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Register a new userOn the connection between angular and partial waves time delays in electron scattering
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M. Ya. Amusia
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Within the framework of a Dirac bubble potential model for the C60 fullerene shell we investigated the angular time delay in slow-electron elastic scattering by C60 as well as average time delay of electrons in this process. It is demonstrated how the angular time delay is connected to the Eisenbud-Wigner-Smith (EWS) time delay. The angular and energy dependences of these times are investigated. The studies conducted shed light to some extent on the specific features of these dependencies.
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We present a joint experimental and theoretical study of spin dynamics of a single $^{88}$Sr$^+$ ion colliding with an ultracold cloud of Rb atoms in various hyperfine states. While spin-exchange between the two species occurs after 9.1(6) Langevin collisions on average, spin-relaxation of the Sr$^+$ ion Zeeman qubit occurs after 48(7) Langevin collisions which is significantly slower than in previously studied systems due to a small second-order spin-orbit coupling. Furthermore, a reduction of the endothermic spin-exchange rate was observed as the magnetic field was increased. Interestingly, we found that, while the phases acquired when colliding on the spin singlet and triplet potentials vary largely between different partial waves, the singlet-triplet phase difference, which determines the spin-exchange cross-section, remains locked to a single value over a wide range of partial-waves which leads to quantum interference effects.
We study the behavior of the Eisenbud-Wigner collisional time delay around Feshbach resonances in cold and ultracold atomic and molecular collisions. We carry out coupled-channels scattering calculations on ultracold Rb and Cs collisions. In the low-energy limit, the time delay is proportional to the scattering length, so exhibits a pole as a function of applied field. At high energy, it exhibits a Lorentzian peak as a function of either energy or field. For narrow resonances, the crossover between these two regimes occurs at an energy proportional to the square of the resonance strength parameter $s_textrm{res}$. For wider resonances, the behavior is more complicated and we present an analysis in terms of multichannel quantum defect theory.
Unconstrained partial-wave amplitudes obtained at discrete energies from fits to complete sets of experimental data may not vary smoothly with energy, and are in principle non-unique. We demonstrate how this behavior can be ascribed to the continuum ambiguity. Starting from the spinless scattering case, we demonstrate how an unknown overall phase depending on energy and angle mixes the structures seen in the associated partial-wave amplitudes making the partial wave decomposition non-unique, and illustrate it on a simple toy model. We then apply these principles to pseudo-scalar meson photoproduction and show that the non-uniqueness effect can be removed through a phase rotation, allowing a consistent comparison with model amplitudes. The effect of this phase ambiguity is also considered for Legendre expansions of experimental observables. 5 pages,
This tutorial presents an introduction to the interaction of light and matter on the attosecond timescale. Our aim is to detail the theoretical description of ultra-short time-delays, and to relate these to the phase of extreme ultraviolet (XUV) light pulses and to the asymptotic phase-shifts of photoelectron wave packets. Special emphasis is laid on time-delay experiments, where attosecond XUV pulses are used to photoionize target atoms at well-defined times, followed by a probing process in real time by a phase-locked, infrared laser field. In this way, the laser field serves as a clock to monitor the ionization event, but the observable delays do not correspond directly to the delay associated with single-photon ionization. Instead, a significant part of the observed delay originates from a measurement induced process, which obscures the single-photon ionization dynamics. This artifact is traced back to a phase-shift of the above-threshold ionization transition matrix element, which we call the continuum-continuum phase. It arises due to the laser-stimulated transitions between Coulomb continuum states. As we shall show here, these measurement-induced effects can be separated from the single-photon ionization process, using analytical expressions of universal character, so that eventually the attosecond time-delays in photoionization can be accessed.
Measuring the photoionization time delay between electrons from different orbitals is one of the most important accomplishments of attosecond science. These measurements are typically done using attosecond pulses to photoionize a target inside a photoelectron spectrometer. In such experiments, the measured delay corresponds to the superposition of all possible paths to ionization and can include multiple sources of delay. These effects can be difficult to deconvolve. Here, we exploit the collision physics nature of recollision and show that, by perturbing recollision dynamics, photorecombination time delays due to electron dynamics and structure can be measured entirely optically and without obfuscation from molecular structure and propagation effects. While we concentrate on photorecombination delays in argon around the Cooper minimum our approach is general. Therefore, our work holds the potential to fundamentally change how attosecond measurement is performed and paves the way for the entirely optical measurement of ultrafast electron dynamics and photorecombination delays due to electronic structure, multielectron interaction, and strong-field driven dynamics in complex molecular systems and correlated solid-state systems.
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