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In this report, we applied annular bright-field and annular dark-field low-energy (30 keV) scanning transmission electron microscopy imaging to a vitreous ice-embedded biological macromolecule, T4 phage, to investigate the applicability of these methods for morphological investigation and sample screening. Multiple camera lengths were examined to find the optimal acceptance angle for both modes. Image clarity differed substantially between the modes, with the presence of ice also strongly influencing the quality of acquired micrographs. In annular dark-field mode, the proper discrimination of electrons scattered by the specimen from those scattered by the background ice was found to be difficult due to the severe overlap of the scattered electrons. The resulting micrographs lacked clarity, and the ice-embedded phage particles could only be discerned after post-processing image adjustment. However, in annular bright-field mode, despite similar overlapping of the scattered electrons, it was possible to assess the morphology and intactness of the specimen in the embedding ice, suggesting that this mode may find utility in low-energy cryo-scanning transmission electron microscopy imaging methods.

In this work, we have explored the factors which govern mean free path values obtained from off-axis electron holography measurements. Firstly, we explore the topic from a theoretical perspective, and show that the mean amplitude reconstructed from off-axis holograms is due to the coherent portion of the direct, central object-transmitted beam only – it is not affected by the presence or absence of other scattered beams. Secondly, we present a detailed experimental study which compares mean free path values obtained from hologram sideband, centreband, EELS, and TEM measurements as a function of optical collection angle and energy-loss-filtering. These results confirm that the coherent portion of the direct beam defines the mean amplitude, and additionally show that the coherent portion corresponds to the conventional energy-filtered signal (with threshold 5 eV in this work). Finally, we present summary measurements from a selection of different materials, and compare the results against a simple electron scattering model. This study reinforces the claim that the mean amplitude is defined by the energy-filtered direct beam, and confirms that the contributions of elastic and inelastic scattering to the total mean free path are broadly in line with theoretical expectations for these different materials. These results in aggregate indicate that neither experimental collection angles nor enhanced sensitivity to low-loss phonon scattering affect the mean amplitude signal arising from off-axis holography reconstructions, nor the associated mean free path values which are derived from this mean amplitude.

For many macromolecular complexes, the inability to uniformly disperse solubilized specimen particles within vitreous ice films precludes their analysis by cryo-electron microscopy (cryo-EM). Here, we introduce a sample preparation process using "perpetually-hydrated" graphene oxide flakes as particle support films, and report vastly improved specimen dispersion. Furthermore, we provide evidence that the presence of graphene oxide flakes in vitreous ice results in a significant reduction in electron beam-induced specimen decomposition. The new method introduced in this study incorporates hydrated graphene oxide flakes into a standard sample preparation regime, without the need for additional tools or devices, making it a cost-effective and easily adoptable alternative to currently available sample preparation approaches.

A new SEM-based in-line electron holography microscope has been under development. The microscope utilizes conventional SEM and BF-STEM functionality to allow for rapid searching of the specimen of interest, seamless interchange between SEM, BF-STEM and holographic imaging modes, and makes use of coherent low-energy in-line electron holography to obtain low-dose, high-contrast images of light element materials. We report here an overview of the instrumentation and first experimental results on gold nano-particles and carbon nano-fibers for system performance tests. Reconstructed images obtained from the holographic imaging mode of the new microscope show substantial image contrast and resolution compared to those acquired by SEM and BF-STEM modes, demonstrating the feasibility of high-contrast imaging via low-energy in-line electron holography. The prospect of utilizing the new microscope to image purified biological specimens at the individual particle level is discussed and electron optical issues and challenges to further improve resolution and contrast are considered.

The scanning transmission electron microscopy (STEM) mode of today's field emission scanning electron microscopes enables sub-nanometer resolution imaging. Graphene is a single-atom thick, electrically conductive material, making it an excellent specimen support for the low voltage STEM imaging of nanometer-sized objects such as viruses. Here we present low voltage STEM images of bacteriophage T4 recorded on highly cleaned graphene films. The results show that ultrathin graphene support films markedly improve image signal at low accelerating voltages. Staining with a low atomic number methylamine vanadate stain combined with the graphene support film enables the clear visualization of the fine structure of the T4 tail by the low voltage STEM technique. Despite the advantages of graphene support films, difficulties are often encountered in placing hydrophilic biological samples on hydrophobic graphene electron microscopy grids. We employed a spin sedimentation sample loading method to overcome this problem.

The mean free path governing the scattering of high-energy electrons in cadmium telluride (CdTe) has been measured and analyzed using off-axis electron holography (OEH). In the first part of the study, the total mean free path value was determined via acquisition and aggregation of a large off-axis holography dataset at 300 kV and room temperature, yielding the value λ O E H = 52 ± 7 nm. This is significantly shorter than some previously reported values obtained via different experimental techniques and theoretical calculations. To confirm the validity of the measurement and to understand the underlying physical scattering processes, the study was extended to systematically investigate the role of electron energy loss, electron scattering angle, and specimen temperature in the overall holography measurement. This allowed the observed mean free path value to be clearly decomposed into terms of electronic (inelastic) and nuclear (elastic) scattering processes in the material and enabled direct measurement of the relevant contributions. Specifically, the determined attenuation coefficients were μ i n e l ( Δ E > 5 eV ) = 5.9 ± 1.2 μ m − 1 and μ e l ( Δ E 3 mrad ) = 13.5 ± 1.2 μ m − 1 (full details in the main text). With appropriate consideration of the relevant scattering mechanisms, the mean free path value determined here from off-axis holography measurements is consistent with prior experimental measurements from other techniques and theoretical calculations. These insights and measurements should be of future value for quantitative holography and electron beam scattering experiments in CdTe.

