制备量子态可控的超冷极性分子离子的研究

The emerging field of ultracold (defined as temperature T ~ 1 mK) molecular matter is probably one of the most vibrant fields in contemporary physics and chemistry. The applicant has developed a novel method to produce ultracold and quantum-state selected APOLAR molecular ions(N2+). The near-uniform selectivity in the preparation of the ground rovibrational levels has been achieved in ion traps using sympathetic cooling and resonance-enhanced threshold photoionization. The measured state lifetimes reach the order of 15 minutes. ("Editors' Suggestion" and "Viewpoint paper" in Phys. Rev. Lett. 105, 143001 (2010)). Building on the extensive expertise of the applicant in the fields of molecular photoionization and the generation of cold molecular ions, the present proposal is centered on the development of an experimental method to produce quantum-sate selected POLAR molecular ions (e.g.HD+). The study will focus on the effect of electric charge-dipole interaction on the quantum states of molecular ions during the sympathetic cooling process. In addition, a cryostat will be employed to extend the state lifetimes of initially prepared quantum states in molecular ions. Finally, POLAR molecular ions with almost stationary (trapping time > hours, T~mK) position in the laboratory space and very long state lifetimes (> minutes) can be obtained. The ultracold molecular ions will be introduced into a wide range of novel and adventurous applications, for example, 1)using sympathetically cooled molecular ions to perform ultra-high precision spectroscopy; 2)using sympathetically cooled molecular ions to study quantum phenomena in ultra-low collisional energy dynamics.

制备并研究超冷（T~1mK）分子离子已经成为目前在超冷物理与超冷化学领域最前沿、最具有挑战性的科学研究方向之一。申请人曾在国际上首次利用协同冷却和阈值光电离相结合的方法，在离子阱中成功制备了单一量子态的非极性超冷氮(N2+)分子离子，其量子态纯度接近100%，量子态寿命达到15分钟以上（Phys. Rev. Lett. 105,143001 (2010)）。本项目将应用此技术于具有永久偶极矩（极性）的分子离子（如HD+）上,研究在协同冷却过程中电荷与电偶极矩之间相互作用对振转量子态的影响,并利用低温恒温器延长振转量子态寿命,最终实现对极性分子离子内部和外部自由度的全面精确控制。所制备的超冷极性分子离子将具有在空间上"静止"（囚禁时间~小时，动能~mK）和较长量子态寿命（~分钟）的特点，这将为测量超高精度分子离子光谱和研究超冷环境下碰撞反应中的量子效应奠定实验基础。

为了实现分子离子振转光谱的精密测量，就必须对其内部与外部自由度进行全面的精确控制。通过本项目的支持，课题组实现了将利用激光冷却的钙离子对具有偶极矩的氢分子HD+进行协同冷却，使其平动能降低至毫开尔文量级，实现HD+在实验室坐标空间上的基本静止状态，消除多普勒效应对其精密振转光谱测量的影响。而HD+的内部振转自由度的控制则是利用多光子的共振加强阈值光电离的方法，将分子束中的中性HD分子电离，并实现制备HD+于的单一振转量子态上，为获得最佳的光谱测量信噪比奠定基础。由以上手段得到的几乎不受外界环境干扰、可稳定囚禁（囚禁寿命〉小时）并具有长量子态寿命（〉分钟）的HD+体系，就可保证其振转光谱频率测量精密进入10E-10量级。在本项目支持的基础上，课题组获得了较好的成绩，建立了超冷分子离子实验平台，并且确立了通过氢分子离子高精度光谱直接测量质子-电子质量比的进一步研究课题。该衍生课题获得了基金委重大研究计划以及中科院战略先导（B类）的进一步支持，目前已经在国际上首次获得了氢分子离子v=0->6的泛频跃迁频率，与理论计算符合。通过进一步提高测量精度，有望在ppb量级精度上实现质子与电子质量比的直接测量。