C $_n^-$（$n=3-5$）的超低温辐射冷却
Ultraslow radiative cooling of C$_n^-$ ($n=3-5$)
摘要：
利用斯德哥尔摩大学的双静电离子环实验装置，测量了三种与天体化学有关的阴离子C-$_n^-$（$n=3-5$）的超低辐射冷却寿命和绝热脱离能. BURREE保持背景压力为$\approx$ 10 $^{-14}$ \，mbar，温度为$\approx$ 13\，K，允许质量选择离子存储数小时，并提供被称为“盒中分子云”的条件。在这里，我们构建了二维（2D）的目标阴离子的光剥离光谱，通过记录光剥离信号作为照射波长和离子存储时间（秒到分钟的时间尺度）的函数。通过跟踪振动热带信号随离子存储时间的消失，从每个离子的2D光剥离光谱中提取与红外辐射发射相关的离子冷却寿命，得到$\frac{1}{e}$的冷却寿命为3.1 $\pm$ 0.1 μ s（C $_3^-$）、6.8 $\pm$ 0.5 μ s（C $_4^-$）和24 $\pm$ 5 μ s（C $_5^-$）。冷离子的光剥离谱的拟合，即那些存储至少30 μ s的光剥离谱的拟合，提供了与来自激光光电子能谱的值良好一致的绝热剥离能。离子冷却寿命的模拟使用一个简单的谐波级联模型，发现与实验很好的协议，并提供一个模式的辐射冷却性能的模式。二维光剥离策略和辐射冷却模型的发展，在这项研究中可以应用于研究的超低温冷却动力学的分子阴离子。
英文摘要：
Ultraslow radiative cooling lifetimes and adiabatic detachment energies for three astrochemically relevant anions, C$_n^-$ ($n=3-5$), are measured using the Double ElectroStatic Ion Ring ExpEriment (DESIREE) infrastructure at Stockholm University. DESIREE maintains a background pressure of $\approx$10$^{-14}$\,mbar and temperature of $\approx$13\,K, allowing storage of mass-selected ions for hours and providing conditions coined a "molecular cloud in a box". Here, we construct two-dimensional (2D) photodetachment spectra for the target anions by recording photodetachment signal as a function of irradiation wavelength and ion storage time (seconds to minute timescale). Ion cooling lifetimes, which are associated with infrared radiative emission, are extracted from the 2D photodetachment spectrum for each ion by tracking the disappearance of vibrational hot-band signal with ion storage time, giving $\frac{1}{e}$ cooling lifetimes of 3.1$\pm$0.1\,s (C$_3^-$), 6.8$\pm$0.5\,s (C$_4^-$) and 24$\pm$5\,s (C$_5^-$). Fits of the photodetachment spectra for cold ions, i.e. those stored for at least 30\,s, provides adiabatic detachment energies in good agreement with values from laser photoelectron spectroscopy. Ion cooling lifetimes are simulated using a Simple Harmonic Cascade model, finding good agreement with experiment and providing a mode-by-mode understanding of the radiative cooling properties. The 2D photodetachment strategy and radiative cooling modeling developed in this study could be applied to investigate the ultraslow cooling dynamics of wide range of molecular anions.
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