Laser Cooling of Molecular Ions

全球定位系统和快速数据通信需要精确测量时间的能力。最精确的定时系统依赖于使用激光从原子或离子中提取能量（所谓的“激光冷却”技术），以消除由于原子或离子运动而产生的不确定性。直到最近，这些激光冷却技术才被证明对分子起作用。该项目的科学目标是演示激光冷却被捕获的分子离子，这可能在未来的精确测量时间和一些基本的自然物理常数（最终定量决定宇宙中一切事物如何行为的数字）方面有潜在的应用。激光冷却的初始目标离子是BH+。（BH是由一个硼原子和一个氢原子结合而成的分子。）激光冷却分子的挑战在于相对于原子离子的额外自由度。BH+具有类似于SrF（一种最近冷却的分子）的能级结构，我们提出了一个类似的方案。（SrF是由一个锶原子和一个氟原子结合而成的分子。）关键的区别在于，预期的BH+的振动衰减速度要快得多。这导致旋转状态分布更广，需要额外的注意来关闭过渡。此外，BH+的激发态可以预解离成基态的非束缚态。据预测，这种解离速率足够慢，激光冷却仍然是可能的。如果成功，这个项目将导致第一次激光冷却被捕获的分子离子。由于预解离导致的项目潜在失败将使我们对缓慢的氢隧穿过程有更多的了解，并作为分子量子动力学理论的测试。理想的结果是形成分子离子库仑晶体，用于冷化学研究和分子振动的精确测量。

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.