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We report experimental results on the diffractive imaging of three-dimensionally aligned 2,5-diiodothiophene molecules. The molecules were aligned by chirped near-infrared laser pulses, and their structure was probed at a photon energy of 9.5 keV (λ ≈ 130 pm) provided by the Linac Coherent Light Source. Diffracted photons were recorded on the Cornell-SLAC pixel array detector, and a two-dimensional diffraction pattern of the equilibrium structure of 2,5-diiodothiophene was recorded. The retrieved distance between the two iodine atoms agrees with the quantum-chemically calculated molecular structure to be within 5%. The experimental approach allows for the imaging of intrinsic molecular dynamics in the molecular frame, albeit this requires more experimental data, which should be readily available at upcoming high-repetition-rate facilities.

Strong-field ionization of carbonyl sulphide (OCS) molecules, induced by a linearly polarized mid-infrared (mid-IR) probe laser pulse is investigated experimentally and theoretically. We focus on the dependence of the single-ionization yield on the alignment of the molecular axis with respect to the probe pulse polarization axis. In the experiment, the OCS molecules are 1-dimensionally adiabatically aligned and ionized by a 12-femtosecond pulse centered at 1850 nm. The alignment-dependent ionization yields are compared to the theory based on the two-step model for strong-field ionization. Overall the measurements are consistent with the theoretical results.

We report a quantitative study of the effect of nonaxial recoil during Coulomb explosion of laser-aligned molecules and introduce a method to remove the blurring caused by nonaxial recoil in the fragment-ion angular distributions. Simulations show that nonaxial recoil affects correlations between the emission directions of fragment ions differently from the effect caused by imperfect molecular alignment. The method, based on analysis of the correlation between the emission directions of the fragment ions from Coulomb explosion, is used to deconvolute the effect of nonaxial recoil from experimental fragment angular distributions. The deconvolution method is then applied to a number of experimental data sets to correct the degree of alignment for nonaxial recoil, to select optimal Coulomb explosion channels for probing molecular alignment, and to estimate the highest degree of alignment that can be observed from selected Coulomb explosion channels.

The "pixel imaging mass spectrometry" camera is used to perform femtosecond laser-induced Coulomb explosion imaging of 3,5-dibromo-3′, 5′-difluoro-4′-cyanobiphenyl molecules prealigned in space. The experiment allows the concurrent detection of the correlated two-dimensional momentum images of all the ionic fragments resulting from fragmentation of multiple molecules in each acquisition cycle. The Coulomb explosion studies provide rich information about the parent molecular structure and fragmentation dynamics, and open new opportunities for real-time imaging of intramolecular processes. © 2014 American Physical Society.

We apply a three-fold covariance imaging method to analyse previously acquired data [C. S. Slater et al., Phys. Rev. A 89, 011401(R) (2014)] on the femtosecond laser-induced Coulomb explosion of spatially pre-aligned 3,5-dibromo-3′,5′-difluoro-4′-cyanobiphenyl molecules. The data were acquired using the “Pixel Imaging Mass Spectrometry” camera. We show how three-fold covariance imaging of ionic photofragment recoil trajectories can be used to provide new information about the parent ion’s molecular structure prior to its Coulomb explosion. In particular, we show how the analysis may be used to obtain information about molecular conformation and provide an alternative route for enantiomer determination.

Coulomb explosion of 1,4--diiodobenzene molecules, isolated or embedded in helium nanodroplets, is induced by irradiation with an intense femtosecond laser pulse. The recoiling ion fragments are probed by time-of-flight measurements and two-dimensional velocity map imaging. Correlation analysis of the emission directions of ${\mathrm{I}}^{+}$ ions recoiling from each end of the molecules reveals significant deviation from axial recoil, i.e., where the ${\mathrm{I}}^{+}$ ions leave strictly along the I-I symmetry axis. For isolated molecules, the relative angular distribution of the ${\mathrm{I}}^{+}$ ions is centered at ${180}^{\ensuremath{\circ}}$, corresponding to perfect axial recoil, but with a full width at half maximum of ${30}^{\ensuremath{\circ}}$. For molecules inside He droplets, the width of the distribution increases to ${45}^{\ensuremath{\circ}}$. These results provide a direct measure of the accuracy of Coulomb explosion as a probe of the spatial orientation of molecules, which is particularly relevant in connection with laser-induced molecular alignment and orientation. In addition, our studies show how it is possible to identify fragmentation pathways of the Coulomb explosion for the isolated 1,4--diiodobenzene molecules. Finally, for the 1,4--diiodobenzene molecules in He droplets, it is shown that the angular correlation between fragments from the Coulomb explosion is preserved after they have interacted with the He environment.

