Molecules In External Fields Research Articles

Non-relativistic and relativistic energy of molecules in external fields with time-dependent moving boundaries

Non-relativistic and relativistic energy of molecules in external fields with time-dependent moving boundaries

An introduction to classical monodromy: Applications to molecules in external fields

An integrable Hamiltonian system presents monodromy if the action-angle variables cannot be defined globally. As a prototype of classical monodromy with azimuthal symmetry, we consider a linear molecule interacting with external fields and explore the topology structure of its phase space. Based on the behavior of closed orbits around singular points or regions of the energy–momentum plane, a semi-theoretical method is derived to detect classical monodromy. The validity of the monodromy test is numerically illustrated for several systems with azimuthal symmetry.

Adsorbed molecules in external fields: Effect of confining potential

We study the rotational excitation of a molecule adsorbed on a surface. As is well known the interaction potential between the surface and the molecule can be modeled in number of ways, depending on the molecular structure and the geometry under which the molecule is being adsorbed by the surface. We explore the effect of change of confining potential on the excitation, which is largely controlled by the static electric fields and continuous wave laser fields. We focus on dipolar molecules and hence we restrict ourselves to the first order interaction in field-molecule interaction potential either through permanent dipole moment or/and the molecular polarizability parameter. It is shown that confining potential shapes, strength of the confinement, strongly affect the excitation. We compare our results for different confining potentials.

Efficient numerical method for locating Feshbach resonances of ultracold molecules in external fields

Collision properties of atoms and molecules in low temperature gases can be controlled by applying an external magnetic or electric field. The external field shifts the energy levels of the colliding particles, which gives rise to Feshbach resonances modifying the scattering cross sections. The resonances occur at particular magnitudes of the external field, where a bound state of the collision complex is degenerate with a scattering state. The positions of the resonances in the external field are usually identified by computing either the scattering cross sections or the bound states of the collision complex as functions of the external field magnitude. We propose a more efficient method for locating Feshbach resonances that requires neither of these computations. In particular, we show that the positions of Feshbach resonances can be identified by computing the log-derivative of the total wave function in a classically allowed region as a function of the external field strength. This procedure is particularly useful for locating narrow Feshbach resonances that may be hard to identify with the other methods.

Solving them-mixing problem for the three-dimensional time-dependent Schrödinger equation by rotations: Application to strong-field ionization ofH2+

We present a very efficient technique for solving the three-dimensional time-dependent Schrodinger equation. Our method is applicable to a wide range of problems where a fullly three-dimensional solution is required, i.e., to cases where no symmetries exist that reduce the dimensionally of the problem. Examples include arbitrarily oriented molecules in external fields and atoms interacting with elliptically polarized light. We demonstrate that even in such cases, the three-dimensional problem can be decomposed exactly into two two-dimensional problems at the cost of introducing a trivial rotation transformation. We supplement the theoretical framework with numerical results on strong-field ionization of arbitrarily oriented H2+ molecules.

Molecular physics: theoretical principles and experimental methods

Preface. 1. Introduction. 2. Molecules Electronic States. 3. Rotation, Vibration, and Potential Curves of Diatomic Molecules. 4. Spectra of Diatomic Molecules. 5. Molecule Symmetries and Group Theory. 6. Rotation and Vibrations of Polyatomic Molecules. 7. Electronic States of Polyatomic Molecules. 8. Spectra of Polyatomic Molecules. 9. Breakdown of the Born-Oppenheimer-Approximation, Perturbations in Molecular Spectra. 10. Molecules in External Fields. 11. Van der Waals Molecules and Clusters. 12. Experimental Techniques in Molecular Physics. Appendix: Charcter Tables of Some Point Groups. Bibliography. Index.

Classical dynamics of polar diatomic molecules in external fields

In this paper, we present the study of the global classical dynamics of a rigid diatomic molecule in the presence of combined electrostatic and nonresonant polarized laser fields. In particular, we focus on the collinear field case, which is an integrable system because the z-component Pφ of the angular momentum is conserved. The study involves the complete analysis of the stability of the equilibrium points, their bifurcations and the evolution of the phase flow as a function of the field strengths and Pφ. Finally, the influence of the bifurcations on the orientation of the quantum states is studied.