Cryo-electron microscopy (cryo-EM) has become the method of choice in the field of structural biology, owing to its unique ability to deduce structures of vitreous ice-embedded, hydrated biomolecules over a wide range of structural resolutions. As cryo-transmission electron microscopes (cryo-TEM) become increasingly specialised for high, near-atomic resolution studies, operational complexity and associated costs serve as significant barriers to widespread usability and adoptability. To facilitate the expansion and accessibility of the cryo-EM method, an efficient, user-friendly means of imaging vitreous ice-embedded biomolecules has been called for. In this study, we present a solution to this issue by integrating cryo-EM capabilities into a commercial scanning electron microscope (SEM). Utilising the principle of low-energy in-line electron holography, our newly developed hybrid microscope permits low-to-moderate resolution imaging of vitreous ice-embedded biomolecules without the need for any form of sample staining or chemical fixation. Operating at 20 kV, the microscope takes advantage of the ease-of-use of SEM-based imaging and phase contrast imaging of low-energy electron holography. This study represents the first reported successful application of low-energy in-line electron holographic imaging to vitreous ice-embedded small biomolecules, the effectiveness of which is demonstrated here with three morphologically distinct specimens.

© 2016 American Physical Society. Ab initio theoretical studies and momentum-imaging experiments are combined to provide a consistent picture of the dynamics of dissociative electron attachment to ammonia through its 5.5- and 10.5-eV resonance channels. The present study clarifies the character and symmetry of the anion states involved and the dynamics that leads to the observed fragment-ion channels, their branching ratios, and angular distributions.

We report on fully differential cross sections for double ionization of H2 by a single circularly polarized photon of 160 eV energy. For an unequal sharing of the energy between the two electrons and a particular geometry where the influence of electron/electron interaction is constant, we find a four-lobe structure in the molecular frame angular distribution of the faster electron. This structure is interpreted to be due to a coherent emission of the electron from the two atomic centers of the molecule. This Young-type interference pattern is lost for other geometries, where electron-electron interaction plays a major role. Furthermore, we show that the interference structure depends strongly on the internuclear distance.

We present a combined experimental and theoretical investigation of the dynamics and angular dependence of dissociative electron attachment to methane. We show that a triply degenerate (T2) Feshbach resonance is responsible for the broad 10 eV dissociation peak in methane. This resonance alone is shown to correlate asymptotically to the various dissociation channels observed experimentally. The molecular-frame entrance amplitude for electron attachment is calculated for each component of the threefold degenerate resonance. By investigating the topology of the anion potential energy surfaces, we deduce the main pathways to two- and three-body breakup channels involving both bond scission and bond formation. The computed fragment angular distributions reproduce the main trends of the experimental measurements.

We have performed open cell transmission electron microscopy experiments through pure water vapor in the saturation pressure regime (>0.6 kPa), in a modern microscope capable of sub-Å resolution. We have systematically studied achievable pressure levels, stability and gas purity, effective thickness of the water vapor column and associated electron scattering processes, and the effect of gas pressure on electron optical resolution and image contrast. For example, for 1.3 kPa pure water vapor and 300kV electrons, we report pressure stability of ± 20 Pa over tens of minutes, effective thickness of 0.57 inelastic mean free paths, lattice resolution of 0.14 nm on a reference Au specimen, and no significant degradation in contrast or stability of a biological specimen (M13 virus, with 6 nm body diameter). We have also done some brief experiments to confirm feasibility of loading specimens into an in situ water vapor ambient without exposure to intermediate desiccating conditions. Finally, we have also checked if water experiments had any discernible impact on the microscope performance, and report pertinent vacuum and electron optical data, for reference purposes.

Ion Momentum Imaging of Dissociative Electron Attachment Dynamics in Methanol D.S. Slaughter, 1, ∗ D.J. Haxton, 1 H. Adaniya, 1 T. Weber, 1 T.N. Rescigno, 1 C.W. McCurdy, 1 and A. Belkacem 1 Lawrence Berkeley National Laboratory, Chemical Sciences, Berkeley, California 94720,USA A combined experimental and theoretical investigation of the dissociative electron attachment (DEA) dynamics in methanol are presented for the Feshbach resonance at 6.5 eV incident electron energy. Highly-differential laboratory-frame momentum distributions have been measured for each fragmentation channel using a DEA reaction microscope. These measurements are combined with calculations of the molecular frame electron attachment probability in order to investigate the dy- namics of the dissociating methanol transient negative anion. In contrast to previous comparisons between water and methanol [1, 2] we find subtle differences in the dissociation dynamics of the two fragment channels that are direct evidence of planar symmetry-breaking of warm methanol in its electronic ground state. We also find that the DEA fragmentation does not strictly follow the axial recoil approximation and we describe the dynamics that enable an accurate prediction of the fragment angular distributions. PACS