U. Eichmann

Acceleration of neutral atoms in strong short-pulse laser fields

A charged particle exposed to an oscillating electric field experiences a force proportional to the cycle-averaged intensity gradient. This so-called ponderomotive force plays a major part in a variety of physical situations such as Paul traps for charged particles, electron diffraction in strong (standing) laser fields (the Kapitza–Dirac effect) and laser-based particle acceleration. Comparably weak forces on neutral atoms in inhomogeneous light fields may arise from the dynamical polarization of an atom; these are physically similar to the cycle-averaged forces. Here we observe previously unconsidered extremely strong kinematic forces on neutral atoms in short-pulse laser fields. We identify the ponderomotive force on electrons as the driving mechanism, leading to ultrastrong acceleration of neutral atoms with a magnitude as high as ∼1014 times the Earth’s gravitational acceleration, g. To our knowledge, this is by far the highest observed acceleration on neutral atoms in external fields and may lead to new applications in both fundamental and applied physics.

Complementarity and Young’s interference fringes from two atoms

The interference pattern of the resonance fluorescence from a

Laser-driven ion acceleration using isolated mass-limited spheres

J=1/2

Excited neutral atomic fragments in the strong-field dissociation of N2 molecules

to

Barium 6pns (J = 1) autoionizing Rydberg states: comparison between experiment and R-matrix calculations

J=1/2

Steering neutral atoms in strong laser fields

transition of two identical atoms confined in a three-dimensional harmonic potential is calculated. The thermal motion of the atoms is included. Agreement is obtained with experiments [U. Eichmann et al., Phys. Rev. Lett. 70, 2359 (1993)]. Contrary to some theoretical predictions, but in agreement with the present calculations, a fringe visibility greater than 50% can be observed with polarization-selective detection. The dependence of the fringe visibility on polarization has a simple interpretation, based on whether or not it is possible in principle to determine which atom emitted the photon.

Laser spectroscopy on the Stark effect of the level of He I

We report on our experiments on laser-driven ion acceleration using fully isolated mass-limited spheres with a diameter down to 8 μm for the first time. Two-dimensional (2D) particle-in-cell (PIC) and hydro-code simulations were used to show that the pre-plasma at both the front and rear sides of the target strongly affect the efficiency of the ion acceleration. The mechanism of the plasma flow around mass-limited targets has not yet been identified for laser-driven ion acceleration. Our models indicate that this effect is the cause of the observed limitation to the ion-beam energy in both previous experiments and in our own.

Unified Time and Frequency Picture of Ultrafast Atomic Excitation in Strong Laser Fields.

Excited neutral N* fragments with energies between 3 eV and 15 eV have been observed from the dissociation of N2 molecules in strong laser fields. The kinetic energy spectrum of the excited neutral atoms corresponds to Coulomb explosion processes involving N+ ions. This supports the assumption that the production of excited neutral fragments stems from a process in which one of the participating ions in the Coulomb explosion captures an electron into a Rydberg state.

Fano line shapes reconsidered: symmetric photoionization peaks from pure continuum excitation.

The authors present a detailed comparison between results of a R-matrix calculation and laser-excited 6pn s (J=1, odd parity) autoionizing Rydberg states in Ba. After combination with standard MQDT methods, the R-matrix results are compared with new and previous experimental results on resonance profiles, electron branching ratios and angular distributions of 6pns (J=1) autoionizing Rydberg states. The overall agreement between theory and experiment is very satisfactory in all the observables and is in line with similar investigations for Ca and Sr.

Barium autoionising Rydberg series in the vicinity of a shape resonance

The seminal strong-field tunnelling theory introduced by L V Keldysh plays a pivotal role. It has shaped our understanding of atomic strong-field processes, where it represents the first step in complex ionisation dynamics and provides reliable tunnelling rates. Tunnelling rates, however, cannot be necessarily equated with ionisation rates. Taking into account the electron dynamics in the Coulomb potential following the tunnelling process, the process of frustrated tunnelling ionisation has been found to lead to excited Rydberg atoms. Here, we excite He atoms in the strong-field tunnelling regime into Rydberg states. A high percentage of these Rydberg atoms survive in high intensity laser fields. We exploit this fact together with their high polarisability to kinematically manipulate the Rydberg atoms with a second elliptically polarised focused strong laser field. By varying the spatial overlap of the two laser foci, we are able to selectively control the deflection of the Rydberg atoms. The results of semi-classical calculations, which are based on the frustrated tunnelling model and on the ponderomotive acceleration, are in accord with our experimental data.