Laser Cooling of Molecular Ions

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Global positioning systems and rapid data communication require the ability to precisely measure time. The most precise timing systems rely on the use of lasers to extract energy from atom or ions (so-called "laser-cooling" techniques) in order to remove uncertainty due to the atom or ion motion. Only relatively recently have these laser-cooling techniques been demonstrated to work on molecules. The scientific goal of this project is to demonstrate the laser cooling of a trapped molecular ion, which could have potential future applications in the precise measurement of time and some of the fundamental physical constants of nature (the numbers which ultimately determine quantitatively how everything in the universe behaves).The initial target ion for laser-cooling is BH+. (BH is a molecule consisting of one boron atom bonded to one hydrogen atom.) The challenge of laser cooling molecules is the extra degrees of freedom relative to atomic ions. BH+ has a level structure similar to SrF, a recently cooled molecule, and we propose to implement a similar scheme. (SrF is a molceule consisting of one stronium atom bonded to one one fluorine atom.) The key difference is that the expected vibrational decay rate of BH+ is significantly faster. This leads to a broader distribution of rotational states and additional care is required to close the transition. In addition, the excited electronic state of BH+ can predissociate to an unbound state of the ground electronic state. This dissociation rate is predicted to be slow enough that laser cooling will still be possible. If successful, this project will result in the first laser cooling of a trapped molecular ion. The potential failure of the project due to predissociation will give us additional insight into slow hydrogen tunneling processes and serve as a test for theories of the quantum dynamics of molecules. An ideal outcome is the formation of a molecular ion Coulomb crystal for the study of cold chemistry and the precision measurement of molecular vibrations.