We introduce two new ideas that allow us to directly access and quantify the effect of nuclear motion on molecular photoionization delays. Whereas the profound impact of electron-nuclear couplings on excited-state dynamics is a direct consequence of the relatively long time scales at play 13, 14, 15, 16, 17, its influence on attosecond photoionization dynamics remains unknown 18, 19. This has also been suggested to be the case in molecular photoionization, in particular in the presence of shape resonances 6, where the coupling between the fast electronic and the slow nuclear motions can be considerably enhanced owing to the extended trapping time of the photoelectron before its escape 7, 8, 9, 10, 11, 12. It is well-established that the inclusion of both electronic and nuclear motions is a prerequisite for even a qualitatively correct description of excited-state dynamics in molecules 5. Prominent manifestations of this effect include, e.g., conical intersections 1, the Jahn-Teller effect (JTE) 2, 3, or electron-phonon coupling in solids 4. The way electrons and nuclei share their energy in a molecule is a central concept in molecular physics and chemistry, since it is at the root of many fundamental properties and dynamical processes in matter. These findings have important consequences for the design and interpretation of attosecond chronoscopy in molecules, clusters, and liquids. These results show that, in the absence of resonances, even the fastest nuclear motion does not substantially influence photoionization delays, but identify a previously unknown signature of nuclear motion in dissociative-ionization channels. Experiment and theory are in quantitative agreement. However, we measure and calculate delays of up to 20 as between the dissociative and non-dissociative photoionization of the highest-occupied molecular orbitals of both molecules. Remarkably, we find no measurable delay between the photoionization of CH 4 and CD 4, neither experimentally nor theoretically. These molecules are known to feature some of the fastest nuclear dynamics following photoionization. Here, we apply attosecond electron-ion coincidence spectroscopy and advanced calculations that include both electronic and nuclear motions to study the photoionization dynamics of CH 4 and CD 4 molecules. To what extent it influences attosecond photoionization delays is an important, still unresolved question. The interplay between electronic and nuclear motions in molecules is a central concept in molecular science.
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