138 research outputs found
Bichromatically driven double well: parametric perspective of the strong-field control landscape reveals the influence of chaotic states
The aim of this work is to understand the influence of chaotic states in
control problems involving strong fields. Towards this end, we numerically
construct and study the strong field control landscape of a bichromatically
driven double well. A novel measure based on correlating the overlap
intensities between Floquet states and an initial phase space coherent state
with the parametric motion of the quasienergies is used to construct and
interpret the landscape features. "Walls" of no control, robust under
variations of the relative phase between the fields, are seen on the control
landscape and associated with multilevel interactions involving chaotic Floquet
states.Comment: 9 pages and 6 figures. Rewritten and expanded version of
arXiv:0707.4547 [nlin.CD]. Accepted for publication in J. Chem. Phys. (2008
Intramolecular vibrational energy redistribution as state space diffusion: Classical-quantum correspondence
We study the intramolecular vibrational energy redistribution (IVR) dynamics
of an effective spectroscopic Hamiltonian describing the four coupled high
frequency modes of CDBrClF. The IVR dynamics ensuing from nearly isoenergetic
zeroth-order states, an edge (overtone) and an interior (combination) state, is
studied from a state space diffusion perspective. A wavelet based
time-frequency analysis reveals an inhomogeneous phase space due to the
trapping of classical trajectories. Consequently the interior state has a
smaller effective IVR dimension as compared to the edge state.Comment: 5 pages, 3 figure
Local phase space control and interplay of classical and quantum effects in dissociation of a driven Morse oscillator
This work explores the possibility of controlling the dissociation of a
monochromatically driven one-dimensional Morse oscillator by recreating
barriers, in the form of invariant tori with irrational winding ratios, at
specific locations in the phase space. The control algorithm proposed by Huang
{\it et al.} (Phys. Rev. A {\bf 74}, 053408 (2006)) is used to obtain an
analytic expression for the control field. We show that the control term,
approximated as an additional weaker field, is efficient in recreating the
desired tori and suppresses the classical as well as the quantum dissociation.
However, in the case when the field frequency is tuned close to a two-photon
resonance the local barriers are not effective in suppressing the dissociation.
We establish that in the on-resonant case quantum dissociation primarily occurs
via resonance-assisted tunneling and controlling the quantum dynamics requires
a local perturbation of the specific nonlinear resonance in the underlying
phase space.Comment: 12 pages, 6 figures (reduced quality), submitted to Phys. Rev.
Semiclassical methods in chemical reaction dynamics
Semiclassical approximations, simple as well as rigorous, are formulated in order to be able to describe gas phase chemical reactions in large systems. We formulate a simple but accurate semiclassical model for incorporating multidimensional tunneling in classical trajectory simulations. This model is based on the existence of locally conserved actions around the saddle point region on a multidimensional potential energy surface. Using classical perturbation theory and monitoring the imaginary action as a function of time along a classical trajectory we calculate state-specific unimolecular decay rates for a model two dimensional potential with coupling. Results are in good comparison with exact quantum results for the potential over a wide range of coupling constants. We propose a new semiclassical hybrid method to calculate state-to-state S-matrix elements for bimolecular reactive scattering. The accuracy of the Van Vleck-Gutzwiller propagator and the short time dynamics of the system make this method self-consistent and accurate. We also go beyond the stationary phase approximation by doing the resulting integrals exactly (numerically). As a result, classically forbidden probabilties are calculated with purely real time classical trajectories within this approach. Application to the one dimensional Eckart barrier demonstrates the accuracy of this approach. Successful application of the semiclassical hybrid approach to collinear reactive scattering is prevented by the phenomenon of chaotic scattering. The modified Filinov approach to evaluating the integrals is discussed, but application to collinear systems requires a more careful analysis. In three and higher dimensional scattering systems, chaotic scattering is suppressed and hence the accuracy and usefulness of the semiclassical method should be tested for such systems
Dynamical tunneling in molecules: Quantum routes to energy flow
Dynamical tunneling, introduced in the molecular context, is more than two
decades old and refers to phenomena that are classically forbidden but allowed
by quantum mechanics. On the other hand the phenomenon of intramolecular
vibrational energy redistribution (IVR) has occupied a central place in the
field of chemical physics for a much longer period of time. Although the two
phenomena seem to be unrelated several studies indicate that dynamical
tunneling, in terms of its mechanism and timescales, can have important
implications for IVR. Examples include the observation of local mode doublets,
clustering of rotational energy levels, and extremely narrow vibrational
features in high resolution molecular spectra. Both the phenomena are strongly
influenced by the nature of the underlying classical phase space. This work
reviews the current state of understanding of dynamical tunneling from the
phase space perspective and the consequences for intramolecular vibrational
energy flow in polyatomic molecules.Comment: 37 pages and 23 figures (low resolution); Int. Rev. Phys. Chem.
