Publications (131)373.98 Total impact
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ABSTRACT: We propose a Gaussian Process (GP) model as an efficient nonparametric method for constructing multidimensional potential energy surfaces (PES) for polyatomic molecules. Using an example of the molecule N$_4$, we show that a realistic GP model of the sixdimensional PES can be constructed with only 240 potential energy points. We construct a series of the GP models and illustrate the accuracy of the resulting surfaces as a function of the number of {\it ab initio} points. We show that the GP model based on 1800 potential energy points achieves the same level of accuracy as the conventional regression fits based on 16,421 points. The GP model of the PES requires no fitting of {\it ab initio} data with analytical functions and can be readily extended to surfaces of higher dimensions.  [Show abstract] [Hide abstract]
ABSTRACT: We consider a problem of extrapolating the collision properties of a large polyatomic molecule A–H to make predictions of the dynamical properties for another molecule related to A–H by the substitution of the H atom with a small molecular group X, without explicitly computing the potential energy surface for A–X. We assume that the effect of the −H →−X substitution is embodied in a multidimensional function with unknown parameters characterizing the change of the potential energy surface. We propose to apply the Gaussian Process model to determine the dependence of the dynamical observables on the unknown parameters. This can be used to produce an interval of the observable values which corresponds to physical variations of the potential parameters. We show that the Gaussian Process model combined with classical trajectory calculations can be used to obtain the dependence of the cross sections for collisions of C6H5CN with He on the unknown parameters describing the interaction of the He atom with the CN fragment of the molecule. The unknown parameters are then varied within physically reasonable ranges to produce a prediction uncertainty of the cross sections. The results are normalized to the cross sections for He — C6H6 collisions obtained from quantum scattering calculations in order to provide a prediction interval of the thermally averaged cross sections for collisions of C6H5CN with He.  [Show abstract] [Hide abstract]
ABSTRACT: We show that a Gaussian Process model can be combined with a small number of scattering calculations to provide an accurate multidimensional dependence of scattering observables on the experimentally controllable parameters (such as the collision energy, temperature or external fields) as well as the potential energy surface parameters. This can be used for solving the inverse scattering problem, the prediction of collision properties of a specific molecular system based on the information for another molecule, the efficient calculation of thermally averaged observables and for reducing the error of the molecular dynamics calculations by averaging over the potential energy surface variations. We show that, trained by a combination of classical and quantum dynamics calculations, the model provides an accurate description of the scattering cross sections, even near scattering resonances. In this case, the classical calculations stabilize the model against uncertainties arising from wildly varying correlations of resonantly enhanced quantum results.  [Show abstract] [Hide abstract]
ABSTRACT: We consider the dynamics of rotational excitations placed on a single molecule in spatially disordered 1D, 2D and 3D ensembles of ultracold molecules trapped in optical lattices. The disorder arises from incomplete populations of optical lattices with molecules. We show that for realistic experimental parameters this specific type of disorder leads to disorderinduced localization in 1D and 2D systems on a time scale $t \sim 1$ sec. For 3D lattices with $55$ sites in each dimension and vacancy concentrations $\leq 90 \%$, the rotational excitations generated in the middle of the lattice diffuse to the edges of the lattice. We observe that the diffusion of rotational excitations in highly disordered 2D and 3D lattices has three distinct time scales. At short times, the rotational excitations diffuse as quantum particles expanding ballistically. At later times, the diffusion character changes to be the same as for the classical particles in Brownian motion. At still later times, the rotational excitations transition to a subdiffusive regime. While the ballistic expansion is brief ($\sim 10$ ms), the classicallike diffusion can last as long as 200300 ms. We also examine the role of the longrange tunnelling amplitudes allowing for transfer of rotational excitations between distant lattice sites. Our results show that the longrange tunnelling has little impact on the dynamics in the diffusive regime but affects significantly the localization dynamics in lattices with large concentrations of vacancies, enhancing the width of the localized distributions in 2D lattices by more than a factor of 2.  [Show abstract] [Hide abstract]
