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arXiv:1111.4706 · The Journal of Chemical Physics

Cold collisions of complex polyatomic molecules

Zhiying Li, Eric J. Heller

Published Feb 6, 2012• 21 citations
arXivHTMLPDF

Abstract

We introduce a method for classical trajectory calculations to simulate collisions between atoms and large rigid asymmetric-top molecules. We investigate the formation of molecule-helium complexes in buffer-gas cooling experiments at a temperature of 6.5 K for molecules as large as naphthalene. Our calculations show that the mean lifetime of the naphthalene-helium quasi-bound collision complex is not long enough for the formation of stable clusters under the experimental conditions. Our results suggest that it may be possible to improve the efficiency of the production of cold molecules in buffer-gas cooling experiments by increasing the density of helium. In addition, we find that the shape of molecules is important for the collision dynamics when the vibrational motion of molecules is frozen. For some molecules, it is even more crucial than the number of accessible degrees of freedom. This indicates that by selecting molecules with suitable shape for buffer-gas cooling, it may be possible to cool molecules with a very large number of degrees of freedom.

DOI
10.1063/1.3682982

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We introduce a method for classical trajectory calculations to simulate collisions between atoms and large rigid asymmetric-top molecules. We investigate the formation of molecule-helium complexes in buffer-gas cooling experiments at a temperature of 6.5 K for molecules as large as naphthalene. Our calculations show that the mean lifetime of the naphthalene-helium quasi-bound collision complex is not long enough for the formation of stable clusters under the experimental conditions. Our results suggest that it may be possible to improve the efficiency of the production of cold molecules in buffer-gas cooling experiments by increasing the density of helium. In addition, we find that the shape of molecules is important for the collision dynamics when the vibrational motion of molecules is frozen. For some molecules, it is even more crucial than the number of accessible degrees of freedom. This indicates that by selecting molecules with suitable shape for buffer-gas cooling, it may be possible to cool molecules with a very large number of degrees of freedom.
Collision cross sections are calculated for the three systems O2-He{\text{O}}_{2}\text{-He}O2​-He, Na2-Na{\text{Na}}_{2}\text{-Na}Na2​-Na and K2-K{\text{K}}_{2}\text{-K}K2​-K at cold and ultracold temperatures. We discuss their relevance for cold molecule formation by buffer-gas cooling and by other cooling methods where collisions between molecules and atoms are important, e.g photoassociation. The molecule is treated as a rigid rotator in the electronic and vibrational ground state. We have calculated ab initio the potential energy surfaces for the three systems with special emphasis on the long range potential which is very important at low energies. With relevance to buffer gas cooling, we have also studied the thermalization process between O2{\text{O}}_{2}O2​ molecules and He atoms as determined by the calculated cross sections.
Cold and dense samples of naphthalene (C(10)H(8)) are produced using buffer gas cooling in combination with rapid, high flow molecule injection. The observed naphthalene density is n approximately 10(11) cm(-3) over a volume of a few cm(3) at a temperature of 6 K. We observe naphthalene-naphthalene collisions through two-body loss of naphthalene with a loss cross section of sigma(N-N) = 1.4 x 10(-14) cm(2). Analysis is presented that indicates that this combination of techniques will be applicable to many comparably sized molecules. This technique can also be combined with cryogenic beam methods(1) to produce cold, high flux, continuous molecular beams.
This thesis describes four types of cold molecular collision systems with increasing complexity: from simple atom-diatomic molecule to complicated polyatomic molecule-polyatomic molecule. The early thesis work is concerned with three open questions pertaining to collision dynamics of cold molecules. We demonstrate the possibility of controlling collisional decoherence of ultracold molecules by tuning an external magnetic field. We then provide insight into the feasibility of evaporative cooling of molecules, and for the first time incorporate the uncertainty analysis of the potential energy surface (PES) into scattering calculations. In addition, we use classical trajectory methods to study the effects of the interaction strength and the geometry of rigid polyatomic molecules on the formation of long-lived collision complexes at low collision energies. The second half of the thesis work is focused on combining statistical methodology and scattering calculations to address two major problems in molecular dynamics calculations: increasing computational complexity and uncertainties due to inaccuracies of PES. Using a small number of scattering calculations, we show that we can build a Gaussian Process (GP) model to statistically approximate collision outcomes for complex molecules, and then perform the uncertainty analysis and the sensitivity analysis. We also demonstrate that trained by a combination of classical and quantum calculations, a GP model can provide an accurate description of the quantum scattering cross sections, even near scattering resonances.
We show that weakly bound He-containing van der Waals molecules can be produced and magnetically trapped in buffer-gas cooling experiments, and provide a general model for the formation and dynamics of these molecules. Our analysis shows that, at typical experimental parameters, thermodynamics favors the formation of van der Waals complexes composed of a helium atom bound to most open-shell atoms and molecules, and that complex formation occurs quickly enough to ensure chemical equilibrium. For molecular pairs composed of a He atom and an S-state atom, the molecular spin is stable during formation, dissociation, and collisions, and thus these molecules can be magnetically trapped. Collisional spin relaxation is too slow to affect trap lifetimes. However, (3)He-containing complexes can change spin due to adiabatic crossings between trapped and untrapped Zeeman states, mediated by the anisotropic hyperfine interaction, causing trap loss. We provide a detailed model for Ag(3)He molecules, using ab initio calculation of Ag-He interaction potentials and spin interactions, quantum scattering theory, and direct Monte Carlo simulations to describe formation and spin relaxation in this system. The calculated rate of spin-change agrees quantitatively with experimental observations, providing indirect evidence for molecular formation in buffer-gas-cooled magnetic traps. Finally, we discuss the possibilities for spectroscopic detection of these complexes, including a calculation of expected spectra for Ag(3)He, and report on our spectroscopic search for Ag(3)He, which produced a null result.
Collision cross sections between oxygen molecules and helium atoms are computed at translational energies between 0.1 K and 10.0 K, motivated by the recently demonstrated cooling of molecules by a helium buffer gas. Detailed calculations based on a rigid-rotor model demonstrate the differences among various isotopic combinations of oxygen atoms and also serve to illustrate resonant features unique to cold molecules. Comparisons of elastic versus spin-changing inelastic collision rates suggest that buffer-gas cooling is likely to be a widely applicable tool for producing cold molecular samples.
Building on recent advances in ultrafast lasers and methods to slow molecules, an experiment is proposed to produce translationally cold CO2{}_{2}2​ super-rotors ($j\ensuremath{\sim}200$) by combining an optical centrifuge with helium-buffer-gas cooling. Quantum mechanical calculations of the complex scattering length for He-CO2{}_{2}2​ collisions demonstrate that the efficiency of rotational quenching decreases rapidly with increasing rotational excitation jjj in the ultracold regime. Extrapolating to helium cryogenic temperatures, rotational quenching is predicted to remain inefficient up to $\ensuremath{\sim}$1 K, allowing for the possible creation of a beam of translationally cold, rotationally hot molecules.