Beutner V, Duffy L M, Meyer H
Department of Physics and Astronomy , The University of Georgia , Athens , Georgia 30602-2451 , United States.
Department of Chemistry & Biochemistry , University of North Carolina at Greensboro , Greensboro , North Carolina 27412 , United States.
J Phys Chem A. 2019 Mar 14;123(10):2153-2162. doi: 10.1021/acs.jpca.8b11528. Epub 2019 Feb 27.
We investigate the potential for sensitively detecting millimeter-wave absorption through multiphoton ionization using the lowest rotational transition in CO (XΣ, J = 1 ← J = 0) at 115 GHz as a diagnostic tool. This transition represents an almost perfect realization of a quantum mechanical two-level system. In the experiment, the output of a powerful continuous millimeter-wave source with sub-kilohertz resolution propagates counter to a pulsed molecular beam, causing Doppler shifted resonances. Absorption is detected by monitoring the involved rotational levels through resonance enhanced multiphoton ionization (REMPI). Observed polarization, saturation and Doppler effects demonstrate that the interaction with the millimeter-wave field is coherent and can be simulated well using the results for the two-level system. The combination of these results indicates that the millimeter-wave field is approximated well by a Gaussian beam with beam waist around 15 mm and constant effective electric field amplitude on the molecular beam axis. Time-correlated frequency modulation in the form of a short rectangular pulse is used to follow the molecular trajectory from the source to the laser interaction region. The experiments clearly demonstrate a great potential of extending high-resolution molecular beam spectroscopy with mass specific REMPI detection into the sub-terahertz or terahertz regimes.
我们研究了利用一氧化碳（XΣ，J = 1 ← J = 0）在115吉赫兹的最低转动跃迁通过多光子电离灵敏检测毫米波吸收的潜力，以此作为一种诊断工具。这种跃迁几乎完美地实现了一个量子力学两能级系统。在实验中，具有亚千赫兹分辨率的强大连续毫米波源的输出与脉冲分子束反向传播，产生多普勒频移共振。通过共振增强多光子电离（REMPI）监测相关转动能级来检测吸收。观察到的偏振、饱和和多普勒效应表明，与毫米波场的相互作用是相干的，并且可以使用两能级系统的结果很好地模拟。这些结果的结合表明，毫米波场可以很好地用束腰约为15毫米且在分子束轴上具有恒定有效电场振幅的高斯光束来近似。采用短矩形脉冲形式的时间相关频率调制来跟踪分子从源到激光相互作用区域所需的轨迹。实验清楚地证明了将具有质量特异性REMPI检测的高分辨率分子束光谱扩展到亚太赫兹或太赫兹频段具有巨大潜力。
