Ionization with low-frequency fields in the tunneling regime

Abstract

Strong-field ionisation surprises with richness beyond current understanding despite decade long investigations. Ionisation with mid-IR light has promptly revealed unexpected kinetic energy structures that seem related to unanticipated quantum trajectories of the electrons. We measure first 3D momentum distributions in the deep tunneling regime (γ = 0.3) and observe surprising new electron dynamics of near-zero momentum electrons and extremely low momentum structures, below the eV, despite very high quiver energies of 95 eV. Such level of high-precision measurements at only 1 meV above the threshold, despite 5 orders higher ponderomotive energies, has now become possible with a specifically developed ultrafast mid-IR light source in combination with a reaction microscope, thereby permitting a new level of investigations into mid-IR recollision physics.

Fine structures in the intensity dependence of excitation and ionization probabilities of hydrogen atoms in intense 800-nm laser pulses

“…Examples of such features are abundant. For example, ATI electrons [1], Freeman resonances [2], "fanlike" angular distributions of low-energy photoelectrons [3][4][5], "holography" in the photoelectron momentum distributions [6][7][8], and the conspicuous low-energy structures (LES) of photoelectrons by midinfrared pulses [9][10][11]. In addition, there are fine features observed only in very carefully detailed measurements.…”

Dynamical Characteristics of Rydberg Electrons Released by a Weak Electric Field

“…Hence, the calculations comprise the formation of Rydberg electrons [2] as well the subsequent Stark dynamics. The experimental results [18] shown in Fig. 1b-d are symmetrized in Π with respect to Π = 0 and show a double peak very close to Π = 0.…”

“…1 we compare classical simulation according to H given in (1) with experimental results [18] obtained for argon atoms in a strong few-cycle pulse with λ = 3200 nm. Those experiments were the first to report a "zero-peak", i. e. an extremely narrow contribution of near-zero momentum electrons [18], subsequently confirmed and attributed to Rydberg electrons [19].…”

“…1 we compare classical simulation according to H given in (1) with experimental results [18] obtained for argon atoms in a strong few-cycle pulse with λ = 3200 nm. Those experiments were the first to report a "zero-peak", i. e. an extremely narrow contribution of near-zero momentum electrons [18], subsequently confirmed and attributed to Rydberg electrons [19]. We use standard tunnel-ionization probabilities [11,20] and propagate electrons according to Newton equations of motion for about 10 7 trajectories with the Hamiltonian (1), i. e., in the attractive Coulomb potential, the driving laser pulse f (t) cos ωt and the extraction field F .…”

“…The second term in (1) describes the interaction with the laser pulse of (strong but otherwise arbitrary) envelope f (t), linearly polarized along or perpendicular to the direction of the extraction field along the z-direction in H. It is this combination of fields which leads to the "zero-energy" structure observed in the strong field photo ionization spectrum with REMIs [18]. However, the role of the strong laser pulse is merely to populate the Rydberg states which serve as an initial condition for the unusual Stark dynamics we are going to discuss.…”