Analyzing intramolecular vibrational energy redistribution via the overlap intensity-level velocity correlator

Numerous experimental and theoretical studies have established that intramolecular vibrational energy redistribution in isolated molecules has a hierarchical 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 correlation, 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. In this work we investigate the usefulness of the correlator in understanding the classical-quantum correspondence of effective spectroscopic Hamiltonians. 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 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.

Femtosecond dynamics and coherent control in atoms, molecules and clusters: Theory and experiments

The understanding and control of the dynamics in atoms, molecules and clusters has been a tremendously growing field in the past decade. This has been acknowledged with the 1999 Nobel prize in chemistry awarded to Ahmed Zewail. The present issue collects some of the newest theoretical and experimental results in the field of ultrafast dynamics and coherent control in the gas phase. The papers are grouped into three categories. The first section contains work on the “Coherent Control with Femtosecond Laser Pulses”. Topics like the general theory of quantum control, the control of electron transfer processes, transformation of chiral molecules and coherent population transfer are treated here. The “Femtosecond Dynamics” taking place in molecules and clusters is the topic of the second section. New insight into the nature of atomic motion within molecules and relaxation processes is provided. The last section collects most recent work on the interaction of ultrashort laser pulses with matter. In particular, high harmonic generation, multi-photon ionization and interference effects, as well as the possible orientation of molecules in external fields are discussed. We think that the present compilation demonstrates that the field of ultrashort pulse spectroscopy is of still growing importance and exciting new phenomena have been revealed in the past and will be discovered in the future.

Molecules in strong magnetic fields: Some perspectives and general aspects

In view of the coming availability of magnetic field strengths in the regime B≈100 T at the National High Magnetic Field Laboratory in Tallahassee, we present a brief overview of some relevant problems associated with molecular systems in strong magnetic fields. Both perspectives for future research as well as general aspects for molecules in external fields are outlined. Subsequently, two major fundamental concepts for molecules in strong magnetic fields are investigated in some detail. First, the pseudoseparation of the collective and internal motion is performed. Relevant dynamical effects occurring due to the coupling of the center of mass and electronic motion are investigated using the hydrogen atom. Second, we review the performance of the adiabatic separation and approximation for molecular systems in magnetic fields. The screening effect and its dynamical origin are discussed to some extent. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 501–511, 1997

Atoms in molecules in external fields

The theory of atoms in molecules is extended to the case where the molecule is in the presence of an electromagnetic field. This theory is based upon a generalization of quantum mechanics to an open system, as obtained through a corresponding extension of Schwinger’s principle of stationary action. The extension of this principle is possible only if the open system satisfies a particular boundary condition, one which is expressed as a constraint on the variation of the action integral. This is the condition that it be bounded by a surface of zero flux in the gradient vector field of the charge density, the definition of an atom in a molecule. It is shown that this boundary constraint again suffices to define an atom as a quantum subsystem when the molecule is in the presence of an electromagnetic field. The mechanics of an open system and its properties are determined by the fluxes in corresponding vector current densities through its surface. As in the fieldfree case, the obtainment of these currents from the variation of the action integral is a direct result of the variation of the atomic surface and of the imposition of the variational constraint on its boundary. The currents in this case consist of a paramagnetic and a diamagnetic contribution, currents whose presence are a necessary requirement for the description of the properties of a system in the presence of external fields. The variational statement of the Heisenberg equation of motion obtained from the principle of stationary action is used to derive the Ehrenfest force and virial theorems for an atom in a molecule in the presence of external electric and magnetic fields. In this case, there are forces acting on the interior of the atom which arise from the magnetic pressures acting on its surface. It is shown that the molecular electric polarizability and magnetic susceptibility, like other properties, are rigorously expressible as a sum of atomic contributions.

The theory of sternheimer shielding in molecules in external fields

A series of tensors is defined to describe the response to external electric and magnetic fields of the electric field gradient at a nucleus in a molecule. Perturbation expressions, symmetry relations and exact results for the hydrogen atom are given. The new tensors are related to derivatives of electric field shieldings with respect to motion of a test point through an electron distribution.

Virial theorem for molecules in external fields

Virial theorem for molecules in external fields