R. Kopold

Quantum path analysis of high-order above-threshold ionization

High-order above-threshold ionization spectra are calculated via an improved Keldysh approximation that takes rescattering into account. An approximate method of evaluating the crucial multidimensional integral proceeds via the saddle point method. The saddle points define complex orbits in position space that depart from the ion and return to it to rescatter. The real parts of these orbits are very closely related to the trajectories of the simple-man model. The spectra are analyzed in terms of these quantum orbits whose constructive and destructive interferences generate the spectrums intricate structures. In most spectral regions, the six trajectories having the shortest travel times between start and return already provide an excellent approximation to the exact calculation. In exceptional cases, more orbits are required. The quantum orbits provide an illuminating illustration of the quantum mechanical path integral.

The Wigner function for tunneling in a uniform static electric field

Abstract The Wigner function is used to study a simple model system for strong-field induced ionization: an electron tunneling out of a zero-range potential in the presence of a uniform static electric field. We derive an analytic expression for an approximate Wigner function describing a stationary situation where the part lost to ionization is continuously replenished. This approach is well justified by comparison with the true time dependent Wigner function obtained by numerically solving the one-dimensional problem. The three- and one-dimensional Wigner functions both suggest that the electron leaves the tunnel with a non-zero velocity.

Auf Feynmans Quantenpfaden: Atome im intensiven Laserfeld

Heute kann man Laserpulse erzeugen, deren Feldstarken diejenigen ubertreffen, welche die Atome zusammen halten. In den letzten Jahren gelangen mithilfe des Feynmanschen Pfadintegrals grose Fortschritte im Verstandnis der Effekte, die durch die Wechselwirkung intensiver Laserpulse mit Atomen entstehen. Die begriffliche Nahe des Pfadintegralformalismus zu klassischen Trajektorien erlaubt in vielen Fallen sogar ein intuitives Verstandnis des physikalischen Mechanismus.