High-Precision Spectroscopy of CH5+: Demystifying the Ro-Vibrational Spectrum

伊利诺伊大学厄巴纳-香槟分校的本杰明·麦考尔获得了化学系化学结构、动力学和机制项目的支持，以实施两个互补的实验来研究CH5+的高分辨率红外光谱。这种离子是乔治·奥拉和他的同事们成功地用于超酸化学的典型碳正离子，也是星际化学中的关键中间体。一个由近1000条谱线组成的高分辨率振动光谱是在十多年前获得的，至今仍完全没有归属。第一个实验旨在利用腔增强型速度调制光谱在室温下以亚MHz的精度记录CH5+光谱，以便首次使用四线组合差法确定基态能级间距。我们的第二个实验旨在使用一台由连续超音速膨胀放电源馈送的新型快速离子束光谱仪记录CH5+的光谱，以提供第一个明确的、高精度的低温CH5+光谱；这反过来又使得实验确定CH5+的最低转动能级的能量成为可能。综上所述，这些实验应该提供了对这种神秘分子的振动结构的第一次了解，并使McCall和他的同事第一次能够构建这种离子的基态振动状态的能级图。最终，这一新的认识也应该能够在密集的星际云中用射电天文探测到这种重要的离子。这项研究和相关活动预计将在加强跨学科研究、培养本科生和研究生以及整合教学和研究方面产生广泛影响。这些努力的首要主题是将化学、天文学和物理界聚集在一起，并鼓励这三个学科之间的相互促进。

Benjamin McCall of the University of Illinois at Urbana-Champaig, is supported by an award from the Chemical Structure, Dynamics and Mechanisms program in the Chemistry division to implement two complementary experiments to study the high-resolution infrared spectrum of CH5+. This ion is the prototypical carbonium ion of the type so successfully used by George Olah and colleagues for superacid chemistry, and is also a critical intermediate in interstellar chemistry. A high-resolution ro-vibrational spectrum consisting of nearly 1000 lines was obtained over a decade ago, and remains completely unassigned. The first experiment aims to record the CH5+ spectrum at room temperature with sub-MHz precision and accuracy using cavity-enhanced velocity modulation spectroscopy, to enable the first experimental determination of ground-state energy level spacings using the method of four-line combination differences. Our second experiment aims to record the CH5+ spectrum using a novel fast ion beam spectrometer fed by a continuous supersonic expansion discharge source, to provide the first unambiguous, high-precision spectrum of CH5+ at low temperature; this in turn enables the experimental determination of the energies of the lowest rotational energy levels of CH5+. Taken together, these experiments should provide the first understanding of the ro-vibrational structure of this enigmatic molecule, and enable McCall and coworkers to construct an energy level diagram of the ground vibrational state of this ion for the first time. Ultimately, this new understanding should also enable the radio-astronomical detection of this important ion in dense interstellar clouds. This research, and associated activities, can be expected to have a broad impact in terms of enhancing interdisciplinary research, training undergraduate and graduate students, and integrating teaching and research. The overarching theme of these efforts is to bring together the chemistry, astronomy, and physics communities, and to encourage cross-fertilization between these three disciplines.