Trapping molecules for quantum chemistry

Abstract

The Search and Discovery piece by Ashley Smart in the April 2017 issue of Physics Today (page 18) reported the excellent work on magnetic stopping and trapping of methyl radicals by the group of Takamasa Momose (University of British Columbia). I am writing to provide a different perspective on the field than what was conveyed by the author. The very first sentence of Smart’s article brings up a whole discussion: “By and large, physicists have succeeded in their quest to tame the atom.”The past few decades have seen remarkable advances in the ability to control and cool atoms in the gas phase. Those advances have been enabled by laser-cooling, which produces ultracold atomic gases. The atoms are then further cooled by evaporation to create quantum degenerate gases. However, those methods are far from ideal, even for atoms.Laser cooling is not a general method. Due to the requirements of a cycling transition and available lasers, it works on only a small set of elements in the periodic table. Evaporative cooling requires the right balance between elastic and inelastic collisions, again severely limiting generality. Furthermore, evaporative cooling leads to a large loss of atoms. For the alkalis, the optimum cases for laser cooling and evaporative cooling, the flux of ultracold atoms has been limited by optical density to around 109 atoms per second.It is natural to ask whether one can invent new methods that work on most elements and that break the current limit on flux. Such a development would open many new possibilities for fundamental science and for real-life applications. The challenges and limitations have motivated several groups to explore alternative approaches. One method is buffer-gas cooling, pioneered by John Doyle at Harvard University.11. J. M. Doyle et al., Phys. Rev. A 52, R2515 (1995). https://doi.org/10.1103/PhysRevA.52.R2515 A different approach was developed independently and in parallel by Frédéric Merkt and his group at ETH Zürich and by my group at the University of Texas at Austin: magnetic stopping and trapping of paramagnetic atoms in a supersonic beam.22. S. D. Hogan et al., Phys. Rev. Lett. 101, 143001 (2008). https://doi.org/10.1103/PhysRevLett.101.143001,33. E. Narevicius et al., Phys. Rev. Lett. 100, 093003 (2008). https://doi.org/10.1103/PhysRevLett.100.093003 That work was summarized in an invited review article I wrote for Science.44. M. G. Raizen, Science 324, 1403 (2009). https://doi.org/10.1126/science.1171506 Since then, many new developments have continued to advance the field.Now that the first sentence of the Search and Discovery piece is out of the way, I can discuss the rest—in particular, the motivation of studying quantum chemistry and the question of whether one actually needs to trap molecules. The goal of studying chemical reactions at ultralow temperatures is to observe unique quantum pathways that can dominate the reaction dynamics. Until recently it has been an elusive goal. Stopping and trapping molecules, either by electrostatic or magnetic forces, has produced a phase-space density that is too low for the study of chemical reactions. Molecules at much higher phase-space density can be produced by starting with Bose–Einstein condensates and “making” molecules with lasers. However, those experiments have so far been limited to alkali chemistry.Work by the Narevicius group at the Weizmann Institute of Science in Israel provided a major breakthrough in quantum chemistry without the need for stopping and trapping. Their approach relies on the merging of two supersonic beams by magnetically deflecting one of them.55. A. B. Henson et al., Science 338, 234 (2012). https://doi.org/10.1126/science.1229141 The copropagating beams have controllable collision energies down to temperatures of several millikelvin. The resulting chemical reactions, observed as a function of energy, reveal striking quantum resonances in the reaction dynamics. That work is continuing, and it demonstrates the power of merged beams, which do not require trapping and cooling. To study slower chemical reactions, trapping at high phase-space density is necessary. New ideas and work along those lines are in progress and will hopefully prove successful.ReferencesSection:ChooseTop of pageReferences <<CITING ARTICLES1. J. M. Doyle et al., Phys. Rev. A 52, R2515 (1995). https://doi.org/10.1103/PhysRevA.52.R2515, Google ScholarCrossref, ISI2. S. D. Hogan et al., Phys. Rev. Lett. 101, 143001 (2008). https://doi.org/10.1103/PhysRevLett.101.143001, Google ScholarCrossref3. E. Narevicius et al., Phys. Rev. Lett. 100, 093003 (2008). https://doi.org/10.1103/PhysRevLett.100.093003, Google ScholarCrossref, ISI4. M. G. Raizen, Science 324, 1403 (2009). https://doi.org/10.1126/science.1171506, Google ScholarCrossref5. A. B. Henson et al., Science 338, 234 (2012). https://doi.org/10.1126/science.1229141, Google ScholarCrossref, ISI© 2017 American Institute of Physics.