M. Lara

Magnetoelectrostatic Trapping of Ground State OH Molecules

We report magnetic confinement of neutral, ground state OH at a density of approximately 3 x 10(3) cm(-3) and temperature of approximately 30 mK. An adjustable electric field sufficiently large to polarize the OH is superimposed on the trap in various geometries, making an overall potential arising from both Zeeman and Stark effects. An effective molecular Hamiltonian is constructed, with Monte Carlo simulations accurately modeling the observed single-molecule dynamics in various trap configurations. Magnetic trapping of cold polar molecules under adjustable electric fields may enable study of low energy dipolar interactions.

Quantum study of the Li+HF→LiF+H reaction

In this work we present a new global fit for the potential energy surface of the LiFH system. This fit is an improvement of a recently published one [Aguado et al., J. Chem. Phys. 106, 1013 (1997)] for which more ab initio points have been calculated (from 644 to 2323). The reaction dynamics is studied using a time dependent treatment in reactant Jacobi coordinates in a body-fixed frame in which the internal coordinates are represented on a grid while Eulerian angles are described in a basis set. The centrifugal sudden approach is tested for total angular momentum J=5 and used to calculate the reaction cross section. The reaction cross section shows oscillations as a function of kinetic energy. This is a consequence of strong interference effects between reactant and product channels and is in agreement with the recent experimental data.

Kinetics and Dynamics of the S(1D2) + H2 → SH + H Reaction at Very Low Temperatures and Collision Energies

We report combined studies on the prototypical S(1D2) + H2 insertion reaction. Kinetics and crossed-beam experiments are performed in experimental conditions approaching the cold energy regime, yielding absolute rate coefficients down to 5.8 K and relative integral cross sections to collision energies as low as 0.68 meV. They are supported by quantum calculations on a potential energy surface treating long-range interactions accurately. All results are consistent and the excitation function behavior is explained in terms of the cumulative contribution of various partial waves.

Quantum stereodynamics of the Li HF v,j reactive collision for different initial states of the reagent

The effect of the reagent initial state excitation on the reactive cross section in the Li+HF(v,j) collision is analyzed for v=0, 1 and j=0, 1, 2 and 3. A wave packet treatment is used within the centrifugal sudden approximation on a global potential energy surface recently proposed [Aguado et al., J. Chem. Phys. 107, 10085 (1997)]. The reaction cross-section for v=0 is in good agreement with the available experimental data, and for low j shows oscillations as a function of the translational energy which are due to the structure of the transition state. For v=1 the reaction cross-section increases by a factor of 10–50 with respect to that of v=0. The influence of the alignment of the initial angular momentum on the reaction cross section is studied.

State-to-state reaction probabilities using bond coordinates: Application to the Li+HF(v,j) collision

In this work we present a time-dependent method to evaluate state-to-state reaction probabilities, based on bond coordinates and an adapted body-fixed frame. Such a method is expected to be rather efficient to describe A+BC→AB+C reactive collisions. In addition, the apparent complexity of the Hamiltonian expressed in these coordinates is reduced when applied to a wave packet represented in grids for the internal coordinates. The efficiency of this method as compared to the use of the most adapted Jacobi coordinates increases as the masses of the satellite atoms approach that of the heavier central atom and, what may be more important, avoids the problems associated with the singularities of the radial kinetic terms in the region of configuration space of interest. This method is used to study the Li+HF(v=0,1,j=0,J=0) reactive collision and the structure of the final state distribution of the LiF products is interpreted in terms of transition state resonances.

Potential energy surface and wave packet calculations on the Li+HF→LiF+H reaction

In this work an analytic fit of previous ab initio points [Aguado, Suarez, and Paniagua, Chem. Phys. 201, 107 (1995)] on the potential energy surface of the LiFH system is presented and the reaction dynamics is studied using a time-dependent treatment based on local coordinates. Three-dimensional wave packet calculations performed for zero total angular momentum indicate that the reactivity for the HF reactant in its ground vibrational state is quite low, in contradiction with previous dynamical calculations using different potential energy surfaces. The differences with previous potential energy surfaces are further analyzed using a bidimensional approach. Finally, the effect of the initial vibrational excitation of the HF reactant on the reactivity is studied using the bidimensional approach.

Ultracold Rb-OH Collisions and Prospects for Sympathetic Cooling

We compute ab initio cross sections for cold collisions of Rb atoms with OH radicals. We predict collision rate constants of order 10(-11) cm3/s at temperatures in the range 10-100 mK at which molecules have already been produced. However, we also find that in these collisions the molecules have a strong propensity for changing their internal state, which could make sympathetic cooling of OH in a Rb buffer gas problematic in magnetostatic or electrostatic traps.

Transition state spectroscopy via infrared excitation of Li⋯HF and Li⋯DF van der Waals precursors

The photoinitiated reactions after infrared excitation from the LiHF and LiDF complexes in the reactant valley are studied as an extension of a recent communication by Paniagua et al. [J. Chem. Phys. 109, 2971 (1998)]. For LiHF two broad bands, associated to Δv=1 and 2 transitions, are obtained at which the probability of forming LiF products is very high, >90%. For LiDF the Δv=1 band consists of several narrow resonances, and some of them are supported by the barrier separating reactant and product valleys. Even at these resonances the reaction probability is relatively high, starting at a value about 30% and increasing rapidly to >90% with increasing energy. This implies the tunneling through the barrier. The reason for the high efficiency in the photoinitiated reaction is that the main excitation corresponds to the HF (or DF) stretch within the complex, which is the “active” mode for the reaction in agreement with the presence of a late barrier. These results are very different from those obtained in L...

Cold collisions between OH and Rb: The field-free case

We have calculated elastic and state-resolved inelastic cross sections for cold and ultracold collisions in the Rb(

TRANSITION STATE SPECTROSCOPY ON THE LI-HF SYSTEM

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