3,511 research outputs found
Controlling vibrational cooling with Zero-Width Resonances: An adiabatic Floquet approach
In molecular photodissociation, some specific combinations of laser
parameters (wavelength and intensity) lead to unexpected Zero-Width Resonances
(ZWR), with in principle infinite lifetimes. Their interest in inducing basic
quenching mechanisms have recently been devised in the laser control of
vibrational cooling through filtration strategies [O. Atabek et al., Phys. Rev.
A87, 031403(R) (2013)]. A full quantum adiabatic control theory based on the
adiabatic Floquet Hamiltonian is developed to show how a laser pulse could be
envelop-shaped and frequency-chirped so as to protect a given initial
vibrational state against dissociation, taking advantage from its continuous
transport on the corresponding ZWR, all along the pulse duration. As compared
with previous control scenarios actually suffering from non-adiabatic
contamination, drastically different and much more efficient filtration goals
are achieved. A semiclassical analysis helps in finding and interpreting a
complete map of ZWRs in the laser parameter plane. In addition, the choice of a
given ZWR path, among the complete series identified by the semiclassical
approach, amounts to be crucial for the cooling scheme, targeting a single
vibrational state population left at the end of the pulse, while all others
have almost completely decayed. The illustrative example, offering the
potentiality to be transposed to other diatomics, is Na2 prepared by
photoassociation in vibrationally hot but translationally and rotationally cold
states.Comment: 15 pages, 14 figure
On-the-fly ab initio semiclassical evaluation of time-resolved electronic spectra
We present a methodology for computing vibrationally and time-resolved
pump-probe spectra, which takes into account all vibrational degrees of freedom
and is based on the combination of the thawed Gaussian approximation with
on-the-fly ab initio evaluation of the electronic structure. The method is
applied to the phenyl radical and compared with two more approximate approaches
based on the global harmonic approximation - the global harmonic method expands
both the ground- and excited-state potential energy surfaces to the second
order about the corresponding minima, while the combined global
harmonic/on-the-fly method retains the on-the-fly scheme for the excited-state
wavepacket propagation. We also compare the spectra by considering their means
and widths, and show analytically how these measures are related to the
properties of the semiclassical wavepacket. We find that the combined approach
is better than the global harmonic one in describing the vibrational structure,
while the global harmonic approximation estimates better the overall means and
widths of the spectra due to a partial cancellation of errors. Although the
full-dimensional on-the-fly ab initio result seems to reflect the dynamics of
only one mode, we show, by performing exact quantum calculations, that this
simple structure cannot be recovered using a one-dimensional model. Yet, the
agreement between the quantum and semiclassical spectra in this simple, but
anharmonic model lends additional support for the full-dimensional ab initio
thawed Gaussian calculation of the phenyl radical spectra. We conclude that the
thawed Gaussian approximation provides a viable alternative to the expensive or
unfeasible exact quantum calculations in cases, where low-dimensional models
are not sufficiently accurate to represent the full system.Comment: Last 6 pages contain the Supplementary Materia
Application of the Mixed Time-averaging Semiclassical Initial Value Representation method to Complex Molecular Spectra
The recently introduced mixed time-averaging semiclassical initial value
representation molecular dynamics method for spectroscopic calculations [M.
Buchholz, F. Grossmann, and M. Ceotto, J. Chem. Phys. 144, 094102 (2016)] is
applied to systems with up to 61 dimensions, ruled by a condensed phase
Caldeira-Leggett model potential. By calculating the ground state as well as
the first few excited states of the system Morse oscillator, changes of both
the harmonic frequency and the anharmonicity are determined. The method
faithfully reproduces blueshift and redshift effects and the importance of the
counter term, as previously suggested by other methods. Differently from
previous methods, the present semiclassical method does not take advantage of
the specific form of the potential and it can represent a practical tool that
opens the route to direct ab initio semiclassical simulation of condensed phase
systems.Comment: 11 figure
Semiclassical instanton formulation of Marcus-Levich-Jortner theory
Marcus-Levich-Jortner (MLJ) theory is one of the most commonly used methods
for including nuclear quantum effects into the calculation of electron-transfer
rates and for interpreting experimental data. It divides the molecular problem
into a subsystem treated quantum-mechanically by Fermi's golden rule and a
solvent bath treated by classical Marcus theory. As an extension of this idea,
we here present a "reduced" semiclassical instanton theory, which is a
multiscale method for simulating quantum tunnelling of the subsystem in
molecular detail in the presence of a harmonic bath. We demonstrate that
instanton theory is typically significantly more accurate than the cumulant
expansion or the semiclassical Franck-Condon sum, which can give
orders-of-magnitude errors and in general do not obey detailed balance. As
opposed to MLJ theory, which is based on wavefunctions, instanton theory is
based on path integrals and thus does not require solutions of the
Schr\"odinger equation, nor even global knowledge of the ground- and
excited-state potentials within the subsystem. It can thus be efficiently
applied to complex, anharmonic multidimensional subsystems without making
further approximations. In addition to predicting accurate rates, instanton
theory gives a high level of insight into the reaction mechanism by locating
the dominant tunnelling pathway as well as providing information on the
reactant and product vibrational states involved in the reaction and the
activation energy in the bath similarly to what would be found with MLJ theory.Comment: 21 pages, 4 figure
Theoretical study of charge exchange dynamics in He + NO collisions
We investigate the charge transfer mechanism in the collisions of helium ions
on nitric oxide using a molecular description framework with consideration of
the orientation of the projectile toward the target. The anisotropy of the
collision process has been analysed in detail in connection with the
non-adiabatic interactions around avoided crossings. Potential energy curves,
radial and rotational coupling matrix elements have been determined by means of
ab initio quantum chemical methods. The collision dynamics is performed in the
[1.-25.] keV collision energy range using a semiclassical approach, and the
total electron transfer cross sections are analysed with regard to available
experimental data.Comment: 16 pages, 3 tables, 6 figure
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