1,983 research outputs found
Plasmon signatures in high harmonic generation
High harmonic generation in polarizable multi-electron systems is
investigated in the framework of multi-configuration time-dependent
Hartree-Fock. The harmonic spectra exhibit two cut offs. The first cut off is
in agreement with the well established, single active electron cut off law. The
second cut off presents a signature of multi-electron dynamics. The strong
laser field excites non-linear plasmon oscillations. Electrons that are ionized
from one of the multi-plasmon states and recombine to the ground state gain
additional energy, thereby creating the second plateau.Comment: Major revision, 12 pages, 5 figures, submitted to J. Phys. B (2005),
accepte
Saddle point localization of molecular wavefunctions
The quantum mechanical description of isomerization is based on bound eigenstates of the molecular potential energy surface. For the near-minimum regions there is a textbook-based relationship between the potential and eigenenergies. Here we show how the saddle point region that connects the two minima is encoded in the eigenstates of the model quartic potential and in the energy levels of the [H, C, N] potential energy surface. We model the spacing of the eigenenergies with the energy dependent classical oscillation frequency decreasing to zero at the saddle point. The eigenstates with the smallest spacing are localized at the saddle point. The analysis of the HCN???HNC isomerization states shows that the eigenstates with small energy spacing relative to the effective (v1, v3, l) bending potentials are highly localized in the bending coordinate at the transition state. These spectroscopically detectable states represent a chemical marker of the transition state in the eigenenergy spectrum. The method developed here provides a basis for modeling characteristic patterns in the eigenenergy spectrum of bound states
Time-resolved photoelectron spectroscopy of proton transfer in the ground state of chloromalonaldehyde: Wave-packet dynamics on effective potential surfaces of reduced dimensionality
We report on a simple but widely useful method for obtaining time-independent potential surfaces of reduced dimensionality wherein the coupling between reaction and substrate modes is embedded by averaging over an ensemble of classical trajectories. While these classically averaged potentials with their reduced dimensionality should be useful whenever a separation between reaction and substrate modes is meaningful, their use brings about significant simplification in studies of time-resolved photoelectron spectra in polyatomic systems where full-dimensional studies of skeletal and photoelectron dynamics can be prohibitive. Here we report on the use of these effective potentials in the studies of dump-probe photoelectron spectra of intramolecular proton transfer in chloromalonaldehyde. In these applications the effective potentials should provide a more realistic description of proton-substrate couplings than the sudden or adiabatic approximations commonly employed in studies of proton transfer. The resulting time-dependent photoelectron signals, obtained here assuming a constant value of the photoelectron matrix element for ionization of the wave packet, are seen to track the proton transfer
Semi-classical generalized Langevin equation for equilibrium and nonequilibrium molecular dynamics simulation
Molecular dynamics (MD) simulation based on Langevin equation has been widely
used in the study of structural, thermal properties of matters in difference
phases. Normally, the atomic dynamics are described by classical equations of
motion and the effect of the environment is taken into account through the
fluctuating and frictional forces. Generally, the nuclear quantum effects and
their coupling to other degrees of freedom are difficult to include in an
efficient way. This could be a serious limitation on its application to the
study of dynamical properties of materials made from light elements, in the
presence of external driving electrical or thermal fields. One example of such
system is single molecular dynamics on metal surface, an important system that
has received intense study in surface science. In this review, we summarize
recent effort in extending the Langevin MD to include nuclear quantum effect
and their coupling to flowing electrical current. We discuss its applications
in the study of adsorbate dynamics on metal surface, current-induced dynamics
in molecular junctions, and quantum thermal transport between different
reservoirs.Comment: 23 pages, 16 figur
Strong-field physics with mid-IR fields
Strong-field physics is currently experiencing a shift towards the use of
mid-IR driving wavelengths. This is because they permit conducting experiments
unambiguously in the quasi-static regime and enable exploiting the effects
related to ponderomotive scaling of electron recollisions. Initial measurements
taken in the mid-IR immediately led to a deeper understanding of
photo-ionization and allowed a discrimination amongst different theoretical
models. Ponderomotive scaling of rescattering has enabled new avenues towards
time resolved probing of molecular structure. Essential for this paradigm shift
was the convergence of two experimental tools: 1) intense mid-IR sources that
can create high energy photons and electrons while operating within the
quasi-static regime, and 2) detection systems that can detect the generated
high energy particles and image the entire momentum space of the interaction in
full coincidence. Here we present a unique combination of these two essential
ingredients, namely a 160\~kHz mid-IR source and a reaction microscope
detection system, to present an experimental methodology that provides an
unprecedented three-dimensional view of strong-field interactions. The system
is capable of generating and detecting electron energies that span a six order
of magnitude dynamic range. We demonstrate the versatility of the system by
investigating electron recollisions, the core process that drives strong-field
phenomena, at both low (meV) and high (hundreds of eV) energies. The low energy
region is used to investigate recently discovered low-energy structures, while
the high energy electrons are used to probe atomic structure via laser-induced
electron diffraction. Moreover we present, for the first time, the correlated
momentum distribution of electrons from non-sequential double-ionization driven
by mid-IR pulses.Comment: 17 pages, 11 figure
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