775 research outputs found
Semiclassical two-step model for ionization of hydrogen molecule by strong laser field
We extend the semiclassical two-step model for strong-field ionization that
describes quantum interference and accounts for the Coulomb potential beyond
the semiclassical perturbation theory to the hydrogen molecule. In the simplest
case of the molecule oriented along the polarization direction of a linearly
polarized laser field, we predict significant deviations of the two-dimensional
photoelectron momentum distributions and the energy spectra from the case of
atomic hydrogen. Specifically, for the hydrogen molecule the electron energy
spectrum falls off slower with increasing energy, and the holographic
interference fringes are more pronounced than for the hydrogen atom at the same
parameters of the laser pulse.Comment: 9 pages, 6 figure
Transfer learning, alternative approaches, and visualization of a convolutional neural network for retrieval of the internuclear distance in a molecule from photoelectron momentum distributions
We investigate the application of deep learning to the retrieval of the
internuclear distance in the two-dimensional H molecule from the
momentum distribution of photoelectrons produced by strong-field ionization. We
study the effect of the carrier-envelope phase on the prediction of the
internuclear distance with a convolutional neural network. We apply the
transfer learning technique to make our convolutional neural network applicable
to distributions obtained for parameters outside the ranges of the training
data. The convolutional neural network is compared with alternative approaches
to this problem, including the direct comparison of momentum distributions,
support-vector machines, and decision trees. These alternative methods are
found to possess very limited transferability. Finally, we use the
occlusion-sensitivity technique to extract the features that allow a neural
network to take its decisions.Comment: 28 pages, 7 figures, 1 tabl
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
A family of thermostable fungal cellulases created by structure-guided recombination
SCHEMA structure-guided recombination of 3 fungal class II cellobiohydrolases (CBH II cellulases) has yielded a collection of highly thermostable CBH II chimeras. Twenty-three of 48 genes sampled from the 6,561 possible chimeric sequences were secreted by the Saccharomyces cerevisiae heterologous host in catalytically active form. Five of these chimeras have half-lives of thermal inactivation at 63°C that are greater than the most stable parent, CBH II enzyme from the thermophilic fungus Humicola insolens, which suggests that this chimera collection contains hundreds of highly stable cellulases. Twenty-five new sequences were designed based on mathematical modeling of the thermostabilities for the first set of chimeras. Ten of these sequences were expressed in active form; all 10 retained more activity than H. insolens CBH II after incubation at 63°C. The total of 15 validated thermostable CBH II enzymes have high sequence diversity, differing from their closest natural homologs at up to 63 amino acid positions. Selected purified thermostable chimeras hydrolyzed phosphoric acid swollen cellulose at temperatures 7 to 15°C higher than the parent enzymes. These chimeras also hydrolyzed as much or more cellulose than the parent CBH II enzymes in long-time cellulose hydrolysis assays and had pH/activity profiles as broad, or broader than, the parent enzymes. Generating this group of diverse, thermostable fungal CBH II chimeras is the first step in building an inventory of stable cellulases from which optimized enzyme mixtures for biomass conversion can be formulated
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