Development of a Trace Gas Detector using Cavity Ring-Down Detection of Two-Photon Absorption

在化学系化学测量和成像（CMI）项目的支持下，弗吉尼亚大学的Kevin Lehmann教授和他的研究小组正在开发和实施一种新的基于激光的痕量气体选择性检测方法。该方法利用了一种称为近共振双光子吸收光谱的独特工艺，以提高在复杂气体混合物中区分不同分子的程度。当一个分子同时吸收两个光子时，其中一个光子的能量与分子的一个振动非常接近，但不完全相同，测量的选择性和灵敏度都显著提高，这是改进的基础。该实验使用一个光学腔来捕获两个镜子之间的光，以增加信号强度。用于痕量气体检测的腔增强技术，特别是称为腔衰荡光谱的方法，允许在许多应用中识别和表征分子，包括超高纯度制造，药物纯度和寿命监测，本地和全球范围的环境监测，医疗诊断，甚至考古样本的时间测定。由Lehmann教授领导的研究小组正在使用新的双光子方法来克服重要的复杂性，这些复杂性限制了使用传统的腔增强方法可以区分的分子的大小，可能导致可以分析的样品的范围和复杂性显着增加。本项目涉及基于红外光谱区近共振双光子吸收腔衰荡光谱（TP-CRDS）的新型痕量气体检测方法的开发。检测小分子（如氧化亚氮和二氧化碳）的测量，为该技术的基础理论和与预测的短噪声极限相比较的检测限提供了重要的测试。测试测量包括检测合成空气样品中的痕量氧化亚氮，这些样品具有各种环境中环境空气中存在的实际微量成分。额外的测量检查更复杂分子的双光子吸收光谱，如反式丁二烯。宽频带扫描首次调查了这些大分子的双光子吸收光谱，提供了低压下纯气体的实验光谱，以便与基于电子结构计算的非调和频率预测的模拟光谱进行比较。宽频带巡天扫描采用两种探测方法，一种基于吸收，另一种基于发射。在第一种情况下，基于反传播波的幅度或频率调制来测量多通赫里奥特电池中红外光的吸收。第二种方法是根据两种情况下信号的相反符号，检测由两个反传播波引起的样品红外发射，以便有选择地区分双光子吸收和兰姆dip。覆盖多原子分子基本C-H拉伸振动的光谱区域的测量有可能揭示碳氢化合物混合物的组成，这些混合物由于光谱重叠而无法通过其单光子吸收光谱来区分。总的来说，这种利用近共振双光子吸收光谱进行痕量气体检测的新方法有望实现更灵敏、更有选择性的检测，包括检测同位素不同的物种，以及在更复杂的混合物中识别大分子，而不是目前的方法。这些发展可能对许多领域产生影响，包括大气科学、工业和环境监测以及放射性碳测年。该研究项目还将为研究生和本科生提供高级培训机会。该奖项反映了美国国家科学基金会的法定使命，并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，被认为值得支持。

With support from the Chemical Measurement and Imaging (CMI) Program in the Division of Chemistry, Professor Kevin Lehmann and his research group at the University of Virginia are working to develop and implement a new laser-based method for the selective detection of trace gases. The method takes advantage of a unique process called near-resonant two-photon absorption spectroscopy in order to improve the extent to which different molecules can be distinguished in complex gas mixtures. The improvements are based on significant increases in both the selectivity and the sensitivity of the measurement that occur when a molecule simultaneously absorbs two photons, with one of the photons having an energy that is very close to, but not exactly the same as, one of the vibrations of the molecule. The experiments use an optical cavity that traps the light between two mirrors in order to increase the signal strength. Cavity-enhanced techniques for trace gas detection, especially a method known as cavity ring-down spectroscopy, allow the identification and characterization of molecules in many applications, including ultrahigh purity manufacturing, drug purity and lifetime monitoring, environmental monitoring on both local and global scales, medical diagnostics, and even the time-dating of archeological samples. The research team led by Professor Lehmann is using the new two-photon approach to overcome important complications that have limited the size of the molecules that can be distinguished using traditional cavity-enhanced methods, potentially leading to significant increases in the range and complexity of samples that can be analyzed. This project involves development of new trace gas detection methods based on near-resonant two-photon absorption cavity ring-down spectroscopy (TP-CRDS) in the infrared spectral region. Measurements to detect small molecules, such as nitrous oxide and carbon dioxide, provide important tests of the underlying theory of the technique and the detection limits compared with the predicted shot-noise limit. Test measurements include detection of trace levels of nitrous oxide in synthetic air samples with realistic minor components that are present in ambient air in various environments. Additional measurements examine the two-photon absorption spectra of more complex molecules, such as trans-butadiene. Wide-bandwidth scans survey the two-photon absorption spectra of these larger molecules for the first time, providing experimental spectra of the pure gases at low pressure for comparison with simulated spectra based on anharmonic frequency predictions from electronic structure calculations. The wide-bandwidth survey scans use two detection methods, one based on absorption and one based on emission. In the first case, absorption of infrared light in a multipass Herriott cell is measured based on amplitude or frequency modulation of a counter-propagating wave. The second method detects infrared emission of the sample induced by two counter-propagating waves in order to selectively distinguish two-photon absorption from Lamb dips, based on opposite sign of the signal in the two cases. Measurements covering the spectral region of the fundamental C-H stretching vibrations of polyatomic molecules have the potential to reveal the composition of mixtures of hydrocarbons that cannot be distinguished by their one-photon absorption spectra due to spectrally overlapping bands. Overall, this new approach to trace gas detection using near-resonant two-photon absorption spectroscopy holds the promise of more sensitive and more selective detection, including detection of isotopically distinct species and identification of larger molecules in more complex mixtures than current methods allow. These developments could have impact across many areas, including atmospheric science, industrial and environmental monitoring, and radiocarbon dating. The research project also will provide advanced training opportunities for graduate and undergraduate research students.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.