arXiv subjects

Manuel Lara

Publications and source records attributed to Manuel Lara.

11 recordsLinked to original sources

arXiv2023

Universal behavior in complex-mediated reactions: Dynamics of S(1D)+ o-D2 --> D + SD at low collision energies

Reactive and elastic cross-sections, and rate coefficients, have been calculated for the S(1D)+ D2 (v=0, j=0) reaction using a modified hyperspherical quantum reactive scattering method. The considered collision energy ranges from the ultracold regime, where only one partial wave is open, up to the Langevin regime, where many of them contribute. This work presents the extension of the quantum calculations, which were compared with the experimental results in a previous work, down to energies in the cold and ultracold domains. Results are analyzed and compared with the universal case of the quantum defect theory by Jachymski et al. [Phys. Rev. Lett. 110, 213202 (2013)]. State-to-state integral and differential cross sections are also shown covering the ranges of low-thermal, cold and ultracold collision energy regimes. It is found that at E/k_B T < 1 K there are substantial departures from the expected statistical behavior, and that dynamical features become increasingly important with decreasing collision energy, leading to vibrational excitation.

physics.chem-ph↗

arXiv2020

Unveiling shape resonances in H + HF collisions at cold energies

Resonances are associated with the trapping of an intermolecular complex, and are characterized by a series of quantum numbers such as the total angular momentum and the parity, representative of a specific partial wave. Here we show how at cold temperatures the rotational quenching of HF(j=1,2) with H is strongly influenced by the presence of manifolds of resonances arising from the combination of a single value of the orbital angular momentum with different total angular momentum values. These resonances give rise up to a two-fold increase in the thermal rate coefficient at the low temperatures characteristic of the interstellar medium. Our results show that by selecting the relative geometry of the reactants by alignment of the HF rotational angular momentum, it is possible to decompose the resonance peak, disentangling the contribution of different total angular momenta to the resonance.

physics.atom-ph↗

arXiv2014

Beyond universality: parametrizing ultracold complex-mediated reactions using statistical assumptions

We have calculated accurate quantum reactive and elastic cross-sections for the prototypical barrierless reaction D$^{+}$ + H$_2$($v$=0, $j$=0) using the hyperspherical scattering method. The considered kinetic energy ranges from the ultracold to the Langevin regimes. The availability of accurate results for this system allows to test the quantum theory by Jachymski et al. [Phys. Rev. Lett. 110, 213202 (2013)] in a nonuniversal case. The short range reaction probability is rationalized using statistical model assumptions and related to a statistical factor. This provides a means to estimate one of the parameters that characterizes ultracold processes from first principles. Possible limitations of the statistical model are considered.

physics.chem-ph↗

arXiv2014

Universality in the cold and ultracold dynamics of the barrierless D$^{+}$+ H$_2$ reaction

We have calculated quantum reactive and elastic cross-sections for D$^{+}+$ para-H$_2$($v$=0, $j$=0) $\rightarrow$ H$^+$ + HD collisons using the hyperspherical quantum reactive scattering method [Chem. Phys. Lett. 1990,169, 473]. The H$_{3}^{+}$ system is the prototype of barrierless ion-molecule reactions, apart from its relevance in astrochemistry. The considered collision energy ranges from the ultracold regime, where only one partial wave is open, up to the Langevin regime, where many of them contribute. At very low kinetic energies, both an accurate description of the long-range (LR) region in the potential energy surface (PES), and long dynamical propagations, up to distances of 10$^{5}$ a0, are required. Accordingly, calculations have been carried out on the PES by Velilla et al. [J. Chem. Phys. 2008, 129,084307] which accurately reproduces the LR interactions. Hyperspherical methodology was recently modified in order to allow the accurate inclusion of LR interactions while minimizing the computational expense. Such implementation is shown particularly suitable for systems involving ions, where the $R^{-4}$ behavior largely extends the range of the potential. We find a reaction rate coefficient which remains almost constant in a kinetic energy range of more than ten orders of magnitude. In particular, the value reached in the Wigner regime, where only one partial wave is open, is paradoxically the Langevin classical value (within a few percent) expected at high energies. Results are discussed in terms of {\em universality} and related to the recently published quantum defect theory [Phys. Rev. Lett. 2013, 110, 213202]. Since the system has small exothermicity and a low number of channels, it is possible to test such model in a case where the loss probability at short range is appreciably far from unity.

physics.chem-ph↗

arXiv2011

On the role of dynamical barriers in barrierless-reactions at low energies: S($^1$D) + H$_2$

Reaction probabilities as a function of total angular momentum and the resulting reaction cross-sections for the collision of open shell S($^1$D) atoms with para-hydrogen have been calculated in the kinetic energy range 0.09--10 meV (1--120 K). The quantum mechanical (QM) hyperspherical reactive scattering method and quasi--classical trajectory (QCT) and statistical quasiclassical trajectory (SQCT) approaches were used. Two different ab initio potential energy surfaces (PESs) have been considered. The widely used RKHS PES by Ho et al. (J. Chem. Phys. 116, 4124, 2002) and the recently published DMBE/CBS PES by Song and Varandas (J. Chem. Phys. 130, 134317, 2009). The calculations at low collision energies reveal very different dynamical behaviors on the two PESs. The reactivity on the RKHS PES is found to be considerably larger than that on the DMBE/CBS PES. The observed differences have their origin in two major distinct topography features. Although both PESs are essentially barrierless for equilibrium H--H distances, when the H--H bond is compressed the DMBE/CBS PES gives rise to a dynamical barrier which limits the reactivity of the system. This barrier is completely absent in the RHKS PES. In addition, the latter PES exhibits a van der Walls well in the entrance channel which reduces the height of the centrifugal barrier and is able to support resonances. As a result a significant larger cross section is found on this PES, with marked oscillations attributable to shape resonances and/or to the opening of partial wave contributions.

