Time-resolved nuclear dynamics in bound and dissociating acetylene

Controlling Fragmentation of the Acetylene Cation in the Vacuum Ultraviolet via Transient Molecular Alignment

An open-loop control scheme of molecular fragmentation based on transient molecular alignment combined with single-photon ionization induced by a short-wavelength free electron laser (FEL) is demonstrated for the acetylene cation. Photoelectron spectra are recorded, complementing the ion yield measurements, to demonstrate that such control is the consequence of changes in the electronic response with molecular orientation relative to the ionizing field. We show that stable C₂H₂⁺ cations are mainly produced when the molecules are parallel or nearly parallel to the FEL polarization, while the hydrogen fragmentation channel (C₂H₂⁺ → C₂H⁺ + H) predominates when the molecule is perpendicular to that direction, thus allowing one to distinguish between the two photochemical processes. The experimental findings are supported by state-of-the art theoretical calculations.

Coincident angle-resolved state-selective photoelectron spectroscopy of acetylene molecules: a candidate system for time-resolved dynamics

The acetylene-vinylidene system serves as a benchmark for investigations of ultrafast dynamical processes where the coupling of the electronic and nuclear degrees of freedom provides a fertile playground to explore the femto- and sub-femto-second physics with coherent extreme-ultraviolet (EUV) photon sources both on the table-top as well as free-electron lasers. We focus on detailed investigations of this molecular system in the photon energy range 19-40 eV where EUV pulses can probe the dynamics effectively. We employ photoelectron-photoion coincidence (PEPICO) spectroscopy to uncover hitherto unrevealed aspects of this system. In this work, the role of excited states of the C2H2+ cation, the primary photoion, is specifically addressed. From photoelectron energy spectra and angular distributions, the nature of the dissociation and isomerization channels is discerned. Exploiting the 4π-collection geometry of the velocity map imaging spectrometer, we not only probe pathways where the efficiency of photoionization is inherently high but also perform PEPICO spectroscopy on relatively weak channels.

Ultrafast Laser-Induced Isomerization Dynamics in Acetonitrile

Isomerization induced by laser ionization in acetonitrile (CH₃CN) was investigated using pump-probe spectroscopy in combination with ion-ion coincident Coulomb explosion imaging. We deduced five primary channels indicating direct C-C breakup, single and double hydrogen migration, and H and H₂ dissociation in the acetonitrile cation. Surprisingly, the hydrogen-migration channels dominate over direct fragmentation. This observation is supported by quantum chemistry calculations showing that isomerization through single and double hydrogen migration leads to very stable linear and ring isomers, with most of them more stable than the original linear structure following ionization of the parent molecule. This is unlike most molecules investigated previously using similar schemes. By varying the delay between the pump and probe pulses, we have also determined the time scales of the corresponding dynamical processes. Isomerization typically occurs in a few hundred femtoseconds, a time scale that is comparable to that found for H and H₂ dissociation and direct molecular fragmentation.

Dependence on the Initial Configuration of Strong Field-Driven Isomerization of C2H2 Cations and Anions

We have investigated the femtosecond laser-induced fragmentation of C₂H₂ ^q ion beam targets in various initial configurations, including acetylene (linear HCCH), vinylidene (H₂CC), and cis/ trans. The initial configuration is shown to have a tremendous impact on the branching ratio of acetylene-like (CH ^q₁ + CH ^q₂) and vinylidene-like (C ^q₁' + CH₂ ^q₂') dissociation of a specific C₂H₂ ^q molecular ion. In particular, whereas C₂H₂⁺ generated from C₂H₂, a linear HCCH target, exhibits comparable levels of acetylene-like and vinylidene-like fragmentation, vinylidene or cis/ trans configuration ion beams preferably undergo vinylidene-like fragmentation, with an acetylene branching ratio ranging from 13.9% to zero.

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

We track the few-femtosecond excited-state dynamics of the acetylene cation through modulations of the C₂H⁺ photofragment yield.