Femtosecond laser-induced Coulomb-explosion imaging of 3,5-dibromo-3′,5′-difluoro-4′-cyanobiphenyl molecules prealigned in space is explored using a pixel-imaging mass-spectrometry (PImMS) camera. The fast-event-triggered camera allows the concurrent detection of the correlated two-dimensional momentum images, or covariance maps, of all the ionic fragments resulting from fragmentation of multiple molecules in each acquisition cycle. Detailed simulation of the covariance maps reveals that they provide rich information about the parent molecular structure and fragmentation dynamics. Future opportunities for imaging the real-time dynamics of intramolecular processes are considered.

The absolute configuration of chiral molecules in the gas phase is determined by femtosecond laser-induced Coulomb explosion and recording of the emission direction of critical ionic fragments. The spatial orientation of the 3,5-dibromo-3′,5′-diflouro-4′-cyanobiphenyl molecules studied is determined by a combination of laser-induced alignment and time-of-flight measurement of F+ ion recoils. Hereby, the enatiomeric state of the molecules could be uniquely identified through analysis of the correlation of the angular distribution of F+ and Br+ ions. The method will apply to a range of molecules and allow imaging of chiral changes on ultrafast time scales.

This paper gives an account of our progress towards performing femtosecond time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe setup combining optical lasers and an X-ray Free-Electron Laser. We present results of two experiments aimed at measuring photoelectron angular distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C8H5F) and dissociating, laseraligned 1,4-dibromobenzene (C6H4Br2) molecules and discuss them in the larger context of photoelectron diffraction on gas-phase molecules. We also show how the strong nanosecond laser pulse used for adiabatically laser-aligning the molecules influences the measured electron and ion spectra and angular distributions, and discuss how this may affect the outcome of future time-resolved photoelectron diffraction experiments., 24 pages, 10 figures, Faraday Discussions 171

The torsional motion of a molecule composed of two substituted benzene rings, linked by a single bond, is coherently controlled by a pair of strong ($3\ifmmode\times\else\texttimes\fi{}1{0}^{13}\text{ }\text{ }\mathrm{W}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$), nonresonant (800 nm) 200-fs-long laser pulses---both linearly polarized perpendicular to the single-bond axis. If the second pulse is sent at the time when the two benzene rings rotate toward (away from) each other the amplitude of the torsion is strongly enhanced (reduced). The torsional motion persists for more than 150 ps corresponding to approximately 120 torsional oscillations. Our calculations show that the key to control is the strong transient modification of the natural torsional potential by the laser-induced dynamic Stark effect.

