利用激光诱导和黑体辐射辅助的对称顶部分子离子的转振动冷却：NH3+和ND3+
Laser-induced, blackbody-radiation-assisted rovibrational cooling of symmetric-top molecular ions: NH3+ and ND3+
AI总结：
该研究通过激光和黑体辐射辅助冷却对称顶部分子离子NH3+和ND3+，发现基态重新分布受BBR驱动，而振动态衰减快，同位素替换减缓动力学。
摘要：
分子离子的量子态制备是精确光谱和受控研究冷离子-分子动力学的前提。尽管这种控制已被广泛开发用于双原子离子，并且已被提出用于线性多原子离子，但对应于对称顶部分子离子的策略仍 largely 未被探索。我们对黑体辐射（BBR）辅助的转振动动力学和激光冷却进行了理论研究，在对称顶部离子NH3+和ND3+中，通过共振增强多光子电离（REMPI）中性前体制备特定的转振动态。计算了状态分辨的辐射寿命和平衡时间，发现振动激发态快速衰减，而基态的重新分布则主要由慢的BBR驱动的转振动转变主导，因为在非极性的NH3+和ND3+离子中纯旋转转变被禁止。通过BBR辅助的激光泵浦利用nu2伞形弯曲模式，能够有效地冷却固定K面内的旋转能级；然而，Delta K = 0选择规则导致瓶颈，限制了某些初始制备态对绝对转振动基态的访问。同位素替换到ND3+进一步减缓了动力学，因为过渡偶极矩减少。在室温下，这些冷却方案能够产生NH3+和ND3+离子所选转振动态中超过90%和85%的粒子数。相比之下，在温度低于100 K时，对于最初制备在转振动基态（v=0, J, K）的离子，BBR诱导的重新分布被强烈抑制，从而在延长的存储时间内冻结粒子数。相比之下，振动激发态的寿命在毫秒级，与BBR场的温度无关。
英文摘要：
Quantum-state preparation of molecular ions is a prerequisite for precision spectroscopy and controlled studies of cold ion-molecule dynamics. While such control has been extensively developed for diatomic ions and proposed for linear polyatomic ions, corresponding strategies for symmetric-top molecular ions remain largely unexplored. We present a theoretical investigation of blackbody-radiation (BBR)-assisted rovibrational dynamics and laser cooling in the symmetric-top ions NH3+ and ND3+, prepared in specific ro-vibrational states by resonance enhanced multiphoton ionization (REMPI) of the neutral precursor. State-resolved radiative lifetimes and equilibration times are computed, revealing that vibrationally excited states decay rapidly, while the ground-state redistribution is dominated by slow BBR-driven ro-vibrational transitions as pure rotational transitions are forbidden in the non-polar NH3+ and ND3+ ions. BBR-assisted laser pumping via the nu2 umbrella-bending mode efficiently cools rotational levels within fixed K manifolds; however, Delta K = 0 selection rules induce a bottleneck, limiting access to the absolute rovibrational ground state for some initially prepared states. Isotopic substitution to ND3+ further slows dynamics due to reduced transition dipoles. At room temperature, these cooling schemes yield more than 90% and 85% of the population in selected rovibrational states of the NH3+ and ND3+ ions, respectively. In contrast, at temperatures below 100 K, BBR-induced redistribution is strongly suppressed for ions initially produced in the rovibrational ground state (v=0, J, K), effectively freezing the population for extended storage times. In comparison, vibrationally excited states exhibit lifetimes on the order of a few milliseconds, independent of the temperature of the BBR field.
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