Diffractive imaging of a rotational wavepacket in nitrogen molecules with femtosecond megaelectronvolt electron pulses

Faraday Discuss., 2014

Advances In Atomic, Molecular, and Optical Physics, 2020

Knowledge of molecular structure is paramount in understanding, and ultimately influencing, chemical reactivity. For nearly a century, diffractive imaging has been used to identify the structures of many biologically-relevant gas-phase molecules with atomic (i.e. Ångstrom, Å; 1Å = 10-10 m) spatial resolution. Unravelling the mechanisms of chemical reactions requires the capability to record multiple wellresolved snapshots of the molecular structure as it is evolving on the nuclear (i.e. femtosecond, fs; 1 fs = 10-15 s) timescale. We present the latest, state-of-the-art ultrafast electron diffraction methods used to retrieve the molecular structure of gas-phase molecules with Ångstrom and femtosecond spatio-temporal resolution. We first provide a historical and theoretical background to elastic electron scattering in its application to structural retrieval, followed by details of field-free and field-dressed ultrafast electron diffraction techniques. We discuss the application of these ultrafast methods to time-resolving chemical reactions in real-time, before providing a future outlook of the field and the challenges that exist today and in the future. I-Introduction 2 II-Elastic electron scattering 4 III-Field-free electron diffraction imaging 10 IIIa-Conventional gas-phase electron diffraction (GED) 10 IIIb-Ultrafast electron microscopy (UEM) 11 IIIc-Ultrafast electron diffraction (UED) 12 IIId-MeV relativistic UED 13 IV-Field-induced electron diffraction imaging 18 IVa-Laser-assisted electron diffraction (LAED) 18 IVb-Extended X-ray absorption fine structure (EXAFS) 19 Ivc-Mid-infrared strong-field physics 23 Ivd-Mid-infrared laser-induced electron diffraction (LIED) 26 V-Future outlook and challenges. 40 References 44 Acknowledgements 48 I-Introduction Chemical reactivity is governed by the location and interaction of electrons and nuclei within atoms and molecules. Pinpointing the location of atoms that typically have an atomic radius of ~0.5 Ångstrom (Å; 1 Å = 10-10 m) is crucial in determining bond lengths in molecules which typically span 1-3 Å [Weast1984]. For example, the H-H, C-H, and CC bond lengths in molecular hydrogen (H2) and ethane (C2H6) are 0.74 Å [Huber1979], 1.09 Å [Herzberg1966], and 1.54 Å [Herzberg1966]. The structure of gas-phase molecules can be determined with atomic (i.e. Ångstrom) spatial resolution using a variety of techniques, such as: microwave rotational spectroscopy [Gordy1970,

Physical Review A, 2013

We demonstrate an experimental method to record snapshot diffraction images of polyatomic gas-phase molecules, which can, in a next step, be used to probe time-dependent changes in the molecular geometry during photochemical reactions with femtosecond temporal and angstrom spatial resolution. Adiabatically laser-aligned 1-ethynyl-4-fluorobenzene (C 8 H 5 F) molecules were imaged by diffraction of photoelectrons with kinetic energies between 31 and 62 eV, created from core ionization of the fluorine (1s)l e v e lb y≈80 fs x-ray free-electronlaser pulses. Comparison of the experimental photoelectron angular distributions with density functional theory calculations allows relating the diffraction images to the molecular structure.

Nature communications, 2015

Laser-induced electron diffraction is an evolving tabletop method that aims to image ultrafast structural changes in gas-phase polyatomic molecules with sub-Ångström spatial and femtosecond temporal resolutions. Here we demonstrate the retrieval of multiple bond lengths from a polyatomic molecule by simultaneously measuring the C-C and C-H bond lengths in aligned acetylene. Our approach takes the method beyond the hitherto achieved imaging of simple diatomic molecules and is based on the combination of a 160 kHz mid-infrared few-cycle laser source with full three-dimensional electron-ion coincidence detection. Our technique provides an accessible and robust route towards imaging ultrafast processes in complex gas-phase molecules with atto- to femto-second temporal resolution.