Benedikt Rudek,1,2,3,* Daniel Rolles,1,3,4,* Sang-Kil Son,5 Lutz Foucar,1,4 Benjamin Erk,1,2 Sascha Epp,1,2 Rebecca Boll,1,2 Denis Anielski,1,2 Christoph Bostedt,6 Sebastian Schorb,6,7 Ryan Coffee,6 John Bozek,6 Sebastian Trippel,5 Tatiana Marchenko,8 Marc Simon,8 Lauge Christensen,9 Sankar De,10,11 Shin-ichi Wada,12 Kiyoshi Ueda,13 Ilme Schlichting,1,4 Robin Santra,5,14 Joachim Ullrich,1,2,15 and Artem Rudenko1,2,16 1Max-Planck-Advanced Study Group at CFEL, DESY, Notkestrase 85, 22607 Hamburg, Germany 2Max-Planck-Institut fur Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany 3DESY, Notkestrase 85, 22607 Hamburg, Germany 4Max-Planck-Institut fur medizinische Forschung, Jahnstrase 29, 69120 Heidelberg, Germany 5Center for Free-Electron Laser Science CFEL, DESY, Notkestrase 85, 22607 Hamburg, Germany 6SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA 7Institut fur Optik und Atomare Physik, Technische Universitat Berlin, Hardenbergstrase 36, 10623 Berlin, Germany 8Laboratoire de Chimie Physique-Matiere et Rayonnement, UPMC and CNRS, 11 rue Pierre et Marie Curie, 75231 Paris, France 9Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark 10Department of Chemistry, Aarhus University, 8000 Aarhus C, Denmark 11Saha Institute of Nuclear Physics, 700064 Kolkata, India 12Department of Physical Science, Hiroshima University, 739-8236 Higashi-Hiroshima, Japan 13IMRAM, Tohoku University, Katahira 2-1-1, Aoba-ku, 980-8577 Sendai, Japan 14Department of Physics, Universitat Hamburg, Jungiusstrase 9, 20355 Hamburg, Germany 15Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany 16J. R. Macdonald Laboratory, Kansas State University, 116 Cardwell Hall, Manhattan, Kansas 66506, USA (Received 18 January 2013; published 19 February 2013)

Journal of physics / Conference Series 388(2), 022108(2012). doi:10.1088/1742-6596/388/2/022108, Fragmentation of small organic molecules by intense few-femtosecond X-ray free-electron laser pulses has been studied using Coulomb explosion imaging. By measuring kinetic energies and emission angles of the ionic fragments in coincidence, we disentangle different fragmentation pathways, for certain cases can reconstruct molecular geometry at the moment of explosion, and show how it depends on LCLS pulse duration., Published by IOP Publ., Bristol

Ionization and fragmentation of methylselenol (${\mathrm{CH}}_{3}\mathrm{SeH}$) molecules by intense ($g{10}^{17}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$) 5 fs x-ray pulses ($\ensuremath{\hbar}\ensuremath{\omega}=2\text{ }\text{ }\mathrm{keV}$) are studied by coincident ion momentum spectroscopy. We contrast the measured charge state distribution with data on atomic Kr, determine kinetic energies of resulting ionic fragments, and compare them to the outcome of a Coulomb explosion model. We find signatures of ultrafast charge redistribution from the inner-shell ionized Se atom to its molecular partners, and observe significant displacement of the atomic constituents in the course of multiple ionization.

Naphthalene molecules are fixed in space by a laser field and rotated, in 2\ifmmode^\circ\else\textdegree\fi{} steps, over 180\ifmmode^\circ\else\textdegree\fi{}. For each orientation, they are ionized by an intense, circularly polarized femtosecond laser pulse, and the 2D projection of the photoelectron momentum distribution is recorded. The molecular-frame 3D momentum distribution is obtained by tomographic reconstruction from all 90 projections. It reveals an anisotropic electron distribution, angularly shifted in the polarization plane, that is not accessible by the 2D momentum images. Our theoretical analysis shows that the magnitude of the angular shift is very sensitive to the exact form of the laser-modified molecular potential.

We demonstrate a novel experimental implementation to strongly align molecules at full repetition rates of free-electron lasers. We utilized the available in-house laser system at the coherent x-ray imaging beamline at the Linac Coherent Light Source. Chirped laser pulses, i. e., the direct output from the regenerative amplifier of the Ti:Sa chirped pulse amplification laser system, were used to strongly align 2,5-diiodothiophene molecules in a molecular beam. The alignment laser pulses had pulse energies of a few mJ and a pulse duration of 94 ps. A degree of alignment of $\left$ = 0.85 was measured, limited by the intrinsic temperature of the molecular beam rather than by the available laser system. With the general availability of synchronized chirped-pulse-amplified near-infrared laser systems at short-wavelength laser facilities, our approach allows for the universal preparation of molecules tightly fixed in space for experiments with x-ray pulses., Comment: 10 pages, 5 figures

Static and time-resolved photoelectron diffraction of laser-aligned polyatomic molecules photoionized by Free-Electron Laser pulses is presented. The results are a proof-of-principle for imaging dynamic structural changes in molecules with femtosecond temporal and Angstrom spatial resolution.