arXiv subjects

Srihari Keshavamurthy

Publications and source records attributed to Srihari Keshavamurthy.

27 records · Page 2Linked to original sources

arXiv2007

Intramolecular vibrational energy redistribution from a high frequency mode in the presence of an internal rotor: Classical thick-layer diffusion and quantum localization

We study the effect of an internal rotor on the classical and quantum intramolecular vibrational energy redistribution (IVR) dynamics of a model system with three degrees of freedom. The system is based on a Hamiltonian proposed by Martens and Reinhardt (J. Chem. Phys. {\bf 93}, 5621 (1990).) to study IVR in the excited electronic state of para-fluorotoluene. We explicitly construct the state space and show, confirming the mechanism proposed by Martens and Reinhardt, that an excited high frequency mode relaxes via diffusion along a thick layer of chaos created by the low frequency-rotor interactions. However, the corresponding quantum dynamics exhibits no appreciable relaxation of the high frequency mode. We attribute the quantum suppression of the classical thick-layer diffusion to the rotor selection rules and, possibly, dynamical localization effects.

physics.chem-ph↗

arXiv2007

On dynamical tunneling and classical resonances

This work establishes a firm relationship between classical nonlinear resonances and the phenomenon of dynamical tunneling. It is shown that the classical phase space with its hierarchy of resonance islands completely characterizes dynamical tunneling and explicit forms of the dynamical barriers can be obtained only by identifying the key resonances. Relationship between the phase space viewpoint and the quantum mechanical superexchange approach is discussed in near-integrable and mixed regular-chaotic situations. For near-integrable systems with sufficient anharmonicity the effect of multiple resonances {\it i.e.,} resonance-assisted tunneling can be incorporated approximately. It is also argued that the, presumed, relation of avoided crossings to nonlinear resonances does not have to be invoked in order to understand dynamical tunneling. For molecules with low density of states the resonance-assisted mechanism is expected to be dominant.

nlin.CD↗

arXiv2007

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.

physics.chem-ph↗

arXiv2005

Resonance-assisted tunneling in three degrees of freedom without discrete symmetry

We study dynamical tunneling in a near-integrable Hamiltonian with three degrees of freedom. The model Hamiltonian does not have any discrete symmetry. Despite this lack of symmetry we show that the mixing of near-degenerate quantum states is due to dynamical tunneling mediated by the nonlinear resonances in the classical phase space. Identifying the key resonances allows us to suppress the dynamical tunneling via the addition of weak counter-resonant terms.

nlin.CD↗

arXiv2004

On dynamical tunneling and classical resonances

This letter establishes a firm relationship between classical nonlinear resonances and the phenomenon of dynamical tunneling. It is shown that the classical phase space with its hierarchy of resonance islands completely characterizes dynamical tunneling. In particular, it is not important to invoke criteria such as the size of the islands and presence or absence of avoided crossings for a consistent description of dynamical tunneling in near-integrable systems.

physics.chem-ph↗

arXiv2004

Parametric perspective on highly excited states: case studies of CHBrClF and C2H2

Considerable insights can be obtained regarding the nature of highly excited states by computing the eigenbasis expectation values of the resonance operators associated with an effective spectroscopic Hamiltonian. The expectation values are related to the parametric derivative of the eigenvalues with respect to specific resonance strengths i.e., level velocities.Sensitivity of the level velocities to the existence of closed orbits in the underlying classical phase space provides for a dimensionality independent route to a dynamical assignment of the states. In this letter, taking CHBrClF (polyad P=5) and the [16,0]^{g+} bend polyad of C_{2}H_{2} as examples, we show that the level velocities can signal the birth of new modes and highlight sequences of localized eigenstates.

physics.chem-ph↗

arXiv2003

Exploring classical phase space structures of nearly integrable and mixed quantum systems via parametric variation

The correlation between overlap intensities and level velocities has been introduced as a sensitive measure capable of revealing phase space localization. Previously applied to chaotic quantum systems, here we extend the theory to near-integrable and mixed quantum systems. This measure is useful in the latter cases because it has the ability to highlight certain phase space structures depending upon the perturbation used to parametrically vary the Hamiltonian. A detailed semiclassical theory is presented relating the correlation coefficient to the phase space weighted derivatives of the classical action. In the process, we confront the question of whether the Hannay-Ozorio de Almeida sum rules are simply extendable to mixed phase space systems. In addition, the hbar-scalings of the correlation coefficient and relevant quantities are derived for nearly integrable systems. Excellent agreement is found between the theory and the results for integrable billiards as well as for the standard map.

nlin.CD↗

arXiv2002

Dynamical tunneling in molecules: role of the classical resonances and chaos

In this letter we study dynamical tunneling in highly excited symmetric molecules. The role of classical phase space structures like resonances and chaos on the tunneling splittings are illustrated using the water molecule as an example. It is argued that the enhancements in the splittings due to resonances (near-integrable phase space) and due to chaos (mixed phase space) are best understood away from the fluctuations associated with avoided crossings. In particular we provide an essential difference between the two mechanisms in terms of high order perturbation theory. The analysis, apart from testing the validity of a perturbative approach, suggests such systems as prime candidates for studying dynamical tunneling.

nlin.CD↗

arXiv2002

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.

nlin.CD↗