ABSTRACT: We use classical trajectory calculations to study the effects of the interaction strength and the geometry of rigid polyatomic molecules on the formation of longlived collision complexes at low collision energies. We first compare the results of the calculations for collisions of benzene molecules with rare gas atoms He, Ne, Ar, Kr, and Xe. The comparison illustrates that the mean lifetimes of the collision complexes increase monotonically with the strength of the atommolecule interaction. We then compare the results of the atombenzene calculations with those for benzenebenzene collisions. The comparison illustrates that the mean lifetimes of the benzenebenzene collision complexes are significantly reduced due to nonergodic effects prohibiting the molecules from sampling the entire configuration space. We find that the thermally averaged lifetimes of the benzenebenzene collisions are much shorter than those for Xe with benzene and similar to those for Ne with benzene.  [Show abstract] [Hide abstract]
ABSTRACT: We develop a theoretical method for solving the quantum mechanical reactive scattering problem in the presence of external fields based on a hyperspherical coordinate description of the reaction complex combined with the total angular momentum representation for collisions in external fields. The method allows us to obtain converged results for the chemical reaction LiF + H > Li + HF in an electric field. Our calculations demonstrate that, by inducing couplings between states of different total angular momenta, electric fields with magnitudes <150 kV/cm give rise to resonant scattering and a significant modification of the total reaction probabilities, product state distributions and the branching ratios for reactive vs inelastic scattering.  [Show abstract] [Hide abstract]
ABSTRACT: We explore the collision dynamics of complex hydrocarbon molecules (benzene, coronene, adamantane, and anthracene) containing carbon rings in a cold buffer gas of (3)He. For benzene, we present a comparative analysis of the fully classical and fully quantum calculations of elastic and inelastic scattering cross sections at collision energies between 1 and 10 cm(1). The quantum calculations are performed using the timeindependent coupled channel approach and the coupledstates approximation. We show that the coupledstates approximation is accurate at collision energies between 1 and 20 cm(1). For the classical dynamics calculations, we develop an approach exploiting the rigidity of the carbon rings and including lowenergy vibrational modes without holonomic constraints. Our results illustrate the effect of the molecular shape and the vibrational degrees of freedom on the formation of longlived resonance states that lead to lowtemperature clustering.  [Show abstract] [Hide abstract]
ABSTRACT: Magnetic Feshbach resonances play a central role in experimental research on atomic gases at ultracold temperatures, as they allow one to control the microscopic interactions between ultracold atoms by tuning an applied magnetic field. These resonances arise due to strong hyperfine interactions between the unpaired electron and the nuclear magnetic moment of the alkali metal atoms. A major thrust of current research is to create an ultracold gas of diatomic alkali metal molecules in the ground rovibrational state of the ground electronic 1Σ state. Unlike alkali metal atoms, 1Σ diatomic molecules have no unpaired electrons. However, the hyperfine interactions of molecules may give rise to magnetic Feshbach resonances. We use quantum scattering calculations to study the possible width of these resonances. Our results show that the widths of magnetic Feshbach resonances in ultracold moleculemolecule collisions for 1Σ molecules may exceed 1 mG, rendering such resonances experimentally detectable. We hope that this work will stimulate the experimental search of these resonances.  [Show abstract] [Hide abstract]
ABSTRACT: We show that the interaction of polar alkalimetal dimers in the quintet spin state leads to the formation of a deeply bound reaction complex. The reaction complex can decompose adiabatically into homonuclear alkalimetal dimers (for all molecules except KRb) and into alkalimetal trimers (for all molecules). We show that there are no barriers for these chemical reactions. This means that all alkalimetal dimers in the a^{3}Σ^{+} state are chemically unstable at ultracold temperature, and the use of an optical lattice to segregate the molecules and suppress losses may be necessary. In addition, we calculate the minimumenergy path for the chemical reactions of alkalimetal hydrides. We find that the reaction of two molecules is accelerated by a strong attraction between the alkalimetal atoms, leading to a barrierless process that produces hydrogen atoms with large kinetic energy. We discuss the unique features of the chemical reactions of ultracold alkalimetal dimers in the a^{3}Σ^{+} electronic state.  [Show abstract] [Hide abstract]
