EAGER: Doppler-free indirect rotational spectroscopy of complex interstellar radicals

自由基是星际气体分子清单的重要组成部分，驱动化学反应，在太空中建立分子复杂性，但由于缺乏准确的实验室数据，许多星际自由基仍然无法识别。 这一探索性项目将开发一种新仪器，用于测量被认为在星际化学中很重要的自由基的高精度红外光谱。 这将允许导出具有与直接亚毫米光谱学相当的精度的自由基的纯旋转频率，从而使它们能够被诸如阿塔卡马大型毫米/亚毫米阵列（阿尔马）的射电望远镜探测到。 该项目将对一名研究生和一名本科生进行高级实验室天体物理学方法的培训，并将围绕激光技术向当地高中进行推广。Crabtree和他的团队将在正柱直流等离子体放电池中进行的中红外噪声免疫腔增强光外差光谱（NICE-OHMS）中添加磁场“塞曼”调制。这将产生“无多普勒”，尖锐的“兰姆凹陷”光谱，当与激光光学频率梳的校准相结合时，将允许对等离子体中产生的自由基的红外振转跃迁进行高度准确和精确的测量。虽然NICE-OHMS本身是成熟的，但为自由基的无多普勒光谱增加塞曼调制是一项创新，具有重大的技术挑战。该项目将在概念验证层面开发这一新能力，以证明其适用于亚毫米天文观测，这需要确定高位跃迁的旋转静止频率优于1 MHz。在塞曼调制设备的建造和测试之后，乙炔基自由基CCH的V1基本带将被记录和分析，以验证这种方法与直接亚毫米旋转光谱相比的准确性。 这项技术的成功演示将为实验室中复杂自由基的宽带间接旋转光谱提供一种强大的新方法，从而使阿尔马和其他亚毫米望远镜能够探测到它们。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

Free radicals are an important component of the molecular inventory of the interstellar gas, driving chemical reactions which build up molecular complexity in space, but many interstellar radicals remain unidentified because of the lack of accurate laboratory data. This exploratory project will develop a new instrument to measure highly precise infrared spectra of radicals postulated to be important in interstellar chemistry. This will allow derivation of pure rotational frequencies of radicals with comparable accuracy to direct submillimeter spectroscopy, enabling their detection by radio telescopes such as the Atacama Large Millimeter/submillimeter Array (ALMA). The project will train a graduate student and an undergraduate student in methods of advanced laboratory astrophysics, and it will conduct outreach centered around laser technology to local high schools.Crabtree and his team will add magnetic-field “Zeeman” modulation to mid-infrared Noise-Immune Cavity-Enhanced Optical Heterodyne Spectroscopy (NICE-OHMS) performed in a positive column direct-current plasma discharge cell. This will yield "Doppler-free", sharp “Lamb dip” spectra, which, when combined with calibration by laser optical frequency combs, will allow highly accurate and precise measurements of infrared rovibrational transitions of radicals generated in the plasma. While NICE-OHMS itself is well-established, the addition of Zeeman modulation for Doppler-free spectroscopy of radicals is an innovation that carries significant technical challenges. This project will develop this new capability at the proof-of-concept level to demonstrate its suitability for submillimeter astronomical observations, which require rotational rest frequencies of high-lying transitions to be determined to better than 1 MHz. Following construction and testing of equipment for Zeeman modulation, the v1 fundamental band of the ethynyl radical, CCH, will be recorded and analyzed to validate the accuracy of this method compared with direct submillimeter rotational spectroscopy. Successful demonstration of this technique will provide a robust new method for wideband indirect rotational spectroscopy of complex radicals in the laboratory, thereby enabling their detection with ALMA and other submillimeter telescopes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.