Spectroscopy of C60+ and Complex Organic Molecular Ions: from Mid-IR Emission to the Diffuse Interstellar Bands

许多与星际介质（ISM）化学有关的分子种类的实验室光谱学仍然是一个巨大的挑战。缺乏可靠的实验数据的后果是许多未识别的星际光谱和长期存在的谜团，如弥散星际带（DIB）和未识别的红外发射（UIR）特征。在该领域已经认识到，在实验室天体物理学方面的巨大努力将是必要的，以提供光谱信息，以了解现代望远镜在未来几年产生的大量数据。一个特别困难的领域是大分子离子如富勒烯和多环芳烃（PAH）的光谱学。虽然PAH离子长期以来一直被怀疑是负责在中红外线观测到的发射特征，实验室发射光谱对这些重要的物种都是几乎不存在的。同样，自20多年来，有一个有趣的说法，C60+是负责两个DIB的转变，但该说法的确认或拒绝需要可靠的气相光谱数据，这到目前为止一直难以捉摸的，我们建议开发一种新型的气相光谱，是基于激光诱导振动发射。我们打算在超高真空的低温离子束阱中储存富勒烯和PAH离子。在阱内，离子将暴露于从近红外到紫外范围的可调谐激光辐射。激光辐射将加热分子，使吸收的能量在其正常模式之间重新分配，并通过中红外振动跃迁重新发射。这与星际介质中复杂有机分子的加热和冷却过程相同。我们将通过使用超灵敏的阻塞杂质带Si：As二极管检测发射的中红外辐射来真实的观察这个过程。探测器采用的技术与空间中红外观测所用的技术相同。通过记录发射的中红外强度作为激光波长的函数，我们将能够获得大分子离子的真实气相光谱。MPIK现有的低温离子束阱在非常低的温度（约10 K）下提供了最终的真空系统（p < 10-12 Ω），这是无背景发射测量的理想环境。虽然我们的第一个目标分子是C60+，但我们打算将该技术扩展到大的PAH离子和碳链。随着静电离子阱在全球范围内的广泛应用，本文提出的技术的成功演示将为复杂有机分子的气相光谱提供一条新的途径，并进一步加深我们对ISM碳平衡的理解。

The laboratory spectroscopy of many molecular species that are relevant for the chemistry of the interstellar medium (ISM) still poses a great challenge. Consequences of the lack of reliable experimental data are numerous unidentified interstellar spectra and long-standing mysteries like the Diffuse Interstellar Bands (DIBs) and the Unidentified Infrared Emission (UIR) features. It has been recognized in the field that a tremendous effort in laboratory astrophysics will be necessary to provide the spectroscopic information to understand the vast amount of data produced by modern telescopes in the coming years. A particularly difficult field is the spectroscopy of large molecular ions like fullerenes and Polycyclic Aromatic Hydrocarbons (PAHs). While PAH ions have long been suspected to be responsible for the observed emission features in the mid-infrared, laboratory emission spectra on these important species are all but non-existent. Likewise, since more than 20 years there is an intriguing claim that C60+ is responsible for two DIB transitions, yet the confirmation or rejection of that claim requires reliable gas phase spectroscopic data, which so far have been elusive.We propose to develop a novel type of gas phase spectroscopy that is based on laser-induced vibrational emission. We intend to store fullerene and PAH ions in a cryogenic ion beam trap at ultra-high vacuum. Inside the trap the ions will be exposed to tunable laser radiation from the near-infrared to the UV range. The laser radiation will heat the molecules, which will redistribute the absorbed energy among their normal modes and re-emit it via vibrational transitions in the mid-IR. This is the same process that is responsible for the heating and cooling of complex organic molecules in the interstellar medium. We will observe this process in real time by detecting the emitted mid-IR radiation using ultra-sensitive blocked-impurity-band Si:As diodes. The detectors are based on the same technique that is used for mid-IR observations in space. By recording the emitted mid-IR intensity as a function of the wavelength of the laser, we will be able to obtain true gas phase spectra of large molecular ions. The existing cryogenic ion beam trap at MPIK provides an ultimate vacuum system (p < 10-12 torr) at very low temperatures (around 10 K), which is an ideal environment for background-free emission measurements. While our first target molecule will be C60+, we intend to extend the technique to large PAH ions and carbon chains. As electrostatic ion traps are gaining acceptance worldwide, the successful demonstration of the technique proposed here will provide a new pathway for gas phase spectroscopy of complex organic molecules, and further our understanding of the carbon balance of the ISM.