194 research outputs found
Influence of asymmetry and nodal planes on high-harmonic generation in heteronuclear molecules
The relation between high-harmonic spectra and the geometry of the molecular
orbitals in position and momentum space is investigated. In particular we
choose two isoelectronic pairs of homonuclear and heteronuclear molecules, such
that the highest occupied molecular orbital of the former exhibit at least one
nodal plane. The imprint of such planes is a strong suppression in the harmonic
spectra, for particular alignment angles. We are able to identify two distinct
types of nodal planes. If the nodal planes are determined by the atomic
wavefunctions only, the angle for which the yield is suppressed will remain the
same for both types of molecules. In contrast, if they are determined by the
linear combination of atomic orbitals at different centers in the molecule,
there will be a shift in the angle at which the suppression occurs for the
heteronuclear molecules, with regard to their homonuclear counterpart. This
shows that, in principle, molecular imaging, which uses the homonuclear
molecule as a reference and enables one to observe the wavefunction distortions
in its heteronuclear counterpart, is possible.Comment: 14 pages, 7 figures. Figs. 3, 5 and 6 have been simplified in order
to comply with the arXiv size requirement
Existence criteria for stabilization from the scaling behaviour of ionization probabilities
We provide a systematic derivation of the scaling behaviour of various
quantities and establish in particular the scale invariance of the ionization
probability. We discuss the gauge invariance of the scaling properties and the
manner in which they can be exploited as consistency check in explicit
analytical expressions, in perturbation theory, in the Kramers-Henneberger and
Floquet approximation, in upper and lower bound estimates and fully numerical
solutions of the time dependent Schroedinger equation. The scaling invariance
leads to a differential equation which has to be satisfied by the ionization
probability and which yields an alternative criterium for the existence of
atomic bound state stabilization.Comment: 12 pages of Latex, one figur
Coulomb-corrected quantum interference in above-threshold ionization: Working towards multi-trajectory electron holography
Using the recently developed Coulomb Quantum Orbit Strong-Field Approximation
(CQSFA), we perform a systematic analysis of several features encountered in
above-threshold ionization (ATI) photoelectron angle-resolved distributions
(PADs), such as side lobes, and intra- and intercycle interference patterns.
The latter include not only the well-known intra-cycle rings and the
near-threshold fan-shaped structure, but also previously overlooked patterns.
We provide a direct account of how the Coulomb potential distorts different
types of interfering trajectories and changes the corresponding phase
differences, and show that these patterns may be viewed as generalized
holographic structures formed by up to three types of trajectories. We also
derive analytical interference conditions and estimates valid in the presence
or absence of the residual potential, and assess the range of validity of
Coulomb-corrected interference conditions provided in the literature.Comment: 17 pages, 11 figures. Some figures have been compressed in order to
comply with the arXiv requirement
Enhancement of bichromatic high-harmonic generation with a high-frequency field
Using a high-frequency field superposed to a linearly polarized bichromatic
laser field composed by a wave with frequency and a wave with
frequency , we show it is possible to enhance the intensity of a
group of high harmonics in orders of magnitude. These harmonics have
frequencies about 30% higher than the monochromatic-cutoff frequency, and,
within the three-step-model framework, correspond to a set of electron
trajectories for which tunneling ionization is strongly suppressed. Particular
features in the observed enhancement suggest that the high-frequency field
provides an additional mechanism for the electron to reach the continuum. This
interpretation is supported by a time-frequency analysis of the harmonic yield.
The additional high frequency field permits the control of this group of
harmonics leaving all other sets of harmonics practically unchanged, which is
an advantage over schemes involving only bichromatic fields.Comment: 6 pages RevTex, 5 figures (ps files), Changes in text, figures,
references and equations include
It is all about phases: ultrafast holographic photoelectron imaging
Photoelectron holography constitutes a powerful tool for the ultrafast
imaging of matter, as it combines high electron currents with subfemtosecond
resolution, and gives information about transition amplitudes and phase shifts.
Similarly to light holography, it uses the phase difference between the probe
and the reference waves associated with qualitatively different ionization
events for the reconstruction of the target and for ascertaining any changes
that may occur. These are major advantages over other attosecond imaging
techniques, which require elaborate interferometric schemes in order to extract
phase differences. For that reason, ultrafast photoelectron holography has
experienced a huge growth in activity, which has led to a vast, but fragmented
landscape. The present review is an organizational effort towards unifying this
landscape. This includes a historic account in which a connection with
laser-induced electron diffraction (LIED) is established, a summary of the main
holographic structures encountered and their underlying physical mechanisms, a
broad discussion of the theoretical methods employed, and of the key challenges
and future possibilities. We delve deeper in our own work, and place a strong
emphasis on quantum interference, and on the residual Coulomb potential.Comment: Review article; 88 pages, 40 figures; the quality of some figures has
been compromised in order to comply with the arxiv requirement
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