1,379 research outputs found
Control of recollision wave packets for molecular orbital tomography using short laser pulses
The tomographic imaging of arbitrary molecular orbitals via high-order
harmonic generation requires that electrons recollide from one direction only.
Within a semi-classical model, we show that extremely short phase-stabilized
laser pulses offer control over the momentum distribution of the returning
electrons. By adjusting the carrier-envelope phase, recollisions can be forced
to occur from mainly one side, while retaining a broad energy spectrum. The
signatures of the semi-classical distributions are observed in harmonic spectra
obtained by numerical solution of the time-dependent Schr\"{o}dinger equation.Comment: 8 pages, 4 figures; v2: Added some extra clarifications; v3: minor
grammatical change
Strong-field approximation for harmonic generation in diatomic molecules
The generation of high-order harmonics in diatomic molecules is investigated
within the framework of the strong-field approximation. We show that the
conventional saddle-point approximation is not suitable for large internuclear
distances. An adapted saddle-point method that takes into account the molecular
structure is presented. We analyze the predictions for the harmonic-generation
spectra in both the velocity and the length gauge. At large internuclear
separations, we compare the resulting cutoffs with the predictions of the
simple-man's model. Good agreement is obtained only by using the adapted
saddle-point method combined with the velocity gauge.Comment: 24 pages, 7 figure
Accurate retrieval of structural information from laser-induced photoelectron and high-harmonic spectra by few-cycle laser pulses
By analyzing ``exact'' theoretical results from solving the time-dependent
Schr\"odinger equation of atoms in few-cycle laser pulses, we established the
general conclusion that differential elastic scattering and photo-recombination
cross sections of the target ion with {\em free} electrons can be extracted
accurately from laser-generated high-energy electron momentum spectra and
high-order harmonic spectra, respectively. Since both electron scattering and
photoionization (the inverse of photo-recombination) are the conventional means
for interrogating the structure of atoms and molecules, this result shows that
existing few-cycle infrared lasers can be implemented for ultrafast imaging of
transient molecules with temporal resolution of a few femtoseconds.Comment: 4 pages, 4 figure
Ultrahigh harmonics from laser-assisted ion-atom collisions
We present a theoretical analysis of high-order harmonic generation from
ion-atom collisions in the presence of linearly polarized intense laser pulses.
Photons with frequencies significantly higher than in standard atomic
high-harmonic generation are emitted. These harmonics are due to two different
mechanisms: (i) collisional electron capture and subsequent laser-driven
transfer of an electron between projectile and target atom; (ii) reflection of
a laser-driven electron from the projectile leading to recombination at the
parent atom.Comment: 5 pages, 4 figure
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
The effect of dressing on high-order harmonic generation in vibrating H molecules
We develop the strong-field approximation for high-order harmonic generation
in hydrogen molecules, including the vibrational motion and the laser-induced
coupling of the lowest two Born-Oppenheimer states in the molecular ion that is
created by the initial ionization of the molecule. We show that the field
dressing becomes important at long laser wavelengths (m),
leading to an overall reduction of harmonic generation and modifying the ratio
of harmonic signals from different isotopes.Comment: 23 pages, 11 figures, submitted to PR
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
Theory of high-order harmonic generation from molecules by intense laser pulses
We show that high-order harmonics generated from molecules by intense laser
pulses can be expressed as the product of a returning electron wave packet and
the photo-recombination cross section (PRCS) where the electron wave packet can
be obtained from simple strong-field approximation (SFA) or from a companion
atomic target. Using these wave packets but replacing the PRCS obtained from
SFA or from the atomic target by the accurate PRCS from molecules, the
resulting HHG spectra are shown to agree well with the benchmark results from
direct numerical solution of the time-dependent Schr\"odinger equation, for the
case of H in laser fields. The result illustrates that these powerful
theoretical tools can be used for obtaining high-order harmonic spectra from
molecules. More importantly, the results imply that the PRCS extracted from
laser-induced HHG spectra can be used for time-resolved dynamic chemical
imaging of transient molecules with temporal resolutions down to a few
femtoseconds.Comment: 10 pages, 5 figure
Strong field double ionization of H2 : Insights from nonlinear dynamics
The uncorrelated (``sequential'') and correlated (``nonsequential'') double
ionization of the H2 molecule in strong laser pulses is investigated using the
tools of nonlinear dynamics. We focus on the phase-space dynamics of this
system, specifically by finding the dynamical structures that regulate these
ionization processes. The emerging picture complements the recollision scenario
by clarifying the distinct roles played by the recolliding and core electrons.
Our analysis leads to verifiable predictions of the intensities where
qualitative changes in ionization occur. We also show how these findings depend
on the internuclear distance
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