61 research outputs found
Keldysh-Rutherford model for attoclock
We demonstrate a clear similarity between attoclock offset angles and
Rutherford scattering angles taking the Keldysh tunnelling width as the impact
parameter and the vector potential of the driving pulse as the asymptotic
velocity. This simple model is tested against the solution of the
time-dependent Schr\"odinger equation using hydrogenic and screened (Yukawa)
potentials of equal binding energy. We observe a smooth transition from a
hydrogenic to 'hard-zero' intensity dependence of the offset angle with
variation of the Yukawa screening parameter. Additionally we make comparison
with the attoclock offset angles for various noble gases obtained with the
classical-trajectory Monte Carlo method. In all cases we find a close
correspondence between the model predictions and numerical calculations. This
suggests a largely Coulombic origin of the attoclock offset angle and casts
further doubt on its interpretation in terms of a finite tunnelling time
Simulation of angular-resolved RABBITT measurements in noble-gas atoms
We simulate angular-resolved RABBITT (reconstruction of attosecond beating by interference of two-photon transitions) measurements on valence shells of noble-gas atoms (Ne, Ar, Kr, and Xe). Our nonperturbative numerical simulation is based on solution of the time-dependent Schrödinger equation (TDSE) for a target atom driven by an ionizing XUV and dressing IR fields. From these simulations we extract the angular-dependent magnitude and phase of the RABBITT oscillations and deduce the corresponding angular anisotropy
β
parameter and Wigner time delay
τ
W
for the single XUV photon absorption that initiates the RABBITT process. Said
β
and
τ
W
parameters are compared with calculations in the random-phase approximation with exchange (RPAE), which includes intershell correlation. This comparison is used to test various effective potentials employed in the one-electron TDSE. In lighter atoms (Ne and Ar), several effective potentials are found to provide accurate simulations of RABBITT measurements for a wide range of photon energies up to 100 eV above the valence-shell threshold. In heavier atoms (Kr and Xe), the onset of strong correlation with the
d
shell restricts the validity of the single active electron approximation to several tens of eV above the valence-shell threshold
Numerical attoclock on atomic and molecular hydrogen
Numerical attoclock is a theoretical model of attosecond angular streaking
driven by a very short, nearly a single oscillation, circularly polarized laser
pulse. The reading of such an attoclock is readily obtained from a numerical
solution of the time-dependent Schr\"odinger equation as well as a
semi-classical trajectory simulation. By making comparison of the two
approaches, we highlight the essential physics behind the attoclock
measurements. In addition, we analyze the predictions of the Keldysh-Rutherford
model of the attoclock [Phys. Rev. Lett. 121, 123201 (2018)]. In molecular
hydrogen, we highlight a strong dependence of the width of the attoclock
angular peak on the molecular orientation and attribute it to the two-center
electron interference. This effect is further exemplified in the weakly bound
neon dimer.Comment: 8 pages, 7 figure
Double ionization of helium by electron-impact: complete pictures of the four-body breakup dynamics
The dynamics of He double ionization by 2 keV electron impact is studied experimentally for a momentum
transfer of 0.6 a.u. at excess energies of 10 and 40 eV. Complete sets of fivefold differential
cross sections are presented for all electron emission angles in coplanar geometry. Contributions beyond
the first Born approximation are identified comparing experimental data with first order convergent
close-coupling calculations which are in considerably better agreement with the present experiment than
with the earlier measurement of Kheifets et al. [J. Phys. B 32, 5047 (1999)]
Confident Predictability: Identifying reliable gene expression patterns for individualized tumor classification using a local minimax kernel algorithm
<p>Abstract</p> <p>Background</p> <p>Molecular classification of tumors can be achieved by global gene expression profiling. Most machine learning classification algorithms furnish global error rates for the entire population. A few algorithms provide an estimate of probability of malignancy for each queried patient but the degree of accuracy of these estimates is unknown. On the other hand <it>local minimax learning </it>provides such probability estimates with best finite sample bounds on expected mean squared error on an individual basis for each queried patient. This allows a significant percentage of the patients to be identified as <it>confidently predictable</it>, a condition that ensures that the machine learning algorithm possesses an error rate below the tolerable level when applied to the confidently predictable patients.</p> <p>Results</p> <p>We devise a new learning method that implements: (i) feature selection using the k-TSP algorithm and (ii) classifier construction by local minimax kernel learning. We test our method on three publicly available gene expression datasets and achieve significantly lower error rate for a substantial identifiable subset of patients. Our final classifiers are simple to interpret and they can make prediction on an individual basis with an individualized confidence level.</p> <p>Conclusions</p> <p>Patients that were predicted confidently by the classifiers as cancer can receive immediate and appropriate treatment whilst patients that were predicted confidently as healthy will be spared from unnecessary treatment. We believe that our method can be a useful tool to translate the gene expression signatures into clinical practice for personalized medicine.</p
Appearance and disappearance of the second Born effects in the (e,3e) reaction in He
We demonstrate, both experimentally and theoretically, clear manifestation of the second Born effects in the
angular distributions of two ejected electrons produced by a 500 eV electron impact on the He atom in the
so-called (e,3e) reaction. The second Born contribution, due to subsequent interaction of the projectile with the
target, is most prominent for glancing collisions with small momentum transfer. However, these effects are
absent for hard knock-out collisions with large momentum transfer
- …