Attosecond time-resolved photoelectron dispersion and photoemission time delays.

We compute spectrograms and relative time delays for laser-assisted photoemission by single attosecond extreme ultraviolet pulses from valence band (VB) and 2p core levels (CLs) of a Mg(0001) surface within a quantum-mechanical model. Comparing the time-dependent dispersion of photoelectron (PE) wave packets for VB and CL emission, we find striking differences in their dependence on the (i) electron mean free path (MFP) in the solid, (ii) screening of the streaking laser field, and (iii) chirp of the attosecond pulse. The relative photoemission delay between VB and 2p PEs is shown to be sensitive to the electron MFP and screening of the streaking laser field inside the solid. Our model is able to reproduce a recent attosecond-streaking experiment [S. Neppl et al., Phys. Rev. Lett. 109, 087401 (2012)], which reveals no relative streaking time delay between VB and 2p PEs.

Steering the nuclear motion in singly ionized argon dimers with mutually detuned laser pulses.

We demonstrate that the vibrational nuclear motion of singly ionized argon dimers can be controlled with two ultrashort laser pulses of different wavelengths. In particular, we observe a striking "gap" in the pump-probe-delay-dependent kinetic-energy release spectrum only if the probe-pulse wavelength exceeds the pump-pulse wavelength. This "frustrated dissociation effect" is reproduced by our two-state quantum mechanical model, validating its interpretation as a pump-pulse-initiated population transfer between dipole-coupled Born-Oppenheimer electronic states of the dissociating Ar(2)(+) molecular ion. Our numerical results also reproduce the measured collapse and fractional revival of the oscillating Ar(2)(+) nuclear wave packet, and, for single-pulse dissociation, the decrease of the kinetic-energy release with increasing laser wavelength.

Electron-nuclear energy sharing in above-threshold multiphoton dissociative ionization of H2.

We report experimental observation of the energy sharing between electron and nuclei in above-threshold multiphoton dissociative ionization of H2 by strong laser fields. The absorbed photon energy is shared between the ejected electron and nuclei in a correlated fashion, resulting in multiple diagonal lines in their joint energy spectrum governed by the energy conservation of all fragment particles.

Control of electron localization in deuterium molecular ions using an attosecond pulse train and a many-cycle infrared pulse.

We demonstrate an experimental control of electron localization in deuterium molecular ions created and dissociated by the combined action of an attosecond pulse train and a many-cycle infrared (IR) pulse. The attosecond pulse train is synthesized using both even and odd high order harmonics of the driving IR frequency so that it can strobe the IR field once per IR cycle. An asymmetric ejection of the deuterium ions oscillates with the full IR period when the APT-IR time-delay is scanned. The observed control is due to the creation of a coherent superposition of 1s sigma{g} and 2p sigma{u} states via interference between one-photon and two-photon dissociation channels.

Ion-energy dependence of asymmetric dissociation of D2 by a two-color laser field.

Two-color (800 and 400 nm) short (45 fs) linearly polarized pulses are used to ionize and dissociate D2 into a neutral deuterium atom and a deuteron. The yields and energies of the ions are measured left and right along the polarization vector. As the relative phase of the two colors is varied, strong yield asymmetries are found in the ion-energy regions traditionally identified as bond softening, above-threshold dissociation and rescattering. The asymmetries in these regions are quite different. A model based on the dynamic coupling by the laser field of the gerade and ungerade states in the molecular ion accounts for many of the observed features.

Understanding the role of phase in chemical bond breaking with coincidence angular streaking.

Electron motion in chemical bonds occurs on an attosecond timescale. This ultrafast motion can be driven by strong laser fields. Ultrashort asymmetric laser pulses are known to direct electrons to a certain direction. But do symmetric laser pulses destroy symmetry in breaking chemical bonds? Here we answer this question in the affirmative by employing a two-particle coincidence technique to investigate the ionization and fragmentation of H2 by a long circularly polarized multicycle femtosecond laser pulse. Angular streaking and the coincidence detection of electrons and ions are employed to recover the phase of the electric field, at the instant of ionization and in the molecular frame, revealing a phase-dependent anisotropy in the angular distribution of H⁺ fragments. Our results show that electron localization and asymmetrical breaking of molecular bonds are ubiquitous, even in symmetric laser pulses. The technique we describe is robust and provides a powerful tool for ultrafast science.

Attosecond photoelectron spectroscopy of metal surfaces.

Recent attosecond-streaking spectroscopy experiments [A. L. Cavalieri, Nature (London) 449, 1029 (2007)10.1038/nature06229] using copropagating extreme ultraviolet (XUV) and infrared (IR) pulses of variable relative delay have measured a delay of approximately 100 attoseconds between photoelectrons emitted by a single XUV photon from localized core states and delocalized conduction-band states of a tungsten surface. We analyze the underlying XUV-photoemission-IR-streaking mechanism by combining a perturbative description of the XUV-photoemission process and the subsequent nonperturbative IR streaking of the photoelectrons. Our calculated time-resolved photoelectron spectra agree with the experiments of Cavalieri et al. and demonstrate that the observed temporal shift is caused by the interference of core-level photoelectrons that originate in different layers of the solid.

Complete characterization of molecular dynamics in ultrashort laser fields.

Reaction Microscope-based, complete, and time-resolved Coulomb explosion imaging of vibrating and dissociating D(2)(+) molecules with femtosecond time-resolution allowed us to perform an internuclear distance (R-)dependent Fourier analysis of the corresponding wave packets. Calculations demonstrate that the obtained two-dimensional R-dependent frequency spectra enable the complete characterization of the wave packet dynamics and directly visualize the field-modified molecular potential curves in intense, ultrashort laser pulses.

Boundary conditions for the Pauli equation: application to photodetachment of Cs-.

We formulate the boundary conditions near the atomic nucleus for solving the Pauli equation, based on the analytic solution of the Dirac equation for a Coulomb potential. We then integrate the Pauli equation using an effective potential that is adjusted to reproduce Dirac R-matrix scattering phase shifts, and find the (3)P(o)(1) resonance contribution to the photodetachment cross section of Cs-. Our photodetachment cross sections agree with recent experiments by Scheer et al. [Phys. Rev. Lett. 80, 684 (1998)] after tuning the resonance position by 2.4 meV. We also provide angle-differential photodetachment cross sections and the corresponding asymmetry parameter beta near the Cs(6s) threshold.

Time-resolved photoimaging of image-potential states in carbon nanotubes.

The first experimental evidence for the existence of image-potential states in carbon nanotubes is presented. The observed features constitute a new class of surface image states due to their quantized centrifugal motion. Measurements of binding energies and the temporal evolution of image state electrons were performed using femtosecond time-resolved photoemission. The associated lifetimes are found to be significantly longer than those of n=1 image state on graphite, indicating a substantial difference in electron decay dynamics between tubular and planar graphene sheets.