Ultrafast electron diffraction imaging of bond breaking in di-ionized acetylene

Abstract

Visualizing chemical reactions as they occur requires atomic spatial and femtosecond temporal resolution. Here, we report imaging of the molecular structure of acetylene 9 fs after ionization. Using mid-infrared laser induced electron diffraction (LIED) we obtain snapshots as a proton departs the [C 2 H 2 ] 2+ ion. By introducing an additional laser field, we also demonstrate control over the ultrafast dissociation process and resolve different bond dynamics for molecules oriented parallel vs. perpendicular to the LIED field. These measurements are in excellent agreement with a quantum chemical description of fielddressed molecular dynamics. One Sentence Summary:We demonstrate space and time imaging of a single acetylene molecule after 9 fs while one of its bonds is broken and a proton departs the molecule.Ultrafast imaging of atomic motion in real time during transitions in molecular structure is prerequisite to disentangling the complex interplay between reactants and products (1, 2) since the motion of all atoms are coupled. Ultrafast absorption and emission spectroscopic techniques have uncovered numerous insights in chemical reaction dynamics (3,4), but are limited by their reliance on local chromophores and their associated ladders of quantum states rather than global structural characterization. Reaction imaging at the molecular level requires a combination of few-femtosecond temporal and picometer spatial measurement resolution (5). Amongst the many techniques that are currently under intense development, x-ray scattering can reach few-femtosecond pulse durations at photon energies of 8.3 keV (1.5 Å) (6) with a demonstrated measurement resolution of 3.5 Å (7). Challenges for such photonbased approaches are the coarse spatial resolution and the low scattering cross-sections, especially for gas phase investigations. Electron scattering (8) provides much larger interaction cross-sections and smaller de Broglie wavelengths, but suffers from space charge broadening which decreases the temporal resolution.Consequently, measurements have demonstrated 7 pm spatial and 100 fs temporal resolution (9, 10) in gas phase experiments. Remedies to improve temporal resolution include relativistic electron acceleration (11) or electron bunch compression (12) with 100 fs and 28 fs limits, respectively. Compared to such incoherent scattering of electrons from an electron source off a molecular target, laser induced electron diffraction (LIED) is a self-imaging method based on coherent electron scattering (13)(14)(15)(16)(17). In LIED, one electron which is liberated from the target molecule through tunnel ionization, it is then accelerated in the field and rescattered of its molecular ion thereby acquiring structural information. The electron recollision process occurs within one optical cycle of the laser field and permits mapping electron momenta to recollision time (18,19).Here, we used LIED to image an entire hydrocarbon molecule (acetylene -C 2 H 2 ) at 9 fs after ionizationtriggered dissociation and visualiz...

Influence of permanent dipole and dynamic core-electron polarization on tunneling ionization of polar molecules

“…In fact, it has been found that all the important strong-field phenomena, which involve rescattering physics, start with an ionization step [1]. Imaging techniques based on rescattering such as laser-induced electron diffraction (LIED) [2,3] and high-order harmonics spectroscopy (HHS) [4][5][6][7] are now capable of probing dynamic molecular structural changes with sub-Ångstrom spatial and few-femtosecond temporal resolutions. Correct interpretation and extraction of target structures depend critically on an accurate description of the ionization step.…”

Diffraction and microscopy with attosecond electron pulse trains

“…Here we advance attosecond metrology to picometre wavelength and sub-atomic resolution by using free-space electrons instead of higher-harmonic photons 1-7 or re-colliding wavepackets [8][9][10][11] . A beam of 70-keV electrons at 4.5-pm de Broglie wavelength is modulated by the electric field of laser cycles into a sequence of electron pulses with sub-optical-cycle duration.…”

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Attosecond spectroscopy1-7 can resolve electronic processes directly in time, but a movie-like space-time recording is impeded by the too long wavelength (~100 times larger than atomic distances) or the source-sample entanglement in re-collision techniques [8][9][10][11] . Here we advance attosecond metrology to picometre wavelength and sub-atomic resolution by using free-space electrons instead of higher-harmonic photons 1-7 or re-colliding wavepackets [8][9][10][11] .

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Optical Parametric Amplification Techniques for the Generation of High-Energy Few-Optical-Cycles IR Pulses for Strong Field Applications

“…They succeeded in capturing the dynamical evolution of oxygen and nitrogen structures under a vibrational excitation [96]. Recently, advances have been reported in LIED experiments performed with mid-infrared few-cycle high repetition rate parametric sources, providing the extension of this technique to the sub-Å imaging of a polyatomic molecule, such as ethylene [97,98], and to the exploration with a sub-fs resolution of a chemical process, such as bond-breaking in acetylene [99].…”