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Chinese Journal of Chemical Physics

Crossed-beam studies of aluminum atom cooling via inelastic collisions with O2 molecules

Dong Yan, Yujie Ma, Ang Xu, Fei Song, Ti Zhou, Zihan Yuan, Xiyu Liu, Fengyan Wang

Published Jun 3, 2025• 1 citation
Publisher

Abstract

The pioneering works have demonstrated that the method of single collisions in crossed molecular beams is an important technique for achieving kinetic cool-ing of atoms or molecules in specific rotational states. In this study, we investigated the elastic and inelastic collisions between Al(2P1/2) metal atoms and O2 molecules at high collision energies in the range of 6.4–14.8 kcal/mol, utilizing the laser-ablation crossed beams in conjunction with time-sliced ion velocity map imaging tech-nique. We observed kinetic cooling of Al(2P1/2) atoms with an upper-limit laboratory-frame root-mean-square velocity of 24±3 m/s, corresponding to a translational temperature of 0.9±0.2 K in the laboratory frame, facilitated by the vibrational excitation of O2 (v′ = 1) in inelastic collisions. The translational cooling of Al atoms in the lab frame enhanced detection probability in the transformation of density-to-flux, as evidenced by the scattering images obtained during the experiments.

DOI
10.1063/1674-0068/cjcp2504039

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The pioneering works have demonstrated that the method of single collisions in crossed molecular beams is an important technique for achieving kinetic cool-ing of atoms or molecules in specific rotational states. In this study, we investigated the elastic and inelastic collisions between Al(2P1/2) metal atoms and O2 molecules at high collision energies in the range of 6.4–14.8 kcal/mol, utilizing the laser-ablation crossed beams in conjunction with time-sliced ion velocity map imaging tech-nique. We observed kinetic cooling of Al(2P1/2) atoms with an upper-limit laboratory-frame root-mean-square velocity of 24±3 m/s, corresponding to a translational temperature of 0.9±0.2 K in the laboratory frame, facilitated by the vibrational excitation of O2 (v′ = 1) in inelastic collisions. The translational cooling of Al atoms in the lab frame enhanced detection probability in the transformation of density-to-flux, as evidenced by the scattering images obtained during the experiments.
We report the cooling of nitric oxide molecules in a single collision between an argon atom and an NO molecule at collision energies of 5.65 {+-} 0.36 kJ/mol and 14.7 {+-} 0.9 kJ/mol in a crossed molecular beam apparatus. We have produced in significant numbers ({approx}10{sup 8} molecules cm{sup -3} per quantum state) translationally cold NO({sup 2}{Pi}{sub 1/2}, {nu}' = 0, j' = 7.5) molecules in a specific quantum state with an upper-limit laboratory-frame rms velocity of 14.8 {+-} 1.1 m/s, corresponding to a temperature of 406 {+-} 28 mK. The translational cooling results from the kinematic collapse of the velocity distribution of the NO molecules after collision. Increasing the collision energy by increasing the velocity of the argon atoms, as we do here, does shift the scattering angle at which the cold molecules appear, but does not result in an experimentally measurable change in the velocity spread of the cold NO. This is entirely consistent with our analysis of the kinematics of the scattering which predicts that the velocity spread will actually decrease with increasing argon atom velocity.
We report the cooling of nitric oxide using a single collision between an argon atom and a molecule of NO. We have produced significant numbers (108 to 109 molecules per cubic centimeter per quantum state) of translationally cold NO molecules in a specific quantum state with an upper-limit root mean square laboratory velocity of 15 plus or minus 1 meters per second, corresponding to a 406 plus or minus 23 millikelvin upper limit of temperature, in a crossed molecular beam apparatus. The technique, which relies on a kinematic collapse of the velocity distributions of the molecular beams for the scattering events that produce cold molecules, is general and independent of the energy of the colliding partner.
We report the preparation and observation of translationally cold atoms using kinematic cooling. In these experiments, krypton atoms are cooled to subkelvin temperatures by elastic collisions in crossed atomic beams. Two independent velocity measurements indicate an upper-bound mean velocity of 13 m/s (Etrans/k=850mK{E}_{\mathrm{trans}}/k=850 \mathrm{mK}Etrans​/k=850mK) and are consistent with a much lower mean velocity of 4 m/s (Etrans/k=80mK{E}_{\mathrm{trans}}/k=80 \mathrm{mK}Etrans​/k=80mK) (kkk is Boltzmann's constant). The density of the cold atoms is measured to be 109{10}^{9}109 atoms/cm3{}^{3}3. Scattering calculations and diffusion models support these velocity and density measurements. The results demonstrate that cold, dense samples of ground-state atoms and molecules can be prepared by elastic collisions between identical collision partners.
[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] This thesis presents the successful implementation of a slice velocity map imaging apparatus capable of performing molecular beam scattering experiments at collision energies from well above the room temperature to below 1 K. The novel techniques introduced here offers a platform to study collisions at a regime where the scattering is mainly governed by quantum mechanics. Experimental developments of low energy collisions will challenge the theoretical description of molecular interactions, and aid in validating the accuracy of the calculated potential energy surfaces. Scattering studies involving two different configurations of the apparatus, intrabeam and near-copropagating beam configurations are reported in this thesis. The intrabeam scattering approach exploits beam slippage, the velocity difference of different species in the same beam, to establish the relative velocity. The beam slippage in the current setup was created by a pulsed electric discharge which induced a velocity upchirp in the excited species. Implementing a unique dual-slit chopper that can separately fix the velocities of the excited and non-excited species at the interaction region, we achieve precise control over the relative velocity and narrow its spread. With this configuration l-changing collisions occurring between Xe Rydberg atoms and Xe ground state atoms at subKelvin temperatures have been observed. The near-copropagating beam apparatus coupled with the stimulated emission pumping method and sliced velocity map imaging detection enabled us to measure the state-to-state differential cross sections for inelastic collisions between vibrationally excited NO molecules and Ar at broadly tunable energies. Experimentally measured rotationally inelastic state-to-state differential cross sections have enjoyed excellent agreement with quantum scattering calculations performed on state-of-the-art coupled-cluster potential energy surfaces. In contrast, state-to-state differential cross sections obtained for collision-induced multiplet changing transitions at collision energies near 1 K have showed better agreement with quantum scattering calculations involving multireference potential energy surfaces compared to coupled-cluster surfaces. This study provided a platform to validate the accuracy of the potential energy surfaces for collisions which are mainly governed by the attractive part of the interaction potential.
This thesis describes an experiment to produce the first laser cooled mixtures of atoms and polar molecules in magneto-optical and magnetic quadrupole traps. Inelastic collision processes between CaF molecules and Rb atoms are studied in both trapping arrangements. When molecules occupy the first rotationally excited state it is found that loss from either trap occurs at near the universal rate due to collisions with co-trapped atoms. When the molecular sample is transferred to the ground rotational state the loss rate is suppressed by at least an order of magnitude. The experimental results are compared to a quantum scattering model based on quantum defect theory and the results seem to be encouraging for observing a high ratio of elastic to inelastic collision rates in this system. In addition, improvements to the experimental apparatus are presented which led to the first measurements of the elastic cross-section for collisions between CaF and Rb. Although at the currently attainable number densities no thermalization is observed, an upper bound on the value of the cross section is placed.