(a) Schematic representation of the fundamental and control fields in multi-dimensional high harmonic spectroscopy
Figure 1. (a) Schematic representation of the fundamental and control fields in multi-dimensional high harmonic spectroscopy. (b) Schematic representation of the evolution in complex time ts = ti + iτ. Quantum orbit enters the barrier at the complex time ts, appears in the continuum at ionization time ti and returns to the core at time t.
High harmonic spectroscopy has the potential to combine attosecond temporal with sub-Angstrom spatial resolution of the early nuclear and multielectron dynamics in molecules. It involves strong-field ionization of the molecule by an infrared (IR) laser field followed by time-delayed recombination of the removed electron with the molecular ion. The time-delay is controlled on the attosecond time scale by the oscillation of the IR field and is mapped into the harmonic number, providing a movie of molecular dynamics between ionization and recombination. One of the challenges in the analysis of a high harmonic signal stems from the fact that the complex dynamics of both ionization and recombination with their multiple observables are entangled in the harmonic signal. Disentangling this information requires a multidimensional approach, capable of mapping ionization and recombination dynamics into different independent parameters. We suggest multidimensional high harmonic spectroscopy as a tool for characterizing ionization and recombination processes separately allowing for simultaneous detection of both the ionization delays and sub-cycle ionization rates. Our method extends the capability of the two-dimensional set-up suggested recently by Shafir et al on reconstructing ionization delays, while keeping the reconstruction procedure as simple as in the original proposal. The scheme is based on the optimization of the high harmonic signal in orthogonally polarized strong fundamental and relatively weak multicolour control fields.