Ultrafast X-ray scattering reveals vibrational coherence following Rydberg excitation

Abstract

The coherence and dephasing of vibrational motions of molecules constitute an integral part of chemical dynamics, influence material properties, and underpin schemes to control chemical reactions. In the present study, we measure coherent structural dynamics in optically excited N-methyl morpholine by scattering with ultrashort X-ray pulses from the Linac Coherent Light Source. The scattering signals are corrected for the different electron density in the excited electronic state of the molecule compared to the ground state. The experiment maps the evolution of the molecular geometry with femtosecond resolution, showing coherent motion that survives electronic relaxation and appears to persist for longer than previously seen using other methods.

Observation of the molecular response to light upon photoexcitation

“…3b drops toward −4.5% as q → 0, corresponding to the removal of 1 of the 44 electrons in CHD. It remains largely parallel to the 3p state signal for q > 1.0 Å −1 , implying that the 3p signal is dominated by the loss of the electron in the molecular core 8,12 . At q > 3.5 Å −1 , the electronic contributions of the ion and the 3p state are largely identical, suggesting that this region is mainly affected by the core electrons.…”

“…The short duration, tunability, and extreme brightness of XFEL pulses allow for the application of sophisticated x-ray techniques in the ultrafast regime. Recent ultrafast non-resonant x-ray scattering experiments have demonstrated that it is possible to track structural changes during molecular vibrations 8 or chemical reactions, with analogous developments in ultrafast electron diffraction 9 . Non-resonant x-ray scattering probes the arrangement of electrons in the sample, and it has been suggested that it might eventually become possible to follow dynamic changes in the electron density upon photoexcitation experimentally [10][11][12][13] .…”

“…In terms of x-ray scattering, excited states have mainly been identified via secondary manifestations, such as the preferential alignment of a molecule with its transition dipole moment 19 or the changes in molecular geometry in an excited state intermediate 3,[20][21][22] . Intriguingly, a recent x-ray scattering study demonstrated that in order to reproduce the correct coherent vibrational motion in an excited molecule, theoretical corrections that account for the change in electron density must be included in the data analysis 8 . The details of the changes in electronic structure, however, were obscured by comparatively large changes in molecular structure.…”

Editorial: Lasers in Accelerator Science and Secondary Emission Light Source Technology

“…The progress of ultra-intense femtosecond lasers, now attaining multi PetaWatt peak power, has recently enabled the demonstration of GeV electron beams in centimeter scale plasma accelerating section [5,6], with the recent world record reaching 8 GeV in 20 cm [7]. As for ultrafast x-ray science, in their relatively short time since their advent, x-ray free electron lasers [8,9] (FEL) have demonstrated the capacity to answer grand fundamental questions in a diverse set of areas in physics, chemistry, and biology, such as revealing vibration coherence in molecules [10], molecular bond formation, charge migration, and dissociation dynamics [11,12], or ultrafast isomerization in biomolecules [13,14], among many others. Further advances in facilities-such as augmented brightness, attosecond duration, or seeded emission-are poised to creating new scientific frontiers in atomic-scale correlated systems and ultrahigh resolution inner shell spectroscopies.…”

Theory of ultrafast x-ray scattering by molecules in the gas phase

“…Moreover, XFELs emit pulsed radiation that allows for investigation of structural changes and chemical reactions in real time. [6][7][8][9][10][11][12][13][14][15][16][17][18] In one remarkable example of these novel experiments, non-resonant scattering of hard xrays from the Linac Coherent Light Source 2 was used to identify reaction paths of the electrocyclic ring-opening of 1,3cyclohexadiene to 1,3,5-hexatriene 7,8 , thus providing insights into chemical reaction mechanisms that are complementary to the information accessible from spectroscopy. 13,19 It is likely that the duration of pulses at XFELs will reduce further in the near future [20][21][22][23][24] , making it possible to study even faster processes such as the rearrangement of electrons during chemical reactions.…”