physics.chem-ph↗

arXiv2010

Kinetics and Dynamics of the S(^1D_2) + H_2 \to SH + H Reaction at Very Low Temperatures and Collision Energies

We report combined studies on the prototypical S(^1D_2) + 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.

physics.chem-ph↗

arXiv2010

Reaching the cold regime: S(^1D) + H_2 and the Role of Long-Range Interactions in Open Shell Reactive Collisions

Reactive cross-sections for the collision of open shell S(^1D) atoms with ortho- and para-hydrogen, in the kinetic energy range 1-120 K, have been calculated using the hyperspherical quantum reactive scattering method developed by Launay et al. [Chem. Phys. Lett. 169, 473 (1990)]. Short-range interactions, described using the {\em ab initio} potential energy surface by Ho et al. were complemented with an accurate description of the long-range interactions, where the main electrostatic (~R^{-5}) and dispersion (~ R^{-6}) contributions were considered. This allows the comparison with recent experimental measurements of rate constants and excitation functions for the title reaction at low temperatures [Berteloite et al., Phys. Rev. Lett., 2010]. The agreement is fairly good. The behavior in the considered energy range can be understood on the average in terms of a classical Langevin (capture) model, where the centrifugal barriers determine the amount of reactive flux which reaches the barrierless transition state. Additionally, the structure of the van der Waals well provides temporal trapping at short distances thus allowing the system to find its way to the reaction at some classically-forbidden energies. Interestingly, the cross-section for para-hydrogen shows clearly oscillating features associated to the opening of new partial waves and to shape resonances which may be amenable to experimental detection.

physics.chem-ph↗

arXiv2008

Loss of molecules in magneto-electrostatic traps due to nonadiabatic transitions

We analyze the dynamics of a paramagnetic, dipolar molecule in a generic "magneto-electrostatic'' trap where both magnetic and electric fields may be present. The potential energy that governs the dynamics of the molecules is found using a reduced molecular model that incorporates the main features of the system. We discuss the shape of the trapping potentials for different field geometries, as well as the possibility of nonadiabatic transitions to untrapped states, i.e., the analog of Majorana transitions in a quadrupole magnetic atomic trap. Maximizing the lifetime of molecules in a trap is of great concern in current experiments, and we assess the effect of nonadiabatic transitions on obtainable trap lifetimes.

physics.atom-ph↗

arXiv2007

Magneto-electrostatic trapping of ground state OH molecules

We report the magnetic confinement of neutral, ground state hydroxyl radicals (OH) at a density of $\sim3\times10^{3}$ cm$^{-3}$ and temperature of $\sim$30 mK. An adjustable electric field of sufficient magnitude to polarize the OH is superimposed on the trap in either a quadrupole or homogenous field geometry. The OH is confined by an overall potential established via molecular state mixing induced by the combined electric and magnetic fields acting on the molecule's electric dipole and magnetic dipole moments, respectively. An effective molecular Hamiltonian including Stark and Zeeman terms has been constructed to describe single molecule dynamics inside the trap. Monte Carlo simulation using this Hamiltonian accurately models the observed trap dynamics in various trap configurations. Confinement of cold polar molecules in a magnetic trap, leaving large, adjustable electric fields for control, is an important step towards the study of low energy dipole-dipole collisions.

physics.atom-ph↗

arXiv2006

Cold collisions of OH and Rb. I: the free collision

We have calculated elastic and state-resolved inelastic cross sections for cold and ultracold collisions in the Rb($^1 S$) + OH($^2 Π_{3/2}$) system, including fine-structure and hyperfine effects. We have developed a new set of five potential energy surfaces for Rb-OH($^2 Π$) from high-level {\em ab initio} electronic structure calculations, which exhibit conical intersections between covalent and ion-pair states. The surfaces are transformed to a quasidiabatic representation. The collision problem is expanded in a set of channels suitable for handling the system in the presence of electric and/or magnetic fields, although we consider the zero-field limit in this work. Because of the large number of scattering channels involved, we propose and make use of suitable approximations. To account for the hyperfine structure of both collision partners in the short-range region we develop a frame-transformation procedure which includes most of the hyperfine Hamiltonian. Scattering cross sections on the order of $10^{-13}$ cm$^2$ are predicted for temperatures typical of Stark decelerators. We also conclude that spin orientation of the partners is completely disrupted during the collision. Implications for both sympathetic cooling of OH molecules in an environment of ultracold Rb atoms and experimental observability of the collisions are discussed.

physics.atom-ph↗

arXiv2005

Isotopic replacement in ionic systems: the 4He2+ + 3He -> 3He4He+ + 4He reaction

Full quantum dynamics calculations have been carried out for the ionic reaction 4He2+ + 3He and state-to-state reactive probabilities have been obtained using both a time-dependent (TD) and a time-independent (TI) approach. An accurate ab-initio potential energy surface has been employed for the present quantum dynamics and the two sets of results are shown to be in agreement with each other. The results for zero total angular momentum suggest a marked presence of atom exchange (isotopic replacement) reaction with probabilities as high as 60%. The reaction probabilities are only weakly dependent on the initial vibrational state of the reactants while they are slightly more sensitive to the degree of rotational excitation. A brief discussion of the results for selected higher total angular momentum values is also presented, while the l-shifting approximation [1] has been used to provide estimates of the total reaction rates for the title process. Such rates are found to be large enough to possibly become experimentally accessible.

physics.chem-ph↗