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New Journal of Physics

Cold TiO(X3Δ)–He collisions

Mei-Ju Lu, Jonathan D. Weinstein

Published May 14, 2009• 22 citations
Publisher

Abstract

We use laser ablation and cryogenic helium buffer-gas cooling to produce large numbers of X3Δ1 TiO molecules at a translational temperature of 5 K. We investigate their cold collisions with helium and measure elastic and inelastic scattering cross-sections. As expected for 3Δ molecules, which have large spin–rotation couplings, TiO's inelastic m-changing collision cross-section is large: on the same order as its momentum transfer cross-section.

DOI
10.1088/1367-2630/11/5/055015

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We use laser ablation and cryogenic helium buffer-gas cooling to produce large numbers of X3Δ1 TiO molecules at a translational temperature of 5 K. We investigate their cold collisions with helium and measure elastic and inelastic scattering cross-sections. As expected for 3Δ molecules, which have large spin–rotation couplings, TiO's inelastic m-changing collision cross-section is large: on the same order as its momentum transfer cross-section.
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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}$1 K, allowing for the possible creation of a beam of translationally cold, rotationally hot molecules.
The close coupling calculation of rotationally inelastic collision of He with Na2 is first performed by employing a recently computed ab initio potential energy surface. The process of rovibrational energy excitation and relaxation of Na2 in collision with He has been studied in detail based on the single and double exitations coupled cluster with a noniterative perturbation treatment of triple excitations CCSD （T） potential energy surface. Our results are in good agreement with the experimental results. Our results show that: （1）The elastic cross section（Δj=0） is much larger than the inelastic cross section. （2）The largest amplitude occurs at small scattering angles. At low Δj transition, the He projectile is predominantly forward scattering. Backward scattering occurs as Δj increases. （3）For inelastic cross section, the most effective transition is that with Δj=2.（4）The fast narrow diffraction oscillations in the cross sections are the obvious characteristics for the low Δj inelastic transitions, which decreases with increasing j. （5）The elastic differential-cross-sections（DCS）provides a useful visualization of rotational rainbows at low translational energy. It is beneficial to reducing the loss in molecular cooling and trapping.
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We produce YbF molecules with a density of 1018{10}^{18}1018 m${}^{\ensuremath{-}3}$ using laser ablation inside a cryogenically cooled cell filled with a helium buffer gas. Using absorption imaging and absorption spectroscopy we study the formation, diffusion, thermalization, and optical pumping of the molecules. The absorption images show an initial rapid expansion of molecules away from the ablation target followed by a much slower diffusion to the cell walls. We study how the time constant for diffusion depends on the helium density and temperature and obtain values for the YbF-He diffusion cross section at two different temperatures. We measure the translational and rotational temperatures of the molecules as a function of time since formation, obtain the characteristic time constant for the molecules to thermalize with the cell walls, and elucidate the process responsible for limiting this thermalization rate. Finally, we make a detailed study of how the absorption of the probe laser saturates as its intensity increases, showing that the saturation intensity is proportional to the helium density. We use this to estimate collision rates and the density of molecules in the cell.