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 approx 10^14 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.

U. Eichmann, T. Nubbemeyer, H. Rottke and W. Sandner
Nature 461,1261 (2009).

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Making the paper , Nature 461, 1171 (2009)

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Two-Source Double-Slit Interference in Angle-Resolved High-Energy Above-Threshold Ionization Spectra of Diatoms

When an electron from a diatomic molecule undergoes tunneling-rescattering ionization, a novel form of destructive interference can be realized that involves all four geometric orbits that are available to the electron when it is freed, because both ionization and rescattering may take place at the same or at different centers. We find experimentally and confirm theoretically that in orientation-averaged angle-resolved high-order above-threshold ionization spectra the corresponding destructive interference is visible for O2 but not for N2. This effect is different from the suppression of ionization that is well known to occur for O2.

M. Okunishi, R. Itaya, K. Shimada, G. Prümper, K. Ueda, M. Busuladžic, A. Gazibegovic-Busuladžic, D. B. Miloševic, and W. Becker, Phys. Rev. Lett, 103, 043001 (2009).

High harmonic interferometry of multi-electron dynamics in molecules  

High harmonic emission occurs when an electron, liberated from a molecule by an incident intense laser field, gains energy from the field and recombines with the parent molecular ion. The emission provides a snapshot of the structure and dynamics of the recombining system, encoded in the amplitudes, phases and polarization of the harmonic light. Here we show with CO2 molecules that high harmonic interferometry can retrieve this structural and dynamic information: by measuring the phases and amplitudes of the harmonic emission, we reveal 'fingerprints' of multiple molecular orbitals participating in the process and decode the underlying attosecond multi-electron dynamics, including the dynamics of electron rearrangement upon ionization. These findings establish high harmonic interferometry as an effective approach to resolving multi-electron dynamics with sub-Ångström spatial resolution arising from the de Broglie wavelength of the recombining electron, and attosecond temporal resolution arising from the timescale of the recombination event.

Olga Smirnova, Yann Mairesse, Serguei Patchkovskii, Nirit Dudovich, David Villeneuve, Paul Corkum, Misha Yu. Ivanov, Nature 460, 972-977 (2009).

Novel Phenomena in Very-Low-Frequency Strong Fields

Atomic ionization by lasers of very low frequency, once thought to be a classical limit or a “tunneling limit”, presents unique spectral features unlike any tunneling phenomenon. The identity of the atom is the controlling factor, leading to photoelectron spectra with well-defined peaks and valleys that persist over wide ranges of field parameters. Such a spectrum was observed 20 years ago in ionization of xenon at 10.6 µm.

H. R. Reiss, Phys. Rev. Lett. 102, 143003 (2009) 

Strong Laser Field Fragmentation of H2: Coulomb Explosion without Double Ionization

We observe fragmentation of H2 molecules exposed to strong laser fields into excited neutral atoms. The measured excited neutral fragment spectrum resembles the ionic fragmentation spectrum including peaks due to bond softening and Coulomb explosion. To explain the occurrence of excited neutral fragments and their high kinetic energy, we argue that the recently investigated phenomenon of frustrated tunnel ionization is also at work in the neutralization of H+ ions into excited H* atoms. In this process the tunneled electron does not gain enough drift energy from the laser field to escape the Coulomb potential and is recaptured. Calculation of classical trajectories as well as a correlated detection measurement of neutral excited H* and H+ ions support the mechanism.

B. Manschwetus, T. Nubbemeyer, K. Gorling, G. Steinmeyer, U. Eichmann, H. Rottke, and W. Sandner, Phys. Rev. Lett. 102, 113002 (2009)

Strong-Field Tunneling without Ionization

In the tunneling regime of strong laser field ionization we measure a substantial fraction of neutral atoms surviving the laser pulse in excited states. The measured excited neutral atom yield extends over several orders of magnitude as a function of laser intensity. Our findings are compatible with the strong-field tunneling-plus-rescattering model, confirming the existence of a widely unexplored neutral exit channel (frustrated tunneling ionization). Strong experimental support for this mechanism as origin of excited neutral atoms stems from the dependence of the excited neutral yield on the laser ellipticity, which is as expected for a rescattering process. Theoretical support for the proposed mechanism comes from the agreement of the neutral excited state distribution centered at n=6–10 obtained from both, a full quantum mechanical and a semiclassical calculation, in agreement with the experimental results.

T. Nubbemeyer, K. Gorling, A. Saenz, U. Eichmann, and W. Sandner, Phys. Rev. Lett. 101, 233001 (2008)

Angle-Resolved High-Order Above-Threshold Ionization of a Molecule: Sensitive Tool for Molecular Characterization

The strong-field approximation for ionization of diatomic molecules by an intense laser field is generalized to include rescattering of the ionized electron off the various centers of its molecular parent ion. The resulting spectrum and its interference structure strongly depend on the symmetry of the ground state molecular orbital. For N2, if the laser polarization is perpendicular to the molecular axis, we observe a distinct minimum in the emission spectrum, which survives focal averaging and allows determination of, e.g., the internuclear separation. In contrast, for O2, rescattering is absent in the same situation.

M. Busuladžic, A. Gazibegovic-Busuladžic, D. B. Miloševic, and W. Becker, Phys. Rev. Lett. 100, 203003(2008)