Aspects of Electron Dynamics in Atoms Exposed to Single Cycle Electromagnetic Pulses
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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 mm - 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.
Driving linearly polarized single-cycle pulse is only bidirectional, indeed the electron is driven mostly to the one direction by the field in the first half of the cycle and to the opposite direction in the second half. Affirmation is given by the observing of the probability density during the field propagation. When some pulse asymmetry is included, then it involves new phenomena, which we have observed at different energy level of initial Rydberg states. While 15-d state may be ionized already in the first half of the pulse, where the sinus pulse has the opposite direction to the second half, lower energy states (6-d and 9-d) are ionized at the second half of the pulse with a higher peak intensity, at the opposite direction. Therefore, 15d electron has lower emission energy compare to lower lying Rydberg states.
We have numerically simulated the experiment published in April 2014 by Li and Sha (University of Virginia), where sodium d-Rydberg atoms have been ionized by single-cycle pulse with the duration 10-100 longer than electron Rydberg period and the ionization probability with increasing field strength has been measured. Curves in our simulation grow sigmoidally with the ionization scaling law n-3 for the field strength. This field strength scaling is inversely proportional to the binding energy of electrons in an atom and is valid for all of the probabilities, since all scaled probability curves meet at the same place on a plot. Explanation of this scaling law and mechanism behind is the main target of this thesis. Ionization of bounded electrons by strong laser pulses occurs most frequently as over the barrier ionization, tunneling or multiphoton ionization. By 3D analysis we concluded that the ionization takes the place mostly during the period around the field maxima. We suppose that the ionization is caused partly by the tunneling ionization for the field strength scaled as n-3 and partly as the over the barrier ionization scaled with the field strength as n-4. Angular distribution confirms that the electron density is located mostly in the direction of the field polarization. Backscattering shows that the part of the wavefunction is scattered back to the nucleus. The third paper comes with the spatial transport of an electron, when an electron is driven by the short strong external pulse(s). To observe this phenomena, the laser pulse frequency and the field intensity must be high enough, so that we can neglect the effect of the Coulomb potential. Then wavefunction is translated almost without any distortion to a well defined distance from the origin. This distance depends just on the set up laser intensity and the frequency.
In quantum mechanics, wavefunction is propagated on the grid by the split-step operator and two-step Euler method. Classical simulations are calculated by the classical Monte Carlo method (CTMC). In this case, the initial state is modelled as the statistical microcanonical ensemble with set up boundary conditions . The classical differential equations are numerically solved by Euler method and Runge-Kutta method.