Th. Ergler

Fragmentation dynamics of molecular hydrogen in strong ultrashort laser pulses

We present the results of a systematic experimental study of dissociation and Coulomb explosion of molecular hydrogen induced by intense ultrashort (7–25 fs) laser pulses. Using coincident recoil-ion momentum spectroscopy we can distinguish the contributions from dissociation and double ionization even if they result in the same kinetic energies of the fragments. The dynamics of all fragmentation channels drastically depends on the pulse duration, and for 7 fs pulses becomes extremely sensitive to the pulse shape.

Wavelength dependence of sub-laser-cycle few-electron dynamics in strong-field multiple ionization

Recoil-ion momentum distributions for double and triple ionization of Ne and Ar, as well as for double ionization of N2 molecule by intense (0.3–0.5 PW cm-2), short (~35–40 fs) laser pulses have been recorded in a so far unexplored long laser-wavelength regime at 1300 nm. Compared to earlier results at 800 nm, the direct (e, ne) ionization pathway during recollision is strongly enhanced manifesting itself in a pronounced double-hump structure in the longitudinal ion momentum spectra not only for Ne, but also surprisingly distinct for Ar and, found for the first time, for molecules. Observed wavelength dependence of the sub-laser-cycle correlated few-electron dynamics might be of paramount importance for possible future applications in attosecond science, in particular, for imaging of ultrafast molecular processes via recollision-induced fragmentation.

EUV-photon-induced multiple ionization and fragmentation dynamics: from atoms to molecules

Multiple ionization (MI), induced by a few EUV photons at energies of 28.2 eV, 38 eV and 44 eV from FLASH (the free-electron laser at Hamburg), has been studied for atoms (He, Ne, Ar) and N2 molecules utilizing our multi-hit coincident technology?the reaction microscope. At comparably low intensities of I 1011?1013 W cm?2 we find the non-sequential (NS) MI mechanism dominating Ar3+ and Ar4+ production. Inspecting recoil ion and electron momentum distributions evidence is provided (i) for preferential back-to-back emission of electrons for NS double ionization of He and (ii) for angular entanglement between two outgoing electrons in sequential ionization (SI). In contrast to atoms, SI is observed to be most effective for MI of N2 molecules at an intensity of ~1013 W cm?2 leading, among others, to N2+2 ? N+ + N+, N3+2 ? N2+ + N+, N4+2 ? N2+ + N2+ Coulomb explosion channels. Fragment ion momentum distributions are investigated and are demonstrated to allow tracing SI pathways.

Single-electron capture in keV Ar15+...18++He collisions

Single-electron capture in 14 keV q(-1) Ar15+...18++He collisions is investigated both experimentally and theoretically. Partial cross sections and projectile scattering angle dependencies have been deduced from the target ion recoil momenta measured by the COLTRIMS technique. The comparison with close-coupling results obtained from a two-centre extension of the basis generator method yields good overall agreement, demonstrating the applicability of close-coupling calculations to collision systems involving highly charged ions in charge states up to 18+.

Ultrafast mapping of H2+ (D2+) nuclear wave packets using time-resolved Coulomb explosion imaging

The time evolution of H2+ (D2+) nuclear wave packets is studied exploiting a combination of coincident Coulomb explosion imaging and femtosecond pump–probe techniques. Using two 25 fs laser pulses, we map the motion of the dissociating molecular ion, observe an enhanced ionization rate at an internuclear separation of ~11 au and resolve trajectories due to the one- and two-photon Floquet channels. With two 7 fs pulses, we are able to visualize the vibrational motion of the bound part of the wave packet, which exhibits counterintuitive quantum behaviour and dephases within about 100 fs, in agreement with recent numerical simulations.

Sequential and direct two-photon double ionization of D2 at FLASH

Sequential and direct two-photon double ionization (DI) of D2 molecule is studied experimentally and theoretically at a photon energy of 38.8 eV. Experimental and theoretical kinetic energy releases of D++D+fragments, consisting of the contributions of sequential DI via the D2+(1sσg) state and direct DI via a virtual state, agree well with each other.

Sequential versus nonsequential two-photon double ionization of the D2 molecule at 38 eV

A simple theoretical model is used to interpret recent experimental results for two-photon double ionization (DI) of D2 at 38 eV. We show that the measured kinetic energy distribution associated with emission of two protons can be interpreted as a sum of two processes: a sequential and an instantaneous absorption of the two incident photons. These processes lead to peaks in different regions of the spaectrum.

Time-resolved Imaging of H2 + (D2 +) Nuclear Wave Packets

The spatio-temporal evolution of H2+ (D2+) nuclear wave packets is visualized using time-resolved Coulomb explosion imaging. We study the motion of the dissociating and bound parts of the wave packet, its dephasing and fractional revivals.

Time-resolved imaging of H₂⁺(D₂⁺) nuclear wave packets

The spatio-temporal evolution of H2+ (D2+) nuclear wave packets is visualized using time-resolved Coulomb explosion imaging. We study the motion of the dissociating and bound parts of the wave packet, its dephasing and fractional revivals.