Session P8: Atomic and Molecular Collisions

Resonances in ultracold reactive collisions of vibrationally excited ortho and para H$_2$ with D

Recent experimental work on collisions of molecular hydrogen with metastable helium showed pronounced shape resonances in the sub-kelvin regime, which have a dramatic effect on the energy dependence of cross sections. In our computational work we will address the question of whether such near threshold resonances exist for the benchmark system H$_2+{}$D. Detailed computations will be performed for an energy range extending from ultracold to about $E=20$~kelvin (to include the temperature regime of astrophysical interest, e.g., relevant to cold molecular clouds). We will explore several initial vibrational states of H$_2$ in order to see how the internal vibrational excitation will influence the presence of shape resonances. We will also investigate the effect of the nuclear spin symmetry of H$_2$ by computing scattering rates for both ortho and para hydrogen, which could give significantly different results at low energies.   

Quantum Defect Theory for Long-range Anisotropic Interactions

Quantum Defect Theory (QDT) is a numerically efficient and accurate tool for studying a wide variety of ultracold atomic collisions, where the asymptotic behavior of the atoms is well described by a set of simple parameters. However, analytic formulas for these parameters only exist for the pure $-1/R^6$ potential. The long-range parameters are given by simple power law equations in the collision energy, and the bound state energies of different partial waves are simply related. We extend these formulas to encompass all potentials of the form $-1/R^n$, where $n>2$. Moreover, the accuracy of QDT is limited by long-range anisotropic interactions, which, for example, play an important role in collisions of dysprosium or erbium atoms. We present our recent developments on numerically treating this type of interaction within perturbation theory.   

Buffer gas cooling of large polyatomic molecules

Cryogenic helium buffer gases have cooled a diversity of molecules to temperatures near 1 K. Motivated by collisional studies as well as mixture and chiral analysis, we are now cooling larger (often biological) molecules. A detailed understanding of helium-molecule sticking is currently unknown, as is any possible molecular size limit of buffer gas cooling. We report on experiments with polyatomic molecules that explore He-molecule collisions at energies well below their binding energy, in which the pair would form a dimer (or a larger cluster) under equilibrium conditions. Molecules from an oven source (300-500 K) flow into a cold (5 K) cell, where they undergo a few hundred collisions with He at a density of $\sim10^{14}$ cm$^{-3}$. We observe translational and internal cooling of both trans-stilbene and the significantly larger molecule Nile Red. Narrow spectral lines in both molecules indicate the gas-phase monomer. In Nile Red, we also see broad spectral features suggestive of molecule-molecule dimer formation. We see no evidence of high He-molecule dimer formation rates and conclude that for these molecules (and smaller molecules\footnote{Patterson, \textit{Molecular Physics} (110), 1757, 2012}) the lifetime of possible helium-molecule dimers is below $\sim$1 microsecond.   

Full-dimensional vibrational quenching of CO in collisions with H$_2$

We report a six-dimensional (6D) potential energy surface (PES) for the CO-H$_2$ collision system calculated using the explicitly correlated coupled-cluster (CCSD(T)-F12B) method as implemented in MOLPRO2010.1 and fitted using an invariant polynomial method. In the fit of the 6D PES, the total polynomial expansion power was restricted to 6 and expansion coefficients were obtained using standard weighted least-squares optimization for potential energies up to 10,000 cm$^{-1}$. Close-coupling scattering calculations have been performed on the new 6D PES for rotational and vibrational quenching of CO in collisions with H$_2$ using the TwoBC code. Cross sections for $j=0 \rightarrow 1$ pure rotational transition in CO, convoluted with a Gaussian kinetic energy distribution, show better agreement with measurement than that obtained on a recently available 4D PES. State-to-state and total quenching cross sections and rate coefficients for the vibrational quenching in CO($v_1=1, j_1=0$)+H$_2$($v_2=0, j_2=0$) $\rightarrow$ CO($v_1^{\prime}=0, j_1^{\prime}$)+H$_2$($v_2^{\prime}=0, j_2^{\prime}=0$) collisions, for $j_1^{\prime}=0, 1, \cdots, 25$ are presented and compared with experimental results and previous calculations using 4D and 5D PESs and various decoupling approximati   

