Left panel: Covariance mapping for the identification of the multiple ionization Coulomb explosion fragmentation channels of N2. Right panel: Pump-probe scheme for the N22+ dissociation lifetimes using Coulomb explosion through the N2+ + N2+ channel.
Whereas tunnel single ionization is an attosecond process taking place within a fraction of an optical cycle, multiple ionization occurs within at least one half-cycle in the case of non sequential multiple ionization or within several cycles. In molecules, multiple ionization leads to the fragmentation into multicharged atomic fragments and is usually labelled Coulomb explosion. In principle Coulomb explosion allows one to image the positions of the atoms within the molecule provided that multiple ionization times remain small in comparison with the timescales of nuclear motions. In these experiments, the time resolution is a major issue in order to compete with accelerator-based techniques such as foil-induced and ion-induced multiple ionizations which occur during a few tens of attoseconds. The advantage of the laser excitation lies in the compactness of the experimental set-ups and more importantly in the possibility to perform pump-probe excitation schemes for imaging excited molecules. Using few-cycle laser pulses to multiply ionize molecules, we study the simultaneous electronic and nuclear relaxations as a powerful probe of the ultrafast ionization sequence at the femtosecond time scale. The molecular systems range from the hydrogen molecule to simple polyatomic molecules. The molecular response is analysed using the multicharged atomic fragments time-of-flight detection, ions correlations, energy analysis, and fluorescence detection.
In 2008, we began developing an alternative imaging technique based on single ionization and diffraction of the returning photoelectron onto the molecular ion core. We expect that the molecular ion structure will be encoded in the angle resolved electron energy spectra which are recorded as a function of the molecular and laser polarisation directions.
We currently work on:
In addition, we collaborate with different theoretical groups for the theoretical aspects of multiple ionization and we provide access time to our kHz femtosecond laser (see Collaborations). Multiple ionization experiments are well-suited for young researchers training in Atomic and Molecular Physics which ranges from ultrafast laser physics, UVX photons and charged particles detection to the resolution of the time-dependent Schrödinger equation (see Photo Gallery).