Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Chem Phys. 2018 Nov 7;149(17):174309. doi: 10.1063/1.5045046.
We report two new experimental schemes to obtain rotationally resolved high-resolution spectra of predissociated S acetylene levels in the 47 000-47 300 cm energy region (∼1200 cm above the predissociation threshold). The two new detection schemes are compared to several other detection schemes (employed at similar laser power, molecular beam temperature, and number of signal averages) that have been used in our laboratory to study predissociated S acetylene levels, both in terms of the signal-to-noise ratio (S/N) of the resultant spectra and experimental simplicity. In the first method, H-atoms from the predissociated S acetylene levels are probed by two-photon laser-induced fluorescence (LIF). The H-atoms are pumped to the 3 level by the two-photon resonance transition at 205.14 nm. The resulting 3-2 fluorescence (654.5 nm) is collected by a photomultiplier. The S/N of the H-atom fluorescence action spectrum is consistently better by ∼3× than that of the more widely used H-atom resonance-enhanced multiphoton ionization (REMPI) detection. Laser alignment is also considerably easier in H-atom fluorescence detection than H-atom REMPI detection due to the larger number-density of molecules that can be used in fluorescence . REMPI detection schemes. In the second method, fluorescence from electronically excited C and CH photofragments of S acetylene is detected. In contrast to the H-atom detection schemes, the detected C and CH photofragments are produced by the UV laser as is used for the acetylene excitation. As a result, laser alignment is greatly simplified for the photofragment fluorescence detection scheme, compared to both H-atom detection schemes. Using the photofragment fluorescence detection method, we are able to obtain action spectra of predissociated S acetylene levels with S/N ∼2× better than the HCCH REMPI detection and ∼10× better than H-atom and HCCH LIF detection schemes.
我们报告了两种新的实验方案，用于获得 47000-47300cm 能量区域（约高于离解阈值 1200cm）内预离解 S 乙炔能级的旋转分辨高分辨率光谱。这两种新的检测方案与我们实验室中用于研究预离解 S 乙炔能级的几种其他检测方案（在类似的激光功率、分子束温度和信号平均次数下使用）进行了比较，比较了两种检测方案的信号噪声比（S/N）和实验的简易性。在第一种方法中，通过双光子激光诱导荧光（LIF）探测预离解 S 乙炔能级的 H 原子。H 原子通过 205.14nm 的双光子共振跃迁被泵浦到 3 能级。所得的 3-2 荧光（654.5nm）被光电倍增管收集。H 原子荧光作用光谱的 S/N 始终比更广泛使用的 H 原子共振增强多光子电离（REMPI）检测好约 3 倍。由于可以在荧光中使用更多数量密度的分子，因此 H 原子荧光检测的激光对准也比 H 原子 REMPI 检测容易得多。在第二种方法中，检测 S 乙炔的电子激发的 C 和 CH 光碎片的荧光。与 H 原子检测方案相反，所检测的 C 和 CH 光碎片是由与用于乙炔激发的 UV 激光一起产生的。因此，与 H 原子检测方案相比，光碎片荧光检测方案的激光对准大大简化。使用光碎片荧光检测方法，我们能够获得预离解 S 乙炔能级的作用光谱，其 S/N 比 HCCH REMPI 检测好约 2 倍，比 H 原子和 HCCH LIF 检测好约 10 倍。
