2,125 research outputs found
Alignment dependent enhancement of the photo-electron cutoff for multi-photon ionization of molecules
The multiphoton ionization rate of molecules depends on the alignment of the
molecular axis with respect to the ionizing laser polarization. By studying
molecular frame photo-electron angular distributions from N, O and
benzene, we illustrate how the angle-dependent ionization rate affects the
photo-electron cutoff energy. We find alignment can enhance the high energy
cutoff of the photo-electron spectrum when probing along a nodal plane or when
ionization is otherwise suppressed. This is supported by calculations using a
tunneling model with a single ion state.Comment: 4 pages, 4 figure
Momentum space tomographic imaging of photoelectrons
We apply tomography, a general method for reconstructing 3-D distributions
from multiple projections, to reconstruct the momentum distribution of
electrons produced via strong field photoionization. The projections are
obtained by rotating the electron distribution via the polarization of the
ionizing laser beam and recording a momentum spectrum at each angle with a 2-D
velocity map imaging spectrometer. For linearly polarized light the tomographic
reconstruction agrees with the distribution obtained using an Abel inversion.
Electron tomography, which can be applied to any polarization, will simplify
the technology of electron imaging. The method can be directly generalized to
other charged particles.Comment: Accepted by J. Phys.
Generation of broad XUV continuous high harmonic spectra and isolated attosecond pulses with intense mid-infrared lasers
We present experimental results showing the appearance of a near-continuum in
the high-order harmonic generation (HHG) spectra of atomic and molecular
species as the driving laser intensity of an infrared pulse increases. Detailed
macroscopic simulations reveal that these near-continuum spectra are capable of
producing IAPs in the far field if a proper spatial filter is applied. Further,
our simulations show that the near-continuum spectra and the IAPs are a product
of strong temporal and spatial reshaping (blue shift and defocusing) of the
driving field. This offers a possibility of producing IAPs with a broad range
of photon energy, including plateau harmonics, by mid-IR laser pulses even
without carrier-envelope phase stabilization.Comment: 7 pages, 5 figures, submitted to J.Phys. B (Oct 2011
Spatiotemporal imaging of valence electron motion
Electron motion on the (sub-)femtosecond time scale constitutes the fastest response in many natural phenomena such as light-induced phase transitions and chemical reactions. Whereas static electron densities in single molecules can be imaged in real space using scanning tunnelling and atomic force microscopy, probing real-time electron motion inside molecules requires ultrafast laser pulses. Here, we demonstrate an all-optical approach to imaging an ultrafast valence electron wave packet in real time with a time-resolution of a few femtoseconds. We employ a pump-probe-deflect scheme that allows us to prepare an ultrafast wave packet via strong-field ionization and directly image the resulting charge oscillations in the residual ion. This approach extends and overcomes limitations in laser-induced orbital imaging and may enable the real-time imaging of electron dynamics following photoionization such as charge migration and charge transfer processes
Waiting and Residence Times of Brownian Interface Fluctuations
We report on the residence times of capillary waves above a given height
and on the typical waiting time in between such fluctuations. The measurements
were made on phase separated colloid-polymer systems by laser scanning confocal
microscopy. Due to the Brownian character of the process, the stochastics vary
with the chosen measurement interval . In experiments, the discrete
scanning times are a practical cutoff and we are able to measure the waiting
time as a function of this cutoff. The measurement interval dependence of the
observed waiting and residence times turns out to be solely determined by the
time dependent height-height correlation function . We find excellent
agreement with the theory presented here along with the experiments.Comment: 5 figure
Degeneracy analysis for a super cell of a photonic crystal and its application to the creation of band gaps
A method is introduced to analyze the degeneracy properties of the band
structure of a photonic crystal making use of the super cells. The band
structure associated with a super cell of a photonic crystal has degeneracies
at the edge of the Brillouin zone if the photonic crystal has some kind of
point group symmetry. Both E-polarization and H-polarization cases have the
same degeneracies for a 2-dimensional (2D) photonic crystal. Two theorems are
given and proved. These degeneracies can be lifted to create photonic band gaps
by changing the transform matrix between the super cell and the smallest unit
cell. The existence of the photonic band gaps for many known 2D photonic
crystals is explained through the degeneracy analysis.Comment: 19 pages, revtex4, 14 figures, p
Unraveling the Mott-Peierls intrigue in vanadium dioxide
Vanadium dioxide is one of the most studied strongly correlated materials. Nonetheless, the intertwining between electronic correlation and lattice effects has precluded a comprehensive description of the rutile metal to monoclinic insulator transition, in turn triggering a longstanding "the chicken or the egg" debate about which comes first, the Mott localization or the Peierls distortion. Here, we suggest that this problem is in fact ill posed: The electronic correlations and the lattice vibrations conspire to stabilize the monoclinic insulator, and so they must be both considered to not miss relevant pieces of the VO2 physics. Specifically, we design a minimal model for VO2 that includes all the important physical ingredients: the electronic correlations, the multiorbital character, and the two components of the antiferrodistortive mode that condense in the monoclinic insulator. We solve this model by dynamical mean-field theory within the adiabatic Born-Oppenheimer approximation. Consistently with the first-order character of the metal-insulator transition, the Born-Oppenheimer potential has a rich landscape, with minima corresponding to the undistorted phase and to the four equivalent distorted ones, and which translates into an equally rich thermodynamics that we uncover by the Monte Carlo method. Remarkably, we find that a distorted metal phase intrudes between the low-temperature distorted insulator and high-temperature undistorted metal, which sheds new light on the debated experimental evidence of a monoclinic metallic phase
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