Nature, 2012

Structural Dynamics, 2021

Visualizing molecular transformations in real-time requires a structural retrieval method with Ångström spatial and femtosecond temporal atomic resolution. Imaging of hydrogen-containing molecules additionally requires an imaging method sensitive to the atomic positions of hydrogen nuclei, with most methods possessing relatively low sensitivity to hydrogen scattering. Laser-induced electron diffraction (LIED) is a table-top technique that can image ultrafast structural changes of gas-phase polyatomic molecules with sub-Ångström and femtosecond spatiotemporal resolution together with relatively high sensitivity to hydrogen scattering. Here, we image the umbrella motion of an isolated ammonia molecule (NH3) following its strong-field ionization. Upon ionization of a neutral ammonia molecule, the ammonia cation (NH3+) undergoes an ultrafast geometrical transformation from a pyramidal (ΦHNH=107°) to planar (ΦHNH=120°) structure in approximately 8 femtoseconds. Using LIED, we retrieve a ...

Physical Review A, 2012

We explore the laser-induced ionization dynamics of N2 and CO2 molecules subjected to a fewcycle, linearly polarized, 800 nm laser pulse using effective two-dimensional single active electron time-dependent quantum simulations. We show that the electron recollision process taking place after an initial tunnel ionization stage results in quantum interference patterns in the energy resolved photo-electron signals. If the molecule is initially aligned perpendicular to the field polarization, the position and relative heights of the associated fringes can be related to the molecular geometrical and orbital structure, using a simple inversion algorithm which takes into account the symmetry of the initial molecular orbital from which the ionized electron is produced. We show that it is possible to extract inter-atomic distances in the molecule from an averaged photon-electron signal with an accuracy of a few percents.

Physical Review A, 2011

We address the feasibility of imaging geometric and orbital structure of a polyatomic molecule on an attosecond time-scale using the laser induced electron diffraction (LIED) technique. We present numerical results for the highest molecular orbitals of the CO2 molecule excited by a near infrared few-cycle laser pulse. The molecular geometry (bond-lengths) is determined within 3% of accuracy from a diffraction pattern which also reflects the nodal properties of the initial molecular orbital. Robustness of the structure determination is discussed with respect to vibrational and rotational motions with a complete interpretation of the laser-induced mechanisms.

Physical Review Letters, 2016

Observing the motion of the nuclear wavepackets during a molecular reaction, in both space and time, is crucial for understanding and controlling the outcome of photoinduced chemical reactions. We have imaged the motion of a vibrational wavepacket in isolated iodine molecules using ultrafast electron diffraction with relativistic electrons. The time-varying interatomic distance was measured with a precision 0.07 Å and temporal resolution of 230 fs full-width at half-maximum (FWHM). The method is not only sensitive to the position but also the shape of the nuclear wavepacket.

Physical Review Letters, 2012

Recently, using mid-infrared laser-induced electron diffraction (LIED), snapshots of a vibrating diatomic molecule on a femtosecond time-scale have been captured [C. I. Blaga et al., Nature 483, 194 (2012)]. In this Letter, a comprehensive treatment for the atomic LIED response is reported, a critical step in generalizing this imaging method. Electron-ion differential cross sections (DCS) of rare gas atoms are extracted from measured angular-resolved, high-energy electron momentum distributions generated by intense mid-infrared lasers. Following strong-field ionization, the highenergy electrons result from elastic rescattering of a field-driven wave packet with the parent ion. For recollision energies ≥100 eV, the measured DCS is indistinguishable for the neutral and ion, illustrating the close collision nature of this interaction. The extracted DCS are found to be independent of laser parameters, in agreement with theory. This study establishes the key ingredients for applying LIED to femtosecond molecular imaging.