Molecules containing a lanthanide atom have sets of electronic states arising from excitation of an inner-shell electron. These states have received little attention but are thought to play an important role in laser cooling of such molecules and may be a useful resource for testing fundamental physics. We study a series of inner-shell excited states in YbF using resonance-enhanced multiphoton ionization spectroscopy. We investigate the excited states of lowest energy, 8474, 9013, and $9090{\mathrm{cm}}^{-1}$ above the ground state, all corresponding to the configuration $4f^{13}6s^2{{}^{2}F}_{7/2}$ of the ${\mathrm{Yb}}^{+}$ ion. They are metastable, since they have no electric dipole allowed transitions to the ground state. We also characterize a state at $31050{\mathrm{cm}}^{-1}$ that is easily excited from both the ground and metastable states, which makes it especially useful for this spectroscopic study. Finally, we study two states at 48 720 and $48729{\mathrm{cm}}^{-1}$, which are above the ionization limit and feature strong autoionizing resonances that prove useful for efficient detection of the molecules and for identifying the rotational quantum number of each line in the spectrum. We resolve the rotational structures of all these states and find that they can all be described by a very simple model based on Hund’s case (c). Our study provides information necessary for laser slowing and magneto-optical trapping of YbF, which is an important species for testing fundamental physics. We also consider whether the low-lying inner-shell states may themselves be useful as probes of the electron’s electric dipole moment or of varying fundamental constants, since they are long-lived states in a laser-coolable molecule featuring closely spaced levels of opposite parity.

中文翻译：

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通过 YbF 4f 空穴态的光谱了解分子内壳激发


含有镧系元素原子的分子具有由内壳电子激发产生的电子态组。这些状态很少受到关注，但被认为在此类分子的激光冷却中发挥着重要作用，并且可能是测试基础物理的有用资源。我们使用共振增强多光子电离光谱研究了 YbF 中的一系列内壳层激发态。我们研究了基态之上的最低能量 8474、9013 和 $9090{\mathrm{cm}}^{-1}$ 的激发态，所有这些都对应于 ${\mathrm{Yb}}^{+}$ 离子的构型 $4f^{13}6s^2{{}^{2}F}_{7/2}$ 。它们是亚稳态的，因为它们不具有允许转变到基态的电偶极子。我们还描述了 $31050{\mathrm{cm}}^{-1}$ 的状态，该状态很容易从基态和亚稳态激发，这使得它对于光谱研究特别有用。最后，我们研究了 48 720 和 $48729{\mathrm{cm}}^{-1}$ 处的两个状态，它们高于电离极限并具有强自电离共振，这证明对于有效检测分子和识别分子中每条线的旋转量子数非常有用。光谱。我们解析了所有这些状态的旋转结构，发现它们都可以通过基于洪德案例（c）的非常简单的模型来描述。我们的研究为 YbF 的激光减速和磁光捕获提供了必要的信息，YbF 是测试基础物理的重要物质。我们还考虑低位内壳态本身是否可用作电子电偶极矩或不同基本常数的探针，因为它们是可激光冷却分子中的长寿命态，具有紧密间隔的相反宇称能级。