Abstract
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.
Highlights
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
The high brightness of XFELs and MeV radio frequency (RF) guns makes it possible to study the photochemistry in low-density gas phase vapors, where the molecular motions can be isolated without interference of nearby molecules as reaction dynamics proceed
We employed their structure pools to perform our structure determination, and we model the signals based on a hypothetical excited state structure that is taken to be the structure of the ionic ground state.[29]
Summary
The determination of molecular structures using x-ray and electron scattering has found many applications in chemistry and related molecular sciences.[1,2,3] The advent of technologies to create ultrafast pulses, first implemented on electron beams,[4,5,6,7] has heralded an age where it is possible to measure molecular systems in transient excited states.[8,9,10,11,12,13,14,15,16,17] More recently, x-ray Free-Electron Lasers (XFELs) and Mega-electron-Volt radio frequency (RF) electron guns have transformed the field, bringing the determination of transient molecular structures into a new era where interatomic distances are measured with sub-Angstr€om spatial resolution and femtosecond time resolution.[10,13,15,17,18,19,20] This makes it possible to capture time-resolved molecular scattering signals revealing the structures of molecules during chemical reactions even in gas-phase small organic molecules.[21,22,23,24] The high brightness of XFELs and MeV RF guns makes it possible to study the photochemistry in low-density gas phase vapors, where the molecular motions can be isolated without interference of nearby molecules as reaction dynamics proceed. Scattering experiments, i.e., ultrafast x-ray scattering and ultrafast electron diffraction (UED), could, in principle, offer direct access to complete molecular structures To measure their time evolution, a pump-probe scheme is the most commonly used experimental technique. The IAM model enables us to isolate contributions from different interatomic distances to the pump-probe scattering patterns This analysis suggests that both x-ray and electron scattering might be able to determine H atom positions. To calculate the probability of photons scattered into a resolution element Dq of the scattering vector, integrated over all azimuthal angles, we integrate Eq (1) over the circular area element between q and q þ Dq in the momentum transfer space (Fig. S1). The probability of photons scattered into a resolution element Dq at the momentum transfer vector q is given by ðq; q P0;X þ
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