8 research outputs found
Few-photon single ionization of cold rubidium in the over-the-barrier regime
Photoionization of the rubidium (Rb) atoms cooled in a magneto-optical trap,
characterized by the coexistence of the ground 5 and the excited
5 states, is investigated experimentally and theoretically with the
400 nm femtosecond laser pulses at intensities of W/cm -
W/cm. Recoil-ion momentum distribution (RIMD) of Rb
exhibits rich ring-like structures and their energies correspond to one-photon
ionization of the 5 state, two-photon and three-photon ionizations of
the 5 state, respectively. With the increasing of , we find that
experimental signals near zero-momentum (NZM) in RIMDs resulted from the
5 state enhance dramatically and its peaked Rb momenta dwindle
obviously while that from the 5 state is maintained. Meanwhile, the
ion-yield ratio of the 5 over the 5 states varies from to
as increases. These features indicate a transition from
perturbative ionization to strong-perturbative ionization for the 5
state. Numerical simulations by solving the time-dependent Schr\"odinger
equation (TDSE) can qualitatively explain the measurements of RIMD, photoion
angular distributions, as well as ion-yield ratio. However, some discrepancies
still exist, especially for the NZM dip, which could stem from the
electron-electron correlation that is neglected in the present TDSE simulations
since we have adopted the single-active-electron approximation
Ellipticity-dependent sequential over-barrier ionization of cold rubidium
We perform high-resolution measurements of momentum distribution on Rb
recoil ions up to charge state , where laser-cooled rubidium atoms are
ionized by femtosecond elliptically polarized lasers with the pulse duration of
35 fs and the intensity of 3.310 W/cm in the over-barrier
ionization (OBI) regime. The momentum distributions of the recoil ions are
found to exhibit multi-band structures as the ellipticity varies from the
linear to circular polarizations. The origin of these band structures can be
explained quantitatively by the classical OBI model and dedicated classical
trajectory Monte Carlo simulations with Heisenberg potential. Specifically,
with back analysis of the classical trajectories, we reveal the ionization time
and the OBI geometry of the sequentially released electrons, disentangling the
mechanisms behind the tilted angle of the band structures. These results
indicate that the classical treatment can describe the strong-field multiple
ionization processes of alkali atoms
Transition of the generation mechanism of high-order harmonics in an extended neon system
Using a time-dependent density functional theory method, we perform a systematic numerical study of the transition of high-order harmonic generation in neon (Ne) systems from an isolated Ne atom to an extended Ne system of solid density. We show that ionized electrons wander in such extended systems until they meet a nearby ion and collide with it. The maximum energy edge for the main feature of the high-order harmonic spectrum in this “wandering electron” picture is determined as Eedge = Ip + 8Up, where Ip is the ionization energy of Ne and Up is the ponderomotive energy delivered by the driving laser. The factor of 8 comes from the maximum kinetic energy of an ionized electron in the driving laser field. Beyond the atomic limit of high-order harmonic spectra, a multiplatform feature is observed, corresponding to re-collisions of ionized electrons with their nearby ions. It is also shown that a Ne simple cubic lattice of appropriate size provides a selection condition for the direction of polarization of high-order harmonics beyond the atomic limit, which may be further used to manipulate the emitted radiation
Multiphoton double ionization of Ar and Ne close to Threshold
In kinematically complete studies we explore double ionization (DI) of Ne and Ar in the threshold regime (I>3×1013W/cm2) for 800 nm, 45 fs pulses. The basic differences are found in the two-electron momentum distributions-"correlation" (CO) for Ne and " anticorrelation" (ACO) for Ar-that can be partially explained theoretically within a 3D classical model including tunneling. Transverse electron momentum spectra provide insight into "Coulomb focusing" and point to correlated nonclassical dynamics. Finally, DI threshold intensities, CO as well as ACO regimes are predicted for both targets. © 2010 The American Physical Society
Electron correlation dynamics of strong-field double ionization of atoms below recollision threshold
In recent combined experimental and theoretical study we have explored nonsequential double ionization of neon and argon atoms in the infrared light field (800nm) below the recollision threshold. We find that the two-electron correlation dynamics depends on atomic structure- "side-by-side emission" (correlation) for Ne and "back-to-back emission" (anticorrelation) for argon atoms. This can be explained theoretically within our three dimensional classical model calculation including tunnelling effect. The multiple recollisions as well as recollision-induced-excitation-tunnelling (RIET) effect dominate the anticorrelation of argon, whereas the laser-assisted instantaneous recollision dominates the correlation of neon