For ultrashort VUV pulses with a pulse length comparable to the orbital time of the bound electrons they couple to we propose a simplified envelope Hamiltonian. It is based on the Kramers-Henneberger representation in connection with a Floquet expans
ion of the strong-field dynamics but keeps the time dependence of the pulse envelope explicit. Thereby, the envelope Hamiltonian captures the essence of the physics, -- light-induced shifts of bound states, single-photon absorption, and non-adiabatic electronic transitions. It delivers quantitatively accurate ionization dynamics and allows for physical insight into the processes occurring. Its minimal requirements for construction in terms of laser parameters make it ideally suited for a large class of atomic and molecular problems.
At the free-electron laser FLASH, multiple ionization of neon atoms was quantitatively investigated at 93.0 eV and 90.5 eV photon energy. For ion charge states up to 6+, we compare the respective absolute photoionization yields with results from a mi
nimal model and an elaborate description. Both approaches are based on rate equations and take into acccout a Gaussian spatial intensity distribution of the laser beam. From the comparison we conclude, that photoionization up to a charge of 5+ can be described by the minimal model. For higher charges, the experimental ionization yields systematically exceed the elaborate rate based prediction.
We formulate the concept of dominant interaction Hamiltonians to obtain an integrable approximation to the dynamics of an electron exposed to a strong laser field and an atomic potential leading to high harmonic generation. The concept relies on loca
l information in phase space to switch between the interactions. This information is provided by classical integrable trajectories from which we construct a semiclassical wave function. The high harmonic spectrum obtained is in excellent agreement with the accurate quantum spectrum. The separation in the atomic potential and laser coupling interactions should facilitate the calculation of high harmonic spectra in complex systems.
The concept of dominant interaction hamiltonians is introduced and applied to classical planar electron-atom scattering. Each trajectory is governed in different time intervals by two variants of a separable approximate hamiltonian. Switching between
them results in exchange of energy between the two electrons. A second mechanism condenses the electron-electron interaction to instants in time and leads to an exchange of energy and angular momentum among the two electrons in form of kicks. We calculate the approximate and full classical deflection functions and show that the latter can be interpreted in terms of the switching sequences of the approximate one. Finally, we demonstrate that the quantum results agree better with the approximate classical dynamical results than with the full ones.
Ultracold quasineutral plasmas generated in the laboratory are generically inhomogeneous and ex- hibit small charge imbalances. As will be demonstrated, via a hydrodynamic theory as well as microscopic simulations, the latter lead to efficient energy
absorption at the plasma boundary. This proposed edge-mode is shown to provide a unified explanation for observed absorption spectra measured in different experiments. Understanding the response of the electronic plasma compo- nent to weak external driving is essential since it grants experimental access to the density and temperature of ultracold plasmas.
We investigate dynamics of atomic and molecular systems exposed to intense, shaped chaotic fields and a weak femtosecond laser pulse theoretically. As a prototype example, the photoionization of a hydrogen atom is considered in detail. The net photoi
onization undergoes an optimal enhancement when a broadband chaotic field is added to the weak laser pulse. The enhanced ionization is analyzed using time-resolved wavepacket evolution and the population dynamics of the atomic levels. We elucidate the enhancement produced by spectrally-shaped chaotic fields of two different classes, one with a tunable bandwidth and another with a narrow bandwidth centered at the first atomic transition. Motivated by the large bandwidth provided in the high harmonic generation, we also demonstrate the enhancement effect exploiting chaotic fields synthesized from discrete, phase randomized, odd-order and all-order high harmonics of the driving pulse. These findings are generic and can have applications to other atomic and simple molecular systems.
A Rydberg and a ground-state atom can form ultralong range diatomic molecules provided the interaction between the ground-state atom and the Rydberg electron is attractive [C. H. Greene, et al., Phys. Rev. Lett. 85, 2458 (2000)]. A repulsive interact
ion does not support bound states. However, as we will show, adding a second ground-state atom, a bound triatomic molecule becomes possible constituting a Borromean Rydberg system.
