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[en] The advent of the first X-ray Free-Electron Laser, the Linac Coherent Light Source (LCLS), opens up a new approach for diffractive imaging of even single molecules that cannot be crystallized into macromolecular crystals of sufficient size necessary for conventional X-ray crystallography. Here, we present the concept, the experimental parametric space that has to be addressed together with first experimental results of X-ray diffractive imaging of single molecules in the gas phase at LCLS. We use a supersonically cooled molecular beam to provide an ensemble of test-molecules, laser-align them, and subsequently probe them with the LCLS in order to get diffraction patterns of single molecules.
[en] We study the focusing of large neutral molecules in a molecular beam using electric fields. Since all quantum states of these molecules are high-field seeking under the practical experimental conditions, alternating gradient (AG) focusing has to be applied. The optimal ac frequency that yields the highest transmission depends on m/μ, where μ is the dipole moment and m denotes the mass. Therefore, an AG focuser can be used to select species with different m/μ ratios, e.g., the conformers of neutral molecules [Phys. Rev. Lett. 100, 133003 (2008)]. Here we demonstrate both theoretically and experimentally how the resolution of such an m/μ selector can be optimized.
[en] Photoelectron momentum distributions from strong-field ionization of carbonyl sulfide with 800 nm central-wavelength laser pulses at various peak intensities from 4.6 to 13 × 1013 W cm−2 were recorded and analyzed regarding resonant Rydberg states and photoelectron orbital angular momentum. The evaluation of the differentials of the momentum distributions with respect to the peak intensity highly suppressed the impact of focal volume averaging and allowed for the unambiguous recognition of Freeman resonances. As a result, previously made assignments of photoelectron lines could be reassigned. An earlier reported empirical rule, which relates the initial state's orbital momentum and the minimum photon expense to ionize an ac Stark shifted atomic system to the observable dominant photoelectron orbital momentum, was confirmed for the molecular target. (paper)
[en] We report on the production of a pulsed molecular beam of metastable NH (a1Δ) radicals and present first results on the Stark deceleration of the NH (a1Δ, J = 2, MΩ = -4) radicals from 550 m s-1 to 330 m s-1. The decelerated molecules are excited on the spin-forbidden A3Π <- a1Δ transition, and detected via their subsequent spontaneous fluorescence to the X3Σ-, v = 0 ground state. These experiments demonstrate the feasibility of our recently proposed scheme (2001 Phys. Rev. A 64 041401) to accumulate ground-state NH radicals in a magnetic trap. In addition, we propose to transfer the NH radicals from the a1Δ state to the X3Σ- state using a cw-laser to allow cooling of the beam during trap loading
[en] We demonstrate feedback control optimization for the Stark deceleration and trapping of neutral polar molecules using evolutionary strategies. In a Stark-decelerator beamline, pulsed electric fields are used to decelerate OH radicals and subsequently store them in an electrostatic trap. The efficiency of the deceleration and trapping process is determined by the exact timings of the applied electric field pulses. Automated optimization of these timings yields an increase of 40% of the number of trapped OH radicals
[en] By miniaturizing electrode geometries high electric fields can be produced using modest voltages. A planar array of 20 μm wide gold electrodes, spaced 20 μm apart, is made on a sapphire substrate. A voltage difference of up to 350 V is applied to adjacent electrodes, generating an electric field that decreases exponentially with distance from the substrate. This microstructured array can be used as a mirror for polar molecules and can be rapidly switched on and off. This is demonstrated by retroreflecting a beam of state-selected ammonia molecules with a forward velocity of about 30 m/s
[en] A decelerated beam of neutral ammonia molecules is injected into an electrostatic storage ring. Electrostatic lenses are used to map the emittance of the decelerator onto the acceptance of the ring. The tangential velocity spread of the package of molecules in the ring is set to less than 1 m/s. The package of molecules can be observed for more than 50 distinct round trips, corresponding to 40 m in circular orbit and almost 0.5 s storage time, sufficiently long for a first investigation of its transversal motion in the ring
[en] Full text: Optical trapping of light absorbing particles in gas environment has shown to be governed by optically induced thermal or photophoretic forces. The applied photophoretic force propels a particle when an incident light beam nonuniformly heats the surface of the particle. Optical guiding of absorbing particles by photophoretic force over large distances in open air was recently realised by an optical pipeline. It was performed using a vortex beam, a donut-like intensity structure with a high-intensity ring of light that surrounds a dark core. Optical vortices create a ring-shaped transverse intensity distribution, while the particles are trapped at the intensity minima. Photophoretic force depends on the particle size relative to the mean-free path of gas molecules, absorption of laser light, and thermal conductivity. In the presented levitation experiments the thermal force is two to three orders of magnitude larger than the radiation–pressure force, rendering opaque particles impervious to conventional optical traps. Here we present the results on quantitative evaluation of the photophoretic force and trapping stiffness by levitating graphite particles and carbon-coated glass shells of calibrated sizes in a vertically directed diverging hollow-core vortex beam. The axial equilibrium position of a suspended particle with known mass in the diverging beam with known intensity distribution immediately provides the axial force acting on the particle. Changing the beam power changes the intensity on the particle surface, which in turn force the particle to find a new equilibrium position. In the transverse plane. The trap’s stiffness was measured using well-established position tracking with a fast CCD. By repeating such experiments with different particle sizes and at different pressure we calibrate the force acting on the particle in the beam. The measurements were conducted at various air pressures from 0.1-1.5 bar and compared with the light pressure forces evaluated for the known intensity illuminating the particle. The forces recovered from our experiments range around ~10-10 N per Watt of total power in the beam, at atmospheric pressures, see Fig. 1, with force increasing near linearly with decrease of pressure. The maximum intensity encountered by the particle is ~30 kW/cm2, given a maximum particle radius to vortex ‘ring’ radius of 1/30. Intensities of this order of magnitude are suitable for handling biological samples without damage. The results provide grounds to link the position and size of a spherical particle in an arbitrary intensity distribution to the value of photophoretic and light pressure forces under various gas pressures. (author)
[en] Many molecules exhibit multiple conformers that often easily interconvert under thermal conditions. Therefore, single conformations are difficult to isolate which renders the study of their distinct chemical reactivities challenging. We have recently reported a new experimental method for the characterization of conformer-specific effects in chemical reactions [Y.-P. Chang, K. Długołęcki, J. Küpper, D. Rösch, D. Wild, and S. Willitsch, “Specific chemical reactivities of spatially separated 3-aminophenol conformers with cold Ca+ ions,” Science 342, 98–101 (2013)]. Different conformers are spatially separated using inhomogeneous electric fields and reacted with a Coulomb crystal of cold, spatially localized ions in a trap. As a first application, we studied reactions between the two conformers of 3-aminophenol and Ca+. We observed a twofold larger rate constant for the cis compared to the trans conformer which was rationalized in terms of the differences in the long-range ion-molecule interactions. The present article provides a detailed description of the new method and a full account of the experimental results as well as the accompanying theoretical calculations