron atom, this will be generic case since configuration highly symmetric. The derived using...

We describe a hybrid molecular dynamics approach for the description of ultracold neutral plasmas, based on an adiabatic treatment electron gas and full simulation ions, which allows us to follow long-time evolution plasma including effect strongly coupled ion motion. The shows rather complex relaxation behavior, connected with temporal as well spatial oscillations temperature. Furthermore, additional laser cooling ions during drastically modifies expansion dynamics, so that crystallization...

Atoms and molecules in highly excited electronic states ('Rydberg atoms') have been the object of broad scientific research for almost a century. Despite this long history, field has never lost its buoyancy, recent years particular seen tremendous revival interest physics Rydberg atoms from many different perspectives. systems touch wide range areas including, among others, ultralong-range molecules, artificial ('designer') atoms, quantum chaos, information, ultracold gases plasmas,...