Mapping degenerate perturbations in atoms onto an asymmetric rotor

Classical dynamics and localization of resonances in the high-energy region of the hydrogen atom in crossed fields

When superimposing the potentials of external fields on the Coulomb potential of the hydrogen atom, a saddle point (called the Stark saddle point) appears. For energies slightly above the saddle point energy, one can find classical orbits that are located in the vicinity of this point. We follow those so-called quasi-Penning orbits to high energies and field strengths, observing structural changes and uncovering their bifurcation behavior. By plotting the stability behavior of those orbits against energy and field strength, the appearance of a stability apex is reported. A cusp bifurcation, located in the vicinity of the apex, will be investigated in detail. In this cusp bifurcation, another orbit of similar shape is found. This orbit becomes completely stable in the observed region of positive energy, i.e., in a region of parameter space, where the Kepler-like orbits located around the nucleus are already unstable. By quantum mechanically exact calculations, we prove the existence of signatures in quantum spectra belonging to those orbits. Husimi distributions are used to compare quantum-Poincaré sections with the extension of the classical torus structure around the orbits. Since periodic orbit theory predicts that each classical periodic orbit contributes an oscillating term to photoabsorption spectra, we finally give an estimation for future experiments, which could verify the existence of the stable orbits.

Ultrafast mid-infrared response of YBa(2)Cu(3)O(7-delta)

Optical spectra of high-transition-temperature superconductors in the mid-infrared display a gap of in-plane conductivity whose role for superconductivity remains unresolved. Femtosecond measurements of the mid-infrared reflectivity of YBa(2)Cu(3)O(7-delta) after nonequilibrium optical excitation are used to demonstrate the ultrafast fill-in of this gap and reveal two gap constituents: a picosecond recovery of the superconducting condensate in underdoped and optimally doped material and, in underdoped YBa(2)Cu(3)O(7-delta), an additional subpicosecond component related to pseudogap correlations. The temperature-dependent amplitudes of both contributions correlate with the antiferromagnetic 41-millielectronvolt peak in neutron scattering, supporting the coupling between charges and spin excitations.

Celestial Mechanics on a Microscopic Scale

Classical and semiclassical methods are unrivaled in providing an intuitive and computationally tractable approach to the study of atomic, molecular, and nuclear dynamics. An important advantage of such methods is their ability to uncover in a single picture underlying structures that may be hard to extract from the profusion of data supplied by detailed quantum calculations. Modern trends in semiclassical mechanics are described, particularly the combination of group theoretical methods with techniques of nonlinear dynamics. Application is made to intramolecular energy transfer and to the electronic structure of atomic Rydberg states in external electric and magnetic fields.