The ionization dynamics of atoms and molecules exposed to short pulses
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The electrons ejected in an ionization event initiated by an incident laser pulse carry detailed information about the structure and the correlation intrinsic to atoms and molecules. This information is essential for our fundamental understanding of such systems. This thesis is a contribution to the study of the ionization dynamics of atoms and molecules exposed to short laser pulses within the field of atomic, molecular and optical sciences. It consists of five papers published in the period 2011-2013, and covers four topics: the ionization dynamics of Rydberg wave packets, high-order harmonic generation, atomic stabilization and two-photon double ionization of the hydrogen molecule. Rydberg atoms are atoms where at least one of the electrons are found in a highly excited state. As the Rydberg electrons are weakly bound, they are easily affected by external electric and magnetic fields. They exhibit exceptionally long lifetimes and may under certain circumstances behave almost like classical particles. In this work we study the ionization dynamics of a Rydberg wave packet formed within a single Stark-split n-shell in hydrogen. The wave packet is resonantly driven between different substates in the shell governed by the action of a rotating microwave field. Superimposed on this process the wave packet is hit by a series of short femtosecond pulses. We study how the number of pulses in the pulse-train and the time-separation between them affect the total and the angular resolved ionization probability of the system. Ionization is also essential in the generation of high-order harmonics. In this process the ionized electron is accelerated before it recombines with the atom, and photons of frequencies that are multiples of the laser frequency are emitted. The present thesis presents a study of high-order harmonics generated when a graphene sheet is exposed to few-cycle femtosecond pulses. Graphene is a single layer of graphite, and was experimentally realized for the first time less than a decade ago. Our study shows that the extended nature of graphene sheets allows for strong harmonic signals as well as maximum harmonics beyond what is observed for atoms and simpler molecules. The third topic to be studied is atomic stabilization. In the limit where the laser intensity is so strong that the force between the nucleus and the electrons is negligible in comparison to the applied forces of the laser, calculations yield an interesting result. Instead of monotonically increasing as a function of the laser intensity, as expected, the ionization probability tends to stabilize below 100% or even start to decline. Although the phenomenon has been extensively studied for more than 20 years, the experimental verifications are scarce and disputed, mainly due to the lack of the laser technology required. Two of the papers included in this work present ab initio calculations on atomic stabilization solving the time-dependent Schrödinger equation from first principles. Firstly, the phenomenon is studied in helium subjected to superintense pulses. In this paper we especially focus on the correlation between the two electrons, and investigate how it influences the stabilization effect. Secondly, we conduct a similar study but in a different atomic system: low-lying circular Rydberg states of hydrogen. In many respects a circular state behaves like a classical particle orbiting the nucleus either clockwise or counter-clockwise. Motivated by this fact, we study how the ionization probability is affected by a circularly polarized electric field which is either coor counter-rotating with respect to the electronic motion. Lastly, the numerical framework, used in the stabilization calculations, is extended to model the hydrogen molecule. We calculate the direct two-photon double ionization resulting from a linearly polarized laser pulse. Both parallel and perpendicular orientation of the laser polarization vector with respect to the internuclear axis is considered. This is a research domain where the results are few, and different approaches are argued for. Our results are shown to be in good agreement with previous studies, and show for the first time the generalized cross section for the two-photon double ionization process beyond 30 eV.
PAPER I: S. A. Sorngard, S. Askeland, R. Nepstad and M. Forre, Multiphoton ionization and stabilization of helium in superintense xuv fields, Physical Review A 83, 033414, 2011. The article is available at: http://hdl.handle.net/1956/7379PAPER II: S. Askeland, S. A. Sorngard, I. Pilskog, R. Nepstad and M. Forre, Stabilization of circular Rydberg atoms by circularly polarized infrared laser fields, Physical Review A 84, 033423, 2011. The article is available at: http://hdl.handle.net/1956/7412PAPER III: A. S. Simonsen, S. A. Sorngard, R. Nepstad and M. Forre, Direct two-photon double ionization of H2, Physical Review A 85, 063404, 2012. The article is available at: http://hdl.handle.net/1956/7448PAPER IV: S. I. Simonsen, S. A. Sorngard, M. Forre and J. P. Hansen, Femtosecond-pulsetrain ionization of Rydberg wave packets, Physical Review A 86, 043423, 2012. The article is available at: http://hdl.handle.net/1956/7449PAPER V: S. A. Sorngard, S. I. Simonsen and J. P. Hansen, High-order harmonic generation from graphene: Strong attosecond pulses with arbitrary polarization, Physical Review A 87, 053803, 2013. The article is available at: http://hdl.handle.net/1956/7450
PublisherThe University of Bergen
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