A tunnelling probe for molecular currents (Vol. 42, No. 2)

Internal electron currents in molecules play a crucial role in chemical and biological processes, like charge transport in cellular respiration and in photosynthesis. Since electron currents can be ultrafast and escape most traditional probes, they are hard to capture. We show that laser-induced tunnel ionization is a powerful probe of internal currents.

An intense infrared laser field acts on a molecule essentially as the tip of a scanning tunnelling microscope (STM): it extracts a weakly bound electron through a tunnelling barrier. The electron is not equally likely to tunnel out in any direction when the orbital has an asymmetric shape - this has led to the development of the molecular STM, a probe of the static electronic structure.

Unlike in the STM, tunnelling in a laser field is an attosecond phenomenon and therefore potentially launches attosecond electron dynamics. Such dynamics has also been inferred from high-harmonic generation, but that method is insensitive to the actual degree of electronic coherence. Here, we show that laser-induced tunnelling directly probes time-dependent deformations of the electron cloud and maps them on two complementary observables: the total tunnelling current and its momentum distribution.

We study spin-orbit dynamics in a rare gas ion, the simplest example of an internal electron current launched by ionization. Such currents are ubiquitous in molecules, where the sudden departure of an electron triggers an internal rearrangement. Laser-induced tunnelling is a powerful probe of such fundamental events.

A new aspect offered by the proposed concept is the control over electronic dynamics and double ionization. We show that the spin state (i.e. the entanglement) of the ejected electron pair can be controlled offering interesting opportunities for quantum control.