V. L. B. de Jesus

Correlated Multielectron Dynamics in Ultrafast Laser Pulse Interactions with Atoms

We present the results of the detailed experimental study of multiple ionization of Ne and Ar by 25 and 7 fs laser pulses. Whereas in multiple ionization of Ar different mechanisms, involving field ionization steps and recollision-induced excitations, play a role, for Ne only one channel, where the highly correlated instantaneous emission of up to four electrons is triggered by a recollisional electron impact, is found to be important. Using few-cycle pulses we are able to suppress those processes that occur on time scales longer than one laser cycle.

Resonant structures in the low-energy electron continuum for single ionization of atoms in the tunnelling regime

We present high-resolution fully differential experimental data on single ionization of He, Ne and Ar by ultra-short (25 fs, 6 fs) 795 nm laser pulses at intensities 0.15–2.0 × 1015 W cm−2. We show that the ATI-like pattern can survive deep in the tunnelling regime and that the atomic structure plays an important role in the formation of the low-energy photoelectron spectra even at high intensities. The absence of ponderomotive shifts, the splitting of the peaks and their degeneration for few-cycle pulses indicate that the observed structures originate from a resonant process.

Atomic structure dependence of nonsequential double ionization of He, Ne and Ar in strong laser pulses

The ion momentum spectra for nonsequential double ionization of rare gases (He, Ne and Ar) in 23 fs 795 nm laser pulses were measured in the intensity range 0.075–1.25 PW cm−2. In the studies published, confusing differences in the shape of momentum distributions among different targets are consistently explained within a recollision scenario: excitation during recollision plus subsequent field ionization, not implemented in most theoretical approaches, evidently plays a decisive role for He and Ar nonsequential double ionization whereas it plays only a minor role for Ne.

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.

Ion-Li collision dynamics studied with a MOTReMi

In a novel experimental approach a magneto-optical trap (MOT) is combined with a Reaction Microscope (ReMi) and implemented in an ion storage ring. Single ionization measurements were performed with 24 MeV O8+ and 6 MeV H+ projectiles and a state prepared lithium target. We obtained single-, double- and fully differential cross sections and observed features that we attribute to the structure and polarization of target initial states.

Measurements of Energy Distribution of Molecular Ions and their Fragments Produced by Electron Impact with a New Spectroscopic Technique

The energy distribution function of molecules and molecular fragments produced by electron impact is measured using a Delayed Extraction Time-Of- Flight (DETOF) spectroscopy. This new technique is able to measure energy distribution of ions from thermal energies up to a few electron volts.

Identification of the CHClF2 molecule fragmentation paths by electron impact

CFC compounds in the atmosphere play a crucial role to the environment, being the main responsible for the enlargement of the ozone hole. Recent studies show that electron impact can be a significant process for the fragmentation of these molecules; for this reason, the collisional processes of CHClF2 by electron impact were studied. Total and partial cross sections have been obtained, showing the predominance of the release of neutral chlorine, which is the main responsible for the breaking down of ozone molecules. There is a strong indication that this chlorine is being released as a result of the ionization of electrons from both chlorine and fluorine orbitals.

Electron localization in fragmentation of H2 with CEP stabilized laser pulses

Fully differential data on ionization and dissociation of H2 in ultra-short (~ 6 fs), linearly polarized, intense (~ 4.1014W/cm2) laser pulses with stabilized carrier-envelope-phase (CEP) have been measured using a reaction microscope. Depending on the CEP of the laser pulses we see a clear asymmetry in the emission direction of the created protons. Contrary to earlier measurements by Kling et al. [1] we observe the highest asymmetry for kinetic energy releases (proton energy) between 0-2 eV. This excludes the electron re-collision mechanism suggested in [1] as dominant excitation channel and requires another explanation.

Direct measurement of supra-thermal energy distribution of fragments following electron-impact ionization of molecules

The energy distribution function of the molecular fragments produced by electron impact is measured using a combination of a delayed pulse extraction, a free-fall degradation of the ionic density along the beam path, and a time-of-flight spectroscopy. This new technique is able to measure energy distribution of ions from thermal energies up to few electron volts and allows measuring the energy distribution of singly charged ions which is difficult to obtain through the more usual method using the increase of the width of the peaks in the time-of-flight spectrum.