We explore time-resolved Coulomb explosion induced by intense, extreme ultraviolet (XUV) femtosecond pulses from a free-electron laser as a method to image photo-induced molecular dynamics in two molecules, iodomethane and 2,6-difluoroiodobenzene. At an excitation wavelength of 267 nm, the dominant reaction pathway in both molecules is neutral dissociation via cleavage of the carbon–iodine bond. This allows investigating the influence of the molecular environment on the absorption of an intense, femtosecond XUV pulse and the subsequent Coulomb explosion process. We find that the XUV probe pulse induces local inner-shell ionization of atomic iodine in dissociating iodomethane, in contrast to non-selective ionization of all photofragments in difluoroiodobenzene. The results reveal evidence of electron transfer from methyl and phenyl moieties to a multiply charged iodine ion. In addition, indications for ultrafast charge rearrangement on the phenyl radical are found, suggesting that time-resolved Coulomb explosion imaging is sensitive to the localization of charge in extended molecules.

In pump-probe experiments employing a free-electron laser (FEL) in combination with a synchronized optical femtosecond laser, the arrival-time jitter between the FEL pulse and the optical laser pulse often severely limits the temporal resolution that can be achieved. Here, we present a pump-probe experiment on the UV-induced dissociation of 2,6-difluoroiodobenzene (C 6 H 3 F 2 I) molecules performed at the FLASH FEL that takes advantage of recent upgrades of the FLASH timing and synchronization system to obtain high-quality data that are not limited by the FEL arrival-time jitter. We discuss in detail the necessary data analysis steps and describe the origin of the timedependent effects in the yields and kinetic energies of the fragment ions that we observe in the experiment.

Laser-induced adiabatic alignment and mixed-field orientation of 2,6-difluoroiodobenzene (C6H3F2I) molecules are probed by Coulomb explosion imaging following either near-infrared strong-field ionization or extreme-ultraviolet multi-photon inner-shell ionization using free-electron laser pulses. The resulting photoelectrons and fragment ions are captured by a double-sided velocity map imaging spectrometer and projected onto two position-sensitive detectors. The ion side of the spectrometer is equipped with the Pixel Imaging Mass Spectrometry (PImMS) camera, a timestamping pixelated detector that can record the hit positions and arrival times of up to four ions per pixel per acquisition cycle. Thus, the time-of-flight trace and ion momentum distributions for all fragments can be recorded simultaneously. We show that we can obtain a high degree of one-and three-dimensional alignment and mixedfield orientation, and compare the Coulomb explosion process induced at both wavelengths. 2 IntroductionUltrafast lasers provide opportunities to image molecular dynamics taking place on the femtosecond timescale [Baumert 1991, Zewail 2000, Chergui 2009. Table-top Ti:Sapphire laser systems are the most commonly used ultrafast laser systems, producing radiation in the near-infrared (NIR) range. High-intensity femtosecond NIR pulses can rapidly remove several valence electrons from a molecule, producing a multiply charged molecular ion that explodes due to the Coulomb repulsion between its components. The resulting recoil velocities and directions of the product ions depend on the position of the atoms in the molecule before ionization, and consequently can provide structural information about the molecule [Vager 1989, Stapelfeldt 1995, Posthumus 1996, Hishikawa 1998, Sanderson 1999. They can also be used to determine the orientation of molecules in the laboratory frame, for example, to probe the degree of molecular alignment induced by intense laser fields [Stapelfeldt 2003], or to probe structural changes of the molecule in time-resolved experiments [Legare 2005, Hishikawa 2007, Matsuda 2011, Bocharova 2011, Ibrahim 2014, Christensen 2014.Absorption of extreme ultraviolet (XUV) and soft X-ray photons can also induce Coulomb explosion when the resulting inner-shell ionization is followed by an Auger process that leads to a multiply charged molecular ion [Muramatsu 2002, Ueda 2005, Ullrich 2012, Erk 2014, Murphy 2014, Ablikim 2016, Ablikim 2017. Free-electron lasers (FELs) produce extremely intense (>10 12 photons/pulse) and ultrashort (few to few hundred fs) pulses of XUV and X-ray radiation [Ackermann 2007, Shintake 2008, Emma 2010, Allaria 2012, Ishikawa 2012, unlocking opportunities to probe ultrafast processes in gas-phase molecules through time-resolved Coulomb explosion imaging experiments [Johnsson 2009, Jiang 2010, Ullrich 2012, Schnorr 2013, Rouzee 2013, Erk 2014, Schnorr 2014, Fang 2014, Rudenko 2015, Liekhus 2015, Picon 2015, Lehmann 2016, Boll 2016. A good understanding of the Coulomb explosion of polyatomic...

Photoelectron Angular Distributions (PADs) resulting from 800 nm and 1300 nm strong field ionization of impulsively aligned CF3I molecules were analyzed using time-dependent density functional theory (TDDFT). The normalized difference between the PADs for aligned and anti-aligned molecules displays large modulations in the high-energy re-collision plateau that are assigned to the diffraction of back-scattered photoelectrons. The TDDFT calculations reveal that, in spite of their 2.6 eV energy difference, ionization from the HOMO-1 orbital contributes to the diffraction pattern on the same footing as ionization from the doubly degenerate HOMO orbital.Following structural changes within single molecules on their natural time and length scales is one of the great challenges in ultrafast molecular physics. Large efforts are currently devoted to the development of techniques for the direct imaging of nuclear motion with atomic resolution. Diffractive imaging methods using ultrashort X-ray pulses available at Free Electron Lasers [1, 2], or using ultrashort electron pulses [3][4][5], have the potential to record structural information with the spatiotemporal resolution required for obtaining "molecular movies" [3,[5][6][7]. In both approaches however, realizing single molecule imaging with sub-10 fs temporal resolution has proven challenging [8,9], since the required synchronization between the visible/ultra-violet laser pulses initiating the molecular dynamics of interest and the Xray/UED probe is difficult to achieve.Fully laser-based molecular self-imaging techniques using strong field ionization by an intense infrared (IR) laser pulse are an alternative and promising route towards the imaging of (time-dependent) molecular structures in the gas phase [10]. In particular, Laser-Induced Electron Diffraction (LIED) [11][12][13][14], where the ionization of a molecule by a strong IR laser field leads to the creation of a photoelectron wavepacket that is accelerated by the laser field to induce a recollision with the parent molecular ion, has already demonstrated fewfemtosecond and sub-Ångström resolution [15][16][17]. The time resolution in LIED is given by the optical cycle of the driving laser field [15,17] and can reach the subfemtosecond timescale, whereas high spatial resolution is possible due to the high kinetic energy of the re-colliding photoelectron, which determines its De Broglie wavelength and can reach values of 0.1Å when using midinfrared laser fields.Retrieval of the molecular structure from an LIED experiment is often done in the framework of the Quan-titative Rescattering Theory (QRT) [13,18,19], which usually assumes that (i) the ionization takes place from the Highest Occupied Molecular Orbital (HOMO) and that (ii) the initial shape of the electron wavepacket is lost during its propagation in the oscillatory laser field, so that the re-colliding electron wavepacket can be approximated by a plane wave. Both of these assumptions may be questioned. Strong field ionization, in particular of polyatomic mol...