11,669 research outputs found
Semiclassical two-step model for strong-field ionization
We present a semiclassical two-step model for strong-field ionization that
accounts for path interferences of tunnel-ionized electrons in the ionic
potential beyond perturbation theory. Within the framework of a classical
trajectory Monte-Carlo representation of the phase-space dynamics, the model
employs the semiclassical approximation to the phase of the full quantum
propagator in the exit channel. By comparison with the exact numerical solution
of the time-dependent Schr\"odinger equation for strong-field ionization of
hydrogen, we show that for suitable choices of the momentum distribution after
the first tunneling step, the model yields good quantitative agreement with the
full quantum simulation. The two-dimensional photoelectron momentum
distributions, the energy spectra, and the angular distributions are found to
be in good agreement with the corresponding quantum results. Specifically, the
model quantitatively reproduces the fan-like interference patterns in the
low-energy part of the two-dimensional momentum distributions as well as the
modulations in the photoelectron angular distributions.Comment: 31 pages, 7 figure
Retrieval of electron-atom scattering cross sections from laser-induced electron rescattering of atomic negative ions in intense laser fields
We investigated the two-dimensional electron momentum distributions of atomic
negative ions in an intense laser field by solving the time-dependent
Schrodinger equation (TDSE) and using the first- and 2nd-order strong-field
approximations (SFA). We showed that photoelectron energy distributions and
low-energy photoelectron momentum spectra predicted from SFA are in reasonable
agreement with the solutions from the TDSE. More importantly, we showed that
accurate electron-atom elastic scattering cross sections can be retrieved
directly from high-energy electron momentum spectra of atomic negative ions in
the laser field. This opens up the possibility of measuring electron-atom and
electron-molecule scattering cross sections from the photodetachment of atomic
and molecular negative ions by intense short lasers, respectively, with
temporal resolutions in the order of femtoseconds.Comment: 6 papges, 5 figure
Maximum information photoelectron metrology
Photoelectron interferograms, manifested in photoelectron angular
distributions (PADs), are a high-information, coherent observable. In order to
obtain the maximum information from angle-resolved photoionization experiments
it is desirable to record the full, 3D, photoelectron momentum distribution.
Here we apply tomographic reconstruction techniques to obtain such 3D
distributions from multiphoton ionization of potassium atoms, and fully analyse
the energy and angular content of the 3D data. The PADs obtained as a function
of energy indicate good agreement with previous 2D data and detailed analysis
[Hockett et. al., Phys. Rev. Lett. 112, 223001 (2014)] over the main spectral
features, but also indicate unexpected symmetry-breaking in certain regions of
momentum space, thus revealing additional continuum interferences which cannot
otherwise be observed. These observations reflect the presence of additional
ionization pathways and, most generally, illustrate the power of maximum
information measurements of this coherent observable
Rotational branching ratios at low photoelectron energies in resonant enhanced multiphoton ionization of NO
We report calculated rotational branching ratios for very low energy (50 meV) photoelectrons resulting from (1+1âČ) resonant enhanced multiphoton ionization (REMPI) via the J_i =1/2, 3/2, 5/2, and 7/2 levels of the P_(11) branch of the Aâ^2ÎŁ^+ (3sÏ) state of NO. Even angular momentum transfer (ÎNâĄN_+âN_i) peaks are dominant in these rotational distributions, in agreement with the selection rule ÎN+l=odd. Angular momentum coupling in the photoelectron wave function arising from the molecular ion potential leads to smaller but appreciable ÎN=odd peaks. The calculated ÎN=0 to ÎN=+2 peak ratios show the same strong decrease when J_i increases from 1/2 to 3/2 as seen in the experimental zeroâkineticâenergy (ZEKE) photoelectron spectra [Sander et al., Phys. Rev. A 36, 4543 (1987)], but do not show the rapid dieâoff of the ÎNâ 0 peaks for higher J_i observed experimentally. The calculated trend in the ÎN=+2 vs ÎN=0 peaks could be understood on the basis of simple angular momentum transfer arguments. These same arguments indicate that this trend in the ÎN=0 and +2 peaks with increasing angular momentum is not generally expected in other branches. Spectra via the R_(21) (âJ) branch are presented to support this assertion. We also present photoelectron angular distributions which show a strong dependence on ÎN reflecting the changing composition of the photoelectron wave function
Rotationally resolved photoelectron angular distributions in resonance enhanced multiphoton ionization of NO
We report calculated ionic rotational branching ratios and associated photoelectron angular distributions for (1+1âČ) resonance enhanced multiphoton ionization (REMPI) via the R_(21)(20.5), P_(21)+Q_(11)(25.5), and P_(11)(22.5) branches of the Aâ^2âÎŁ^+(3sÏ) state of NO. The branching ratios are dominated by even angular momentum transfer peaks, in agreement with the ÎN+l=odd (ÎNâĄN+âNi ) selection rule. Whereas the calculated photoelectron angular distributions are very branch dependent alignment, the ionic branching ratios are found to be less so. The present calculated results agree well with the experimental results of Allendorf et al
Molecular frame photoelectron angular distribution for oxygen 1s photoemission from CO_2 molecules
We have measured photoelectron angular distributions in the molecular frame (MF-PADs) for O 1s photoemission from CO2, using photoelectron-O+âCO+ coincidence momentum imaging. Results for the molecular axis at 0, 45 and 90° to the electric vector of the light are reported. The major features of the MF-PADs are fairly well reproduced by calculations employing a relaxed-core HartreeâFock approach. Weak asymmetric features are seen through a plane perpendicular to the molecular axis and attributed to symmetry lowering by anti-symmetric stretching motion
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