(Review to appear in Oct. 2007
Analyzing intramolecular vibrational energy redistribution via the overlap intensity-level velocity correlator
Numerous experimental and theoretical studies have established that
intramolecular vibrational energy redistribution (IVR) in isolated molecules
has a heirarchical tier structure. The tier structure implies strong
correlations between the energy level motions of a quantum system and its
intensity-weighted spectrum. A measure, which explicitly accounts for this
correaltion, was first introduced by one of us as a sensitive probe of phase
space localization. It correlates eigenlevel velocities with the overlap
intensities between the eigenstates and some localized state of interest. A
semiclassical theory for the correlation is developed for systems that are
classically integrable and complements earlier work focusing exclusively on the
chaotic case. Application to a model two dimensional effective spectroscopic
Hamiltonian shows that the correlation measure can provide information about
the terms in the molecular Hamiltonian which play an important role in an
energy range of interest and the character of the dynamics. Moreover, the
correlation function is capable of highlighting relevant phase space structures
including the local resonance features associated with a specific bright state.
In addition to being ideally suited for multidimensional systems with a large
density of states, the measure can also be used to gain insights into the phase
space transport and localization. It is argued that the overlap intensity-level
velocity correlation function provides a novel way of studying vibrational
energy redistribution in isolated molecules. The correlation function is
ideally suited to analyzing the parametric spectra of molecules in external
fields.Comment: 16 pages, 13 figures (low resolution
Diagnostic utility of fecal calprotectin as a biomarker of gut inflammation in neonates to predict necrotizing enterocolitis: A prospective study
Background: Necrotizing enterocolitis (NEC) is a neonatal emergency that affects preterm newborns during the 1st weeks of life.Diagnosis is made mainly by clinical criteria since no specific diagnostic tests are available. Objective: The objective was to evaluatefecal calprotectin (fCal) as a biomarker of gut inflammation to predict NEC in preterm neonates. Methods: Design: Diagnostic testevaluation. Inclusion criteria: 102 preterm neonates <36 weeks gestation and within 7 days of birth admitted in Level III neonatalintensive care unit (NICU) were recruited from January 2010 to May 2011. Exclusion criteria: Congenital anomalies and overt infection.Paired stool samples at day 3 and 7 were analyzed by lateral quantum blue rapid calprotectin assay. Cut-off values of fCal weredetermined among 30 term healthy infants. A structured questionnaire which included gestational age, symptoms at admission, andmodified Bell’s staging was used to record NEC episodes on day 3 and 7 of admission. Septic screen and radiological tools were doneas per NICU protocol. Results: 48% were above 34 weeks gestation; 31.3% were of very low birth weight. As per modified Bell’sstaging on day 3 and 7, 22 and 11 neonates had 1a or above stage, respectively. 15 had features of NEC; of these, 12 were managedappropriately and discharged and 3 died. In the receiver-operated curve with fCal >279 μg/g as cutoff, the area under the curve was0.652 (95% confidence interval: 0.516-0.789). Day 3 fCal levels were high in 65.7% neonates. Using NEC as outcome, sensitivity of thetest was 93.3%; specificity was 39%; positive predictive value was 20.8% and negative predictive value was 97.14%. Conclusion: fCalhas high sensitivity for diagnosing NEC in preterm neonates. However, further research is needed to establish its clinical usefulness
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