The dynamics of ultra-slow electrons in the combined potential of an ionic core and a static electric field is discussed. With state-of-the-art detection it is possible to create such electrons through strong intense-field photo-absorption and to detect them via high-resolution time-of-flight spectroscopy despite their very low kinetic energy. The characteristic feature of their momentum spectrum, which emerges at the same position for different laser orientations, is derived and could be revealed experimentally with an energy resolution of the order of 1 meV. PACS numbers: 33.60.+q, 32.80.Ee, 33.80.Rv Extracting excited electrons from an atom or molecule, even a plasma with a constant electric field is an established technique which can reveal the minimal (Rydberg)-excitation by tracking the electron yield as a function of applied field strength [1]. In the time domain, very small extraction fields of about F = 1 . . . 10 V/cm (with 5.142 V/cm = 10 −9 atomic units) and high, pulsed Rydberg excitation lead to intricate electron dynamics despite the fact that a hydrogenic problem in an electric field is separable (e.g., in semi-parabolic coordinates). Clearly, electrons which are capable to escape under such conditions must be highly excited and this is achieved in the experiment by a preceding excitation with a short intense laser pulse [2]. Interestingly, the details of this excitation are irrelevant for the dynamics described here.The relevant questions of this problem are: Can one describe the dynamics classically or semi-classically? We will demonstrate, in comparison to experiment, that a classical description is very accurate and we will explain why this is the case. A second question regards the momentum spectrum of these electrons: Is it structured or smooth? We will demonstrate that it is characterized by a main peak, offset from zero by a well defined momentum, in very good agreement with experiment. The dependence of the peak position on the field strength F is deduced analytically and compared to experimental results without any free parameters.The kind of Stark dynamics discussed here with a focus on the differential momentum distribution of ionized electrons has not been investigated before, since ionization of Rydberg states in static or pulsed weak electric fields has mainly served the purpose to extract details of specific quantum states ranging up to principal quantum number n ≈ 100 by either tunneling or over-barrier ionization (ZEKE spectroscopy) [1,3,4]. Individual classical trajectories and their contributions to the electron yield, on the other hand, have been investigated both in the context of astrophysics, where the same Stark Hamiltonian arises from a combined gravitational and constant driving field [5], and in an atomic setting. For the latter, the inclusion of phases to account for interference effects of different pathways to a detector [6-8] even lead beyond a classical treatment.The results presented here are also relevant for all experiments using electric-field extraction te...

We use a pump-probe scheme to measure the time evolution of the C K-edge x-ray absorption spectrum from CO=Ruð0001Þ after excitation by an ultrashort high-intensity optical laser pulse. Because of the short duration of the x-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first picosecond after the pump can be resolved with unprecedented time resolution. By comparing with density functional theory spectrum calculations, we find high excitation of the internal stretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization of the system in the picosecond regime. The ∼100 fs initial excitation of these CO vibrational modes is not readily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e.g., electronic frictions based on first order electron-phonon coupling or transient population of adsorbate resonances. We suggest that coupling of the adsorbate to nonthermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes.

Studying surface reactions using ultrafast optical pump and x-ray probe experiments relies on accurate calculations of x-ray spectra of adsorbates for the correct identification of the spectral signatures and their dynamical evolution. We show that experimental x-ray absorption can be well reproduced for different binding sites in a static prototype system CO/Ni(100) at a standard density functional theory generalized-gradient-approximation level of theory using a plane-wave basis and pseudopotentials. This validates its utility in analyzing ultrafast x-ray probe experiments. The accuracy of computed relative core level binding energies is about 0.2 eV, representing a lower limit for which spectral features can be resolved with this method. We also show that the commonly used Z + 1 approximation gives very good core binding energy shifts overall. However, we find a discrepancy for CO adsorbed in the hollow site, which we assign to the significantly stronger hybridization in hollow bonding than in on-top.

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