46 research outputs found
Wave packet retrieval by multi-photon quantum beat spectroscopy in helium
We show that we can probe the components of an attosecond bound electron wave packet by mapping the quantum beat signal produced by a synchronized delayed few-cycle infrared pulse into the continuum. In addition, spectrally overlapping peaks that result from one-, two- or three-photon processes from more or less deeply bound states can in principle be interferometrically resolved with high resolution
Interpreting Attoclock Measurements of Tunnelling Times
Resolving in time the dynamics of light absorption by atoms and molecules,
and the electronic rearrangement this induces, is among the most challenging
goals of attosecond spectroscopy. The attoclock is an elegant approach to this
problem, which encodes ionization times in the strong-field regime. However,
the accurate reconstruction of these times from experimental data presents a
formidable theoretical challenge. Here, we solve this problem by combining
analytical theory with ab-initio numerical simulations. We apply our theory to
numerical attoclock experiments on the hydrogen atom to extract ionization time
delays and analyse their nature. Strong field ionization is often viewed as
optical tunnelling through the barrier created by the field and the core
potential. We show that, in the hydrogen atom, optical tunnelling is
instantaneous. By calibrating the attoclock using the hydrogen atom, our method
opens the way to identify possible delays associated with multielectron
dynamics during strong-field ionization.Comment: 33 pages, 10 figures, 3 appendixe
Electron correlation in two-photon double ionization of helium from attosecond to FEL pulses
We investigate the role of electron correlation in the two-photon double
ionization of helium for ultrashort XUV pulses with durations ranging from a
hundred attoseconds to a few femtoseconds. We perform time-dependent ab initio
calculations for pulses with mean frequencies in the so-called "sequential"
regime (photon energy above 54.4 eV). Electron correlation induced by the time
correlation between emission events manifests itself in the angular
distribution of the ejected electrons, which strongly depends on the energy
sharing between them. We show that for ultrashort pulses two-photon double
ionization probabilities scale non-uniformly with pulse duration depending on
the energy sharing between the electrons. Most interestingly we find evidence
for an interference between direct ("nonsequential") and indirect
("sequential") double photo-ionization with intermediate shake-up states, the
strength of which is controlled by the pulse duration. This observation may
provide a route toward measuring the pulse duration of FEL pulses.Comment: 9 pages, 6 figure
Isolated terawatt attosecond hard X-ray pulse generated from single current spike
Isolated terawatt (TW) attosecond (as) hard X-ray pulse is greatly desired for four-dimensional investigations of natural phenomena with picometer spatial and attosecond temporal resolutions. Since the demand for such sources is continuously increasing, the possibility of generating such pulse by a single current spike without the use of optical or electron delay units in an undulator line is addressed. The conditions of a current spike (width and height) and a modulation laser pulse (wavelength and power) is also discussed. We demonstrate that an isolated TW-level as a hard X-ray can be produced by a properly chosen single current spike in an electron bunch with simulation results. By using realistic specifications of an electron bunch of the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL), we show that an isolated, >1.0 TW and similar to 36 as X-ray pulse at 12.4 keV can be generated in an optimized-tapered undulator line. This result opens a new vista for current XFEL operation: the attosecond XFEL
Observation of molecular dipole excitations by attosecond self-streaking
We propose a protocol to probe the ultrafast evolution and dephasing of coherent electronic excitation in molecules in the time domain by the intrinsic streaking field generated by the molecule itself. Coherent electronic motion in the endohedral fullerene Ne@C60 is initiated by a moderately intense femtosecond UV-VIS pulse leading to coherent oscillations of the molecular dipole moment that persist after the end of the laser pulse. The resulting time-dependent molecular near-field is probed through the momentum modulation of photoemission from the central neon atom by a time-delayed attosecond XUV pulse. Our ab-initio time-dependent density functional theory and classical trajectory simulations predict that this self-streaking signal accurately traces the molecular dipole oscillations in real time. We discuss the underlying processes and give an analytical model that captures the essence of our ab-initio simulations
The White Rabbit's clock
Without a very precise timer one can never catch up with the electron released in photoemission. Attosecond streaking spectroscopy allows such a chronometer clock to be set to zero and reveals the role of electron correlations
Probing molecular environment through photoemission delays
Attosecond chronoscopy has revealed small but measurable delays in photoionization, characterized by the ejection of an electron on absorption of a single photon. Ionization-delay measurements in atomic targets provide a wealth of information about the timing of the photoelectric effect, resonances, electron correlations and transport. However, extending this approach to molecules presents challenges, such as identifying the correct ionization channels and the effect of the anisotropic molecular landscape on the measured delays. Here, we measure ionization delays from ethyl iodide around a giant dipole resonance. By using the theoretical value for the iodine atom as a reference, we disentangle the contribution from the functional ethyl group, which is responsible for the characteristic chemical reactivity of a molecule. We find a substantial additional delay caused by the presence of a functional group, which encodes the effect of the molecular potential on the departing electron. Such information is inaccessible to the conventional approach of measuring photoionization cross-sections. The results establish ionization-delay measurements as a valuable tool in investigating the electronic properties of molecules. Ionization delays from ethyl iodide around a giant dipole resonance are measured by attosecond streaking spectroscopy. Using theoretical knowledge of the iodine atom as a reference, the contribution of the functional ethyl group can be obtained