A simulation study of Methane by proton at low energies

Proton impact molecular collisions have received considerable attentions over last few decades due to wide applications in various fields such as plasma physics, astrophysics, material science, and radiation therapy. Methane is the simplest hydrocarbon and has recently been detected in the atmosphere of the outer planets. In addition to provide the fundamental information, the charge exchange studies [1] remain critical for understanding the phenomena in studies of comets, the solar wind, and space weather. The charge exchange processes in recent years have been used as diagnostics for temperature and transport. Using the time dependent density functional theory [2] our results for both the elastic and inelastic scattering will be presented. \\[4pt] [1] B. H. Bransden and M R C MacDowell, \textit{Charge Exchange and the Theory of Ion-Atom Collisions }(Oxford Univ. Press, NY 1992).\\[0pt] [2] E. Runge and E. K. U. Gross, Phys. Rev. Lett 52, 997 (1984).   

Threshold resonance effects for $1/R^3$ long-range potentials

We investigate systematically the threshold behavior of scattering cross sections for attractive potentials which have an asymptotic dependence of the type $V(R)\approx-C_3/R^3$. We present numerical results and a theoretical analysis in terms of Jost functions, and we pay close attention to the effects due to near threshold resonances (NTR). Although we restrict our analysis to the single channel case, we show that it is also relevant for the case of weakly coupled many channel problems; in particular, we show that the NTR effects for inelastic scattering can be deduced from the single channel case. Our study could help to identify universal physics in the two-body and three-body systems.   

Exchange of wavefunction in particle collisions

We consider the collision of two equal-mass particles in one dimension, interacting through a hard core repulsive potential, and represented by possibly different wavepackets. Classically it is known that the particles exchange their velocities. We show that quantum mechanically they exchange their wavefunctions as well, making it look even more like they just ``passed through each other.'' We argue that this phenomenon---which is independent of the statistics obeyed by the particles---is a coherent quantum effect, and provide a simple explanation for it.   

Ps-Ps scattering via the correlated Gaussian hyperspherical method

There is renewed interest in systems of electrons and positrons since it may be possible to create a Bose-Einstein condensate of spin-triplet positronium atoms [P. M. Platzman and A. P. Mills, Jr., Phys. Rev. B {\bf 49}, 454 (1994)]. We study the four-body system consisting of two positrons and two electrons. Using a basis of correlated Gaussians at fixed hyperradius, we utilize a new technique [K. M. Daily and C. H. Greene, Phys. Rev. A {\bf 89}, 012503 (2014)] to efficiently calculate the adiabatic potentials and non-adiabatic couplings as a function of the hyperradius. The R-matrix is propagated to large hyperradius and scattering properties are derived.   

Giant molecules composed of polar molecules and atoms in mixed dimensions

Two or three polar molecules, confined to one or two dimensions, can form stable bound states with a single atom living in three dimensions, if the molecule and the atom can interact resonantly such that their mixed dimensional scattering length is large. We call these bound states ``giant molecules'' since it's a molecule composed of smaller molecules and atoms. We study their properties using techniques including exact numerical solution, exact qunatum diffusion Monte Carlo (QMC), Born-Oppenheimer approximation (BOA), and semiclassical approximation. These bound states have a hierarchical structure reminiscent of the celestial systems.   

Semi-Classical and Quantum-Field Descriptions for the Non-Linear Electromagnetic Response of Many-Electron Atoms

Semi-classical and quantum-field descriptions for the non-linear electromagnetic response relevant to resonant pump-probe optical phenomena in quantized many-electron systems are formulated within a general reduced-density-matrix framework. Time-domain (equation-of-motion) and frequency-domain (resolvent-operator) formulations are developed in a unified and self-consistent manner. A preliminary semi-classical perturbation treatment of the electromagnetic interaction is adopted, in which the electromagnetic field is described as a classical field satisfying the Maxwell equations. It is emphasized that a quantized-field approach is essential for a fully self-consistent quantum-mechanical formulation. Compact Liouville-space operator expressions are obtained for the general (n'th order) non-linear electromagnetic-response tensors describing moving many-electron atomic systems. The tetradic matrix elements of the Liouville-space self-energy operators are evaluated for environmental collisional and radiative interactions.