Resolving gas phase molecular motions with simultaneous spatial and temporal resolution is rapidly coming within the reach of x-ray Free Electron Lasers (XFELs) and Mega-electron-Volt (MeV) electron beams. These two methods enable scattering experiments that have yielded fascinating new results, and while both are important methods for determining transient molecular structures in photochemical reactions, it is important to understand their relative merits. In the present study, we evaluate the respective scattering cross sections of the two methods and simulate their ability to determine excited state molecular structures in light of currently existing XFEL and MeV source parameters. Using the example of optically excited N-methyl morpholine and simulating the scattering patterns with shot noise, we find that the currently achievable signals are superior with x-ray scattering for equal samples and on a per-shot basis and that x-ray scattering requires fewer detected signal counts for an equal fidelity structure determination. Importantly, within the independent atom model, excellent structure determinations can be achieved for scattering vectors only to about 5 Å⁻¹, leaving larger scattering vector ranges for investigating vibrational motions and wavepackets. Electron scattering has a comparatively higher sensitivity toward hydrogen atoms, which may point to applications where electron scattering is inherently the preferred choice, provided that excellent signals can be achieved at large scattering angles that are currently difficult to access.

中文翻译：

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自由分子的超快 X 射线和电子散射：比较评估。

X 射线自由电子激光器 (XFEL) 和兆电子伏 (MeV) 电子束正在快速实现以同时空间和时间分辨率解析气相分子运动。这两种方法使得散射实验能够产生令人着迷的新结果，虽然这两种方法都是确定光化学反应中瞬态分子结构的重要方法，但了解它们的相对优点也很重要。在本研究中，我们评估了两种方法各自的散射截面，并根据当前现有的 XFEL 和 MeV 源参数模拟了它们确定激发态分子结构的能力。以光激发 N-甲基吗啉为例，并用散粒噪声模拟散射模式，我们发现，对于相同样本和每次散粒来说，当前可实现的信号优于 X 射线散射，并且 X 射线散射需要用于同等保真度结构确定的检测到的信号计数更少。^{重要的是，在独立原子模型中，只需约 5 Å -1}的散射矢量即可实现出色的结构确定，从而为研究振动运动和波包留下更大的散射矢量范围。电子散射对氢原子具有相对较高的灵敏度，这可能表明电子散射本质上是首选的应用，前提是可以在目前难以获得的大散射角下获得优异的信号。