arXiv subjects

U. Eichmann

Publications and source records attributed to U. Eichmann.

8 recordsLinked to original sources

arXiv2022

Isolated-core quadrupole excitation of highly excited autoionizing Rydberg states

The structure and photoexcitation dynamics of high lying doubly excited states of the strontium atom with high angular momenta are studied in the vicinity of the Sr$^+(N=5)$ threshold. The spectra recorded using resonant multiphoton isolated core excitation are analyzed with calculations based on configuration interaction with exterior complex scaling, which treats the correlated motion of the two valence electrons of Sr from first principles. The results are rationalized with a model based on multichannel quantum-defect theory and transition dipole moments calculated with a perturbative treatment of electron correlations. Together, both approaches reveal that most of the lines observed in the spectra arise from the interaction of a single optically active state, coupled to the initial state by an electric-dipole transition, with entire doubly-excited Rydberg series. The long-range electron correlations responsible for this interaction unexpectedly vanish for identical values of the initial and final principal quantum numbers, a fact related to the quasi hydrogenic nature of the high-$l$ Rydberg electron. This special situation, and in particular the vanishing interaction, leads to the surprising observation of an electric \emph{quadrupole} isolated-core excitation with a similar intensity as the neighboring electric dipole transitions.

physics.atom-ph↗

arXiv2020

Dissecting strong-field excitation dynamics with atomic-momentum spectroscopy

Observation of internal quantum dynamics relies on correlations between the system being observed and the measurement apparatus. We propose using the center-of-mass (c.m.) degrees of freedom of atoms and molecules as a "built-in" monitoring device for observing their internal dynamics in non-perturbative laser fields. We illustrate the idea on the simplest model system - the hydrogen atom in an intense, tightly-focused infrared laser beam. To this end, we develop a numerically-tractable, quantum-mechanical treatment of correlations between internal and c.m. dynamics. We show that the transverse momentum records the time excited states experience the field, allowing femtosecond reconstruction of the strong-field excitation process. The ground state becomes weak-field seeking, an unambiguous and long sought-for signature of the Kramers-Henneberger regime.

physics.atom-ph↗

arXiv2011

Two-electron effects pervading the formation of highly excited neutral atoms in the break-up of strongly driven H$_{2}$

We present the first theoretical treatment of the formation of highly excited neutral H atoms (H$^{*}$) for strongly driven H$_{2}$. This process, with one electron ionizing and one captured in a Rydberg state, was recently reported in an experimental study in Phys. Rev. Lett {\bf 102} 113002 (2009). We show that two mechanisms underlie this process: a non-sequential one resembling non-sequential double ionization, and a sequential one resembling double ionization through enhanced ionization. We also predict a new feature, asymmetric energy sharing between H$^{*}$ and H$^{+}$ with increasing intensity. This feature is a striking demonstration of the influence the electron has on the nuclear motion.

physics.atom-ph↗

arXiv2003

Fano Lineshapes Revisited: Symmetric Photoionization Peaks from Pure Continuum Excitation

In a photoionization spectrum in which there is no excitation of the discrete states, but only the underlying continuum, we have observed resonances which appear as symmetric peaks, not the commonly expected window resonances. Furthermore, since the excitation to the unperturbed continuum vanishes, the cross section expected from Fano's configuration interaction theory is identically zero. This shortcoming is removed by the explicit introduction of the phase shifted continuum, which demonstrates that the shape of a resonance, by itself, provides no information about the relative excitation amplitudes to the discrete state and the continuum.

cond-mat↗

arXiv1999

Coherent photon bremsstrahlung and dynamics of heavy-ion collisions: comparison of different models

Differential spectra of coherent photon bremsstrahlung in relativistic heavy ion collisions are calculated within various schematic models of the projectile-target stopping. Two versions of the degradation length model, based on a phenomenological deceleration law, are considered. The simple shock wave model is studied analytically. The predictions of these models agree in the soft photon limit, where the spectrum is determined only by the final velocity distribution of charged particles. The results of these models in the case of central Au+Au collisions at various bombarding energies are compared with the predictions of the microscopic transport model UrQMD. It is shown that at the AGS energy the coherent photon bremsstrahlung exceeds the photon yield from $π^0$-decays at photon energies $ω\loo 50$ MeV.

hep-ph↗

arXiv1998

Coulomb Effects on Electromagnetic Pair Production in Ultrarelativistic Heavy-Ion Collisions

We discuss the implications of the eikonal amplitude on the pair production probability in ultrarelativistic heavy-ion transits. In this context the Weizsäcker-Williams method is shown to be exact in the ultrarelativistic limit, irrespective of the produced particles' mass. A new equivalent single-photon distribution is derived which correctly accounts for the Coulomb distortions. As an immediate application, consequences for unitarity violation in photo-dissociation processes in peripheral heavy-ion encounters are discussed.

nucl-th↗

arXiv1998

Electron Propagation in the Field of Colliding Nuclei at Ultrarelativistic Energies

We calculate the asymptotic high-energy amplitude for electrons scattering at one ion as well as at two colliding ions, respectively, by means of perturbation theory. We show that the interaction with one ion eikonalizes and that the interaction with two ions causally decouples. We are able to put previous results on perturbative grounds and propose further applications for the obtained rules for interactions on the light cone. The formalism will be of use for the calculation of Coulomb corrections to electron-positron pair creation in heavy ion collisions. Finally we discuss the results and inherent dangers of the employed approximations.

nucl-th↗

arXiv1997

Complementarity and Young's interference fringes from two atoms

The interference pattern of the resonance fluorescence from a J=1/2 to J=1/2 transition of two identical atoms confined in a three-dimensional harmonic potential is calculated. Thermal motion of the atoms is included. Agreement is obtained with experiments [Eichmann et al., Phys. Rev. Lett. 70, 2359 (1993)]. Contrary to some theoretical predictions, but in agreement with the present calculations, a fringe visibility greater than 50% can be observed with polarization-selective detection. The dependence of the fringe visibility on polarization has a simple interpretation, based on whether or not it is possible in principle to determine which atom emitted the photon.

quant-ph↗