Electron wave packet after tunnel ionization (Vol. 44 No. 6)

The dynamics of electron ionization are an important topic in attosecond science. Applying a strong laser pulse, electrons can quantum mechanically tunnel through the potential barrier created by the combined Coulomb field of the atom and the laser field. At the tunnel exit, it is commonly assumed that the electron velocity parallel to the electric field is zero, contrary to the well-described distribution of transverse momenta.

After ionization, electrons propagate in the remainder of the laser pulse, where they acquire a momentum spread due to the different phases of the field at their individual exit time. However, the longitudinal momentum spread measured in experiments on helium is considerably larger than that.

Monte Carlo simulations with zero initial longitudinal momentum agree with the theoretical predictions of acquired spread, while simulations that include a longitudinal momentum spread at the tunnel exit are compatible with experimental data. The authors introduced a new method to investigate electron velocity spreads after ionization. Applying this method to experimental data lead to a more accurate reconstruction of the electron wave packet immediately after tunnelling.