This paper presents an analysis of near-resonant, ro-vibrational two-photon spectroscopy and the use of cavity ring-down spectroscopy for its detection. Expressions are derived for the photon absorption rate of a three-level system, correct to all orders and the simpler expressions that result from various approximations. The analysis includes the angular momentum projection degeneracies and linear or circular polarization of the exciting field. Expressions are derived for the rate of two-photon power loss for light inside a resonant cavity. Explicit calculations are made for excitation of the $ν_3$ mode of $^{12}\textrm{C}^{16}\textrm{O}_2$ for which the two-photon excitation spectrum is dominated by a single $v_3 = 0 \rightarrow 2, Q(16)$ line at $\tildeν = 2335.826$\,cm$^{-1}$. This transition has an intermediate $v_3 = 0 \rightarrow 1,P(16)$ one-photon transition that is off resonance by 0.093 cm$^{-1}$ (2.8 GHz). At 1\,torr total pressure, the Q(16) two-photon transition has a calculated cross-section of $2.99 \cdot 10^{-38}$\,cm$^4$s per CO$_2$ molecule in the $J = 16$ state or $2.24 \cdot 10^{-39}$\,cm$^4$s per CO$_2$ molecule at 300\,K is calculated. Analysis of the sensitivity limits for 2-photon cavity ring-down spectroscopy predicts a theoretical detection limit of 32\,ppq ($10^{-15}$) Hz$^{-1/2}$ for $^{12}\textrm{C}^{16}\textrm{O}_2$, higher sensitivity than has been realized using one-photon absorption. The analysis predicts that most polyatomic molecules will have sparse, Doppler-Free two-photon absorption spectra, which will dramatically increase the selectivity of trace gas detection of samples with multiple components with overlapping absorption bands. This is demonstrated by the predicted mid-IR two-photon absorption spectrum of butadiene using theoretical spectroscopic constants.
Absorption cross-sections for the 5th and 6th vibrational overtones in a series of short chained alcohols using incoherent broadband cavity enhanced-absorption spectroscopy (IBBCEAS)
Anharmonic vibrational spectroscopy of Polycyclic Aromatic Hydrocarbons (PAHs)
虽然存在强大的技术来准确地解释振动分子光谱中的非谐性，但它们在计算上非常昂贵，并且不能常规地用于大物种和/或在非零振动温度下。受空间多环芳烃（PAH）发射研究的启发，我们开发了一个新的编码，它考虑了所有的模式，可以描述所有的红外跃迁，包括由于共振而变得活跃的谱带以及泛音、组合和差异谱带。在这篇文章中，我们描述了实施的方法，并讨论了如何克服主要困难，以保持问题易于处理。对小分子进行高水平计算的基准测试。我们进行了具体的收敛测试两个原型多环芳烃，芘（C $_{16}$ H $_{10}$）和晕苯（C $_{24}$ H $_{12}$），旨在优化可调参数，以达到可接受的精度和计算成本为这类分子。然后，我们报告在0 K的芘和晕苯得到的结果，比较计算的光谱与现有的实验数据。发现与谐波密度泛函理论（DFT）计算相比，理论能带位置得到显著改善。谱带强度与实验结果相当吻合，主要的限制是四次力场计算的准确性。这是第一步，计算多环芳烃和其他类似的刚性分子使用蒙特卡罗采样的适度高温光谱。
Intensities of all fine-structure resolved rovibrational electric quadrupole absorption lines in $^{16}$O$_2$($X^{3}Σ^{-}_{g}$) calculated with a new $\textit{ab initio}$ quadrupole moment curve
Infrared Spectroscopy of Cyanonaphthalenes under Interstellar Relevant Conditions and Their Potential Connection with Astronomical Aromatic Infrared Bands
HFML-FELIX, Nijmegen, The Netherlands（荷兰奈梅亨高磁场实验室FELIX）
