119 research outputs found
Interfering one-photon and two-photon ionization by femtosecond VUV pulses in the region of an intermediate resonance
The electron angular distribution after atomic photoionization by the fundamental frequency and its second harmonic is analyzed for a case when the frequency of the fundamental scans the region of an intermediate atomic state. The angular distribution and its left-right asymmetry, due to the two-pathway interference between nonresonant one-photon and resonant two-photon ionization, sharply change as a function of the photon energy. The phenomenon is exemplified by both solving the time-dependent Schr\"odinger equation on a numerical space-time grid and by applying perturbation theory for ionization of the hydrogen atom in the region of the 1s\text{\ensuremath{-}}2p transition for femtosecond pulses as well as an infinitely long exposure to the radiation. Parametrizations for the asymmetry and the anisotropy coefficients, obtained within perturbation theory, reveal general characteristics of observable quantities as functions of the parameters of the radiation beam
Quantum coherent control of the photo\-electron angular distribution in bichromatic ionization of atomic neon
We investigate the coherent control of the photo\-electron angular
distribution in bichromatic atomic ionization. Neon is selected as target since
it is one of the most popular systems in current gas-phase experiments with
free-electron lasers (FELSs). In particular, we tackle practical questions,
such as the role of the fine-structure splitting, the pulse length, and the
intensity. Time-dependent and stationary perturbation theory are employed, and
we also solve the time-dependent Schr\"odinger equation in a single-active
electron model. We consider neon ionized by a FEL pulse whose fundamental
frequency is in resonance with either or excitation. The
contribution of the non\-resonant two-photon process and its potential
constructive or destructive role for quantum coherent control is investigated.Comment: 10 pages, 6 figure
Photoelectron angular distribution in two-pathway ionization of neon with femtosecond XUV pulses
We analyze the photoelectron angular distribution in two-pathway interference
between non\-resonant one-photon and resonant two-photon ionization of neon. We
consider a bichromatic femtosecond XUV pulse whose fundamental frequency is
tuned near the atomic states of neon. The time-dependent
Schr\"odinger equation is solved and the results are employed to compute the
angular distribution and the associated anisotropy parameters at the main
photoelectron line. We also employ a time-dependent perturbative approach,
which allows obtaining information on the process for a large range of pulse
parameters, including the steady-state case of continuous radiation, i.e., an
infinitely long pulse. The results from the two methods are in relatively good
agreement over the domain of applicability of perturbation theory
Displacement effect in strong-field atomic ionization by an XUV pulse
We study strong-field atomic ionization driven by an XUV pulse with a
non\-zero displacement, the quantity defined as the integral of the pulse
vector potential taken over the pulse duration. We demonstrate that the use of
such pulses may lead to an extreme sensitivity of the ionization process to
subtle changes of the parameters of a driving XUV pulse, in particular, the
ramp-on/off profile and the carrier envelope phase. We illustrate this
sensitivity for atomic hydrogen and lithium driven by few-femto\-second XUV
pulses with intensity in the range. We argue that the
observed effect is general and should modify strong-field ionization of any
atom, provided the ionization rate is sufficiently high.Comment: 5 pages, 7 figure
Experimental ionization of atomic hydrogen with few-cycle pulses
We present the first experimental data on strong-field ionization of atomic
hydrogen by few-cycle laser pulses. We obtain quantitative agreement at the 10%
level between the data and an {\it ab initio} simulation over a wide range of
laser intensities and electron energies
Diffuse versus square-well confining potentials in modelling @C atoms
Attention: this version- of the manuscript differs from its previously
uploaded version- (arXiv:1112.6158v1) and subsequently published in 2012 J.
Phys. B \textbf{45} 105102 only by a removed typo in Eq.(2) of version-;
there was the erroneous factor "2" in both terms in the right-hand-side of the
Eq.(2) of version-. Now that the typo is removed, Eq.(2) is correct.
A perceived advantage for the replacement of a discontinuous square-well
pseudo-potential, which is often used by various researchers as an
approximation to the actual C cage potential in calculations of
endohedral atoms @C, by a more realistic diffuse potential is
explored. The photoionization of endohedral H@C and Xe@C is
chosen as the case study. The diffuse potential is modelled by a combination of
two Woods-Saxon potentials. It is demonstrated that photoionization spectra of
@C atoms are largely insensitive to the degree of diffuseness
of the potential borders, in a reasonably broad range of 's.
Alternatively, these spectra are found to be insensitive to discontinuity of
the square-well potential either. Both potentials result in practically
identical calculated spectra. New numerical values for the set of square-well
parameters, which lead to a better agreement between experimental and
theoretical data for @C spectra, are recommended for future studies.Comment: 11 pages, 4 figure
Measurement of laser intensities approaching 10 15 W/cm 2 with an accuracy of 1%
Accurate knowledge of the intensity of focused ultrashort laser pulses is crucial to the correct interpretation of experimental results in strong-field physics. We have developed a technique to measure laser intensities approaching 1015W/cm2 with an accu
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