Deb Nabanita, Heazlewood Brianna R, Rennick Christopher J, Softley Timothy P
Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Road, Oxford OX1 3TA, United Kingdom.
J Chem Phys. 2014 Apr 28;140(16):164314. doi: 10.1063/1.4870644.
The laser-induced blackbody-assisted rotational cooling of a linear polyatomic ion, C2H2(+), in its (2)Π ground electronic state in the presence of the blackbody radiation field at 300 K and 77 K is investigated theoretically using a rate-equations model. Although pure rotational transitions are forbidden in this non-polar species, the ν5 cis-bending mode is infrared active and the (1-0) band of this mode strongly overlaps the 300 K blackbody spectrum. Hence the lifetimes of state-selected rotational levels are found to be short compared to the typical timescale of ion trapping experiments. The ν5 (1-0) transition is split by the Renner-Teller coupling of vibrational and electronic angular momentum, and by the spin-orbit coupling, into six principal components and these effects are included in the calculations. In this paper, a rotational-cooling scheme is proposed that involves simultaneous pumping of a set of closely spaced Q-branch transitions on the (2)Δ5/2 - (2)Π3/2 band together with two Q-branch lines in the (2)Σ(+) - (2)Π1/2 band. It is shown that this should lead to >70% of total population in the lowest rotational level at 300 K and over 99% at 77 K. In principle, the multiple Q-branch lines could be pumped with just two broad-band (∼Δν = 0.4-3 cm(-1)) infrared lasers.
利用速率方程模型，对线性多原子离子C2H2(+)在300 K和77 K黑体辐射场存在下，处于其(2)Π基电子态时的激光诱导黑体辅助旋转冷却进行了理论研究。尽管在这种非极性物种中纯旋转跃迁是被禁止的，但ν5顺式弯曲模式是红外活性的，并且该模式的(1-0)带与300 K黑体光谱强烈重叠。因此，发现状态选择的旋转能级的寿命与离子俘获实验的典型时间尺度相比很短。ν5(1-0)跃迁通过振动和电子角动量的伦纳-泰勒耦合以及自旋-轨道耦合分裂为六个主要成分，并且这些效应包含在计算中。在本文中，提出了一种旋转冷却方案，该方案涉及同时泵浦(2)Δ5/2-(2)Π3/2带上一组紧密间隔的Q分支跃迁以及(2)Σ(+)-(2)Π1/2带上的两条Q分支线。结果表明，这将导致在300 K时最低旋转能级中的总粒子数超过70%，在77 K时超过99%。原则上，只需两台宽带（~Δν = 0.4-3 cm(-1)）红外激光器就可以泵浦多条Q分支线。
