Strong Fields

Atoms in Strong Fields

This was rationalized by L.V. Keldysh1 in the simple model where the electric field of a laser was assumed slowly varying compared to the electron motion inside the potential. The one, most weakly bound electron is tunneling through the essentially static barrier formed by the superposition of the Coulomb core potential with the laser electric field. The tunneling probability of the electron through the classically forbidden region depends exponentially on the area (height x width) under the barrier, so the tunneling may be considered to occur only at the turning points (i.e. maximal barrier suppression) of the field oscillation. For the noble gases the theory2 predictions are in agreement with experiment over 17 orders of magnitude of laser intensities.

The agreement is less striking in case of transition metals where the theory overestimates ionization rates by a factor sometimes as high as 104. This was studied in detail for V, Nb, Ta, Ni and Pd in ref3. Unlike noble gases, transition metal atoms have many low-lying electronic states and one should not restrict himself in considering only one weakly bound electron. Transition metals were shown to exhibit so-called multi active electron (MAE) response to the applied laser field. Effect of many equivalent electrons is even more prominent in metal clusters and large conjugated molecules.

1: L.V. Keldysh, Sov.Phys. JETP 20, 1307 (1965)
2: Sov. Phys. JETP 64, 1191 (1986)
3: PRL 93, 213003 (2004)