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)化学相关的分子物种的实验室光谱仍然是一个巨大的挑战。缺乏可靠的实验数据的后果是无数不明的星际光谱和长期存在的谜团，如漫射星际带(DIB)和不明红外发射(UIR)特征。该领域已经认识到，在未来几年，需要在实验室天体物理学方面做出巨大努力，提供光谱信息，以了解现代望远镜产生的大量数据。一个特别困难的领域是像富勒烯和多环芳烃(PAHs)这样的大分子离子的光谱。虽然多环芳烃离子长期以来一直被怀疑是中红外观测到的发射特征的原因，但这些重要物种的实验室发射光谱几乎不存在。同样，自20多年以来，有一种耐人寻味的说法认为，C60+是导致两个DIB转变的原因，但对这一说法的证实或否认需要可靠的气相光谱数据，而到目前为止，这些数据一直难以捉摸。我们建议开发一种基于激光诱导振动发射的新型气相光谱。我们打算将富勒烯和多环芳烃离子储存在超高真空的低温离子束陷阱中。在陷阱内，离子将暴露在从近红外到紫外线范围内的可调谐激光辐射中。激光辐射将加热分子，分子将吸收的能量重新分配到它们的简正模之间，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是相同的。我们将使用超灵敏的阻挡杂质带的Si：作为二极管，通过探测发射的中红外辐射来实时观察这一过程。探测器基于用于空间中红外观测的相同技术。通过记录发射的中红外强度随激光波长的变化，我们将能够获得大分子离子的真实气相光谱。MPIK现有的低温离子束陷阱在非常低的温度(约10K)下提供了极限真空系统(p&lt；10-12torr)，这是进行无背景辐射测量的理想环境。虽然我们的第一个目标分子将是C60+，但我们打算将该技术扩展到大的多环芳烃离子和碳链。随着静电离子陷阱技术在世界范围内的广泛应用，该技术的成功应用将为复杂有机分子的气相光谱分析提供一条新的途径，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.