Ultrafast isomerization in acetylene dication after carbon K-shell ionization

Abstract

Ultrafast proton migration and isomerization are key processes for acetylene and its ions. However, the mechanism for ultrafast isomerization of acetylene [HCCH]2+ to vinylidene [H2CC]2+ dication remains nebulous. Theoretical studies show a large potential barrier ( > 2 eV) for isomerization on low-lying dicationic states, implying picosecond or longer isomerization timescales. However, a recent experiment at a femtosecond X-ray free-electron laser suggests sub-100 fs isomerization. Here we address this contradiction with a complete theoretical study of the dynamics of acetylene dication produced by Auger decay after X-ray photoionization of the carbon atom K shell. We find no sub-100 fs isomerization, while reproducing the salient features of the time-resolved Coulomb imaging experiment. This work resolves the seeming contradiction between experiment and theory and also calls for careful interpretation of structural information from the widely applied Coulomb momentum imaging method.

Excited-state dynamics of molecules with classically driven trajectories and Gaussians

“…AIMS has been successfully employed to elucidate the photochemistry of numerous molecules (see Refs. [7,117,119,[143][144][145][146][147] for example and Refs. [136,148] for a more detailed list of applications).…”

Perspective: Ultrafast Imaging of Molecular Dynamics Using Ultrafast Low-Frequency Lasers, X-ray Free Electron Laser and Electron Pulses

“…And this in most cases hinders direct reconstruction of molecular structure, unless a full time reversed propagation of molecular fragment trajectories is made possible. The required accuracy of momenta measurement is not within reach of current experimental techniques [31].…”

Ultrafast nuclear dynamics of the acetylene cation C2H2+ and its impact on the infrared probe pulse induced C–H bond breaking efficiency

“…Thus, it has been widely used to examine fundamental processes, such as charge migration, structural rearrangement, dissociation and isomerization on ultrafast timescales. [12][13][14][15][16][17][18][19][20] In a recent study, the nuclear dynamics following strong-field ionization of the acetylene molecule was investigated using the Coulomb explosion technique. 18 The elaborated experimental setup and advanced calculations allowed the extraction of information about bond length oscillations on the ground state of the acetylene cation initiated by the excitation process and to connect them with the observed fragment yields.…”

“…It is known that the variations of the yields of different fragmentation channels with pump-probe delay reflect the fast nuclear motion occurring on the few-tens-of-femtosecond timescale. [3][4][5]18,19,21 This letter focuses on the nuclear motion after ionization leading to the modulation of the C 2 H + fragment yield probed by IR irradiation. Using electronic structure computations and dynamical simulations, we were able to identify the different reaction pathways leading to C-H dissociation and assess the impact of nuclear rearrangements on the different IR-induced mechanisms leading to either an enhancement or a quenching of the C 2 H + fragment yield.…”