ABSTRACT: We calculate the cross sections for elastic scattering and Zeeman relaxation in binary collisions of molecules in the rovibrational ground state of a $^2\Sigma$ electronic state and the Zeeman state with the electron spin projection $M_S=1/2$ on the magnetic field axis. This is the lowestenergy state of $^2\Sigma$ molecules confined in a magnetic trap. The results are averaged over calculations with multiple molecule  molecule interaction potentials, which yields the expectation intervals for the cross sections and the elastictoinelastic cross section ratios. We find that the elastictoinelastic cross section ratios under conditions corresponding to trapped molecular ensembles at $T \sim 10^{3}$ K exceed 100 for the majority of $^2\Sigma$ molecules. The range of $^2\Sigma$ molecules expected to be collisionally unstable in magnetic traps at $T < 10^{3}$ K is limited to molecules with the spinrotation interaction constant $\gamma_{\rm SR} > 0.5$ cm$^{1}$ and the rotational constant $B_e < 4$ cm$^{1}$.  [Show abstract] [Hide abstract]
ABSTRACT: The goal of the present article is to review the major developments that have led to the current understanding of moleculefield interactions and experimental methods for manipulating molecules with electromagnetic fields. Moleculefield interactions are at the core of several, seemingly distinct, areas of molecular physics. This is reflected in the organization of this article, which includes sections on Field control of molecular beams, External field traps for cold molecules, Control of molecular orientation and molecular alignment, Manipulation of molecules by nonconservative forces, Ultracold molecules and ultracold chemistry, Controlled manybody phenomena, Entanglement of molecules and dipole arrays, and Stability of molecular systems in highfrequency superintense laser fields. The article contains 853 references.  [Show abstract] [Hide abstract]
ABSTRACT: We determine the phase diagram of a polaron model with mixed breathingmode and SuSchriefferHeeger couplings and show that it has two sharp transitions, in contrast to pure models which exhibit one (for SuSchriefferHeeger coupling) or no (for breathingmode coupling) transition. We then show that ultracold molecules trapped in optical lattices can be used as a quantum simulator to study precisely this mixed Hamiltonian, and that the relative contributions of the two couplings can be tuned with external electric fields. The parameters of current experiments place them in the region where one of the transitions occurs. We also propose a scheme to measure the polaron dispersion using stimulated Raman spectroscopy.  [Show abstract] [Hide abstract]
ABSTRACT: An elementary excitation in an aggregate of coupled particles generates a collective excited state. We show that the dynamics of these excitations can be controlled by applying a transient external potential which modifies the phase of the quantum states of the individual particles. The method is based on an interplay of adiabatic and sudden time scales in the quantum evolution of the manybody states. We show that specific phase transformations can be used to accelerate or decelerate quantum energy transfer and spatially focus delocalized excitations onto different parts of arrays of quantum particles. We consider possible experimental implementations of the proposed technique and study the effect of disorder due to the presence of impurities on its fidelity. We further show that the proposed technique can allow control of energy transfer in completely disordered systems. 
Article: Collisional decoherence of superposition states in an ultracold gas near a Feshbach resonance
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ABSTRACT: We present expressions demonstrating that collisional decoherence of ultracold atoms or molecules in a coherent superposition of nondegenerate quantum states is suppressed when both the real and imaginary parts of the scattering lengths for the states in the coherent superposition are equal. We show that the rate of collisional decoherence can be enhanced or suppressed by varying an external magnetic field near a Feshbach resonance. For some resonances, the suppression is very dramatic. We propose a method for measuring the scattering length of ultracold particles in excited quantum states exhibiting Feshbach resonances.  [Show abstract] [Hide abstract]
ABSTRACT: Ultracold polar molecules trapped on an optical lattice is a manybody system that, under appropriate conditions, may support collective excitations reminiscent of excitons in solid state crystals. Here, we discuss the rotational excitations of molecules on an optical lattice leading to rotational Frenkel excitons. Apart from solid hydrogen, there is no other natural system that exhibits rotational excitons. The rotational excitons have unique properties that can be exploited for tuning nonlinear exciton interactions and excitonimpurity scattering by applying an external electric field. We show that this can be used to explore the competing role of the dynamical and kinematic excitonexciton interactions in excitonic energy transfer and to study quantum localization in a dynamically tunable disordered potential. The rotational excitons can also be used as a basis for quantum simulation of condensed matter models that cannot be realized with ultracold atoms. As an example, we discuss the possibility of engineering the Holstein model with polar molecules on an optical lattice. 
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ABSTRACT: We study the growth dynamics of ordered structures of strongly interacting polar molecules in optical lattices. Using a dipole blockade of microwave excitations, we map the system onto an interacting spin1/2 model possessing ground states with crystalline order, and describe a way to prepare these states by nonadiabatically driving the transitions between molecular rotational levels. The proposed technique bypasses the need to cross a phase transition and allows for the creation of ordered domains of considerably larger size compared to approaches relying on adiabatic preparation.  [Show abstract] [Hide abstract]
ABSTRACT: We show that the cross sections for moleculemolecule collisions in the presence of an external field can be computed efficiently using a total angular momentum basis, defined either in the bodyfixed frame or in the spacefixed coordinate system. This method allows for computations with much larger basis sets than previously possible. We present calculations for (15)NH(15)NH collisions in a magnetic field. Our results support the conclusion of the previous study that the evaporative cooling of rotationally ground (15)NH molecules in a magnetic trap has a prospect of success.  [Show abstract] [Hide abstract]
ABSTRACT: Magnetic Feshbach scattering resonances play a central role in experimental research of atomic gases at ultracold temperatures. A major thrust of current research is to create an ultracold gas of diatomic alkalimetal molecules in the ground rovibrational state of the ground electronic ^1σ state. Can ultracold ^1σ molecules be controlled by means of magnetic Feshbach resonances? Unlike alkali metal atoms, ^1σ diatomic molecules have no unpaired electrons. The response of ^1σ molecules to an external magnetic field is determined entirely by the spin structure of the atomic nuclei. We present the first calculations of moleculemolecule collisions for ^1σ molecules in a magnetic field. In particular, we calculate the rates of hyperfine relaxation in molecule  molecule collisions and explore the possibility of tuning magnetic Feshbach resonances in an ultracold gas of ^87Rb^133Cs(X^1&+circ;) molecules.  [Show abstract] [Hide abstract]
ABSTRACT: Rotational excitation of ultracold polar molecules trapped on an optical lattice produces rotational Frenkel excitons (collective rotational excitations) [1]. We show that nonlinear interactions between these excitons can be tuned by applying a dc electric field. We show that, at electric fields greater than a critical value, rotational Frenkel excitons form bound pairs  biexcitons [2]. Frenkel biexcitons are strongly correlated states of two collective excitations in a molecular crystal, which are exceedingly hard to create and observe in solidstate crystals. We demonstrate that the binding energy of the rotational biexcitons can be controlled by tuning the angle between the applied field and the molecular array. Frenkel biexcitons can be used for many applications ranging from the controlled preparation of entanglement between quasiparticles to the study of bipolarons. [4pt] [1] ``Frenkel biexcitons in optical lattices with polar molecules,'' Ping Xiang, M. Litinskaya, R. V. Krems; condmat/1112.3942. [0pt] [2] ``Tunable disorder in a crystal of cold polar molecules,'' F. Herrera, M. Litinskaya, R. V. Krems, Phys. Rev. A 82, 033428 (2010).
Publication Stats
3k  Citations  
373.98  Total Impact Points  
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Institutions

20062015

University of British Columbia  Vancouver
 Department of Chemistry
Vancouver, British Columbia, Canada


2013

University of California, Santa Barbara
 Kavli Institute for Theoretical Physics
Santa Barbara, California, United States


20022012

HarvardSmithsonian Center for Astrophysics
 Institute for Theoretical Atomic, Molecular and Optical Physics
Cambridge, Massachusetts, United States


2010

Radboud University Nijmegen
 Department of Theoretical Chemistry
Nijmegen, Provincie Gelderland, Netherlands


2008

Università degli Studi di Perugia
 Department of Chemistry
Perugia, Umbria, Italy


20032005

Harvard University
 Department of Physics
Cambridge, Massachusetts, United States


2000

Moscow State Forest University
Mytishi, Moskovskaya, Russia
