In recent experiments of single-cycle field ionization of excited Na(nd) atoms with principal quantum number (Li and Jones 2014 Phys. Rev. Lett. 112 143006) it was shown that the maximum field intensity necessary to ionize 10% of the atoms decreases with increasing n according to an power law dependence. This scaling property at the same ionization probability was confirmed in classical trajectory Monte Carlo calculations. In this work we note that the scaling relation in the experiment is much more general, it is in fact valid for all ionization probabilities. When applied to the emitted electron energies it places a very wide distribution of electron momenta from different initial states onto a narrow range. These aspects are investigated in a one-dimensional model with a 3D hydrogen-like spectrum. Calculations confirm the general scaling relation for the ionization probability and that this particular scaling of the kinetic emission spectrum puts the ejected electron momenta on a narrow common scale. The ionization mechanism itself is identified as quantum mechanical tunneling and the nature of the tunneling process is the direct origin of the scaling law.

Excited atoms, or nanotip surfaces, exposed to strong single-cycle terahertz radiation emit electrons with energies strongly dependent on the characteristics of the initial state. Here we consider scaling properties of the ionization probability and electron momenta of H(nd) atoms exposed to a single-cycle pulse of duration 0.5-5 ps, with n = 9,12,15. Results from three-dimensional quantum and classical calculations are in good agreement for long pulse lengths, independent of pulse strength. However, differences appear when the two approaches are compared at the most detailed level of density distributions. For the longest pulse lengths a mixed power law, n-scaling relation, αn −4 + (1 − α)n −3 is shown to hold. Our quantum calculations show that the scaling relation puts its imprint on the momentum distribution of the ionized electrons as well: By multiplying the emitted electron momenta of varying initial n level with the appropriate scaling factor the spectra fall onto a common momentum range. Furthermore, the characteristic momenta of emitted electrons from a fixed n level are proportional to the pulse strength of the driving field.

This thesis covers the topic in atomic physics: Interaction of a strong external field with Rydberg hydrogen atom. In three scientific publications, we have targeted physical processes such as the field ionization in the strong terahertz field, back-scattering in the Coulomb field and spatial transport of electrons. First two of them deal with the study of the ionization of the Rydberg atoms in the terahertz field. Rydberg atoms are highly excited stabilized states with very big dipole moments which makes them very sensitive to the external field. As external field we use THz radiation, submillimeter radiation in the range of 100 µm -1 mm, which generators are in the state-of-the-art development. Specifically, we treat with linearly polarized single-cycle pulses with high intensity and picosecond duration. High intensity and low frequency brings us to the strong field, where the field is so strong, that Coulomb potential may be deformed and field ionization is possible.