School of Chemistry, Norwich Research Park, University of East Anglia, Norwich NR4 7TJ, United Kingdom.
School of Chemistry, University of Melbourne, Parkville, VIC 3010, Australia.
J Chem Phys. 2019 Sep 21;151(11):114304. doi: 10.1063/1.5114678.
Ultraslow radiative cooling lifetimes and adiabatic detachment energies for three astrochemically relevant anions, C (n = 3-5), are measured using the Double ElectroStatic Ion Ring ExpEriment (DESIREE) infrastructure at Stockholm University. DESIREE maintains a background pressure of ≈10 mbar and temperature of ≈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 time scale). 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 1e cooling lifetimes of 3.1 ± 0.1 s (C ), 6.8 ± 0.5 s (C ), and 24 ± 5 s (C ). Fits of the photodetachment spectra for cold ions, i.e., those stored for at least 30 s, provide adiabatic detachment energies in good agreement with values from laser photoelectron spectroscopy on jet-cooled anions, confirming that radiative cooling has occurred in DESIREE. 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 a wide range of molecular anions.
使用斯德哥尔摩大学的双静电离子环实验（DESIREE）基础设施，测量了三个天文化学相关阴离子（C （n = 3-5））的超慢辐射冷却寿命和绝热离解能。DESIREE 维持 ≈10 毫巴的背景压力和 ≈13 K 的温度，允许对质量选择的离子进行数小时的存储，并提供了一种“盒子里的分子云”的条件。在这里，我们通过记录光解离信号作为辐照波长和离子存储时间（秒到分钟时间尺度）的函数，为目标阴离子构建二维（2D）光解离谱。与红外辐射发射相关的离子冷却寿命是通过跟踪离子存储时间与振动热带信号的消失从每个离子的 2D 光解离谱中提取出来的，给出了 1e 冷却寿命为 3.1 ± 0.1 s（C ）、6.8 ± 0.5 s（C ）和 24 ± 5 s（C ）。对冷离子（即至少存储 30 s 的离子）的光解离谱进行拟合，提供了与喷射冷却阴离子激光光电子能谱很好一致的绝热离解能，证实了辐射冷却已经在 DESIREE 中发生。使用简单的谐和级联模型模拟离子冷却寿命，发现与实验吻合良好，并提供了对辐射冷却特性的模式理解。本研究中开发的 2D 光解离策略和辐射冷却建模可以应用于研究广泛的分子阴离子的超慢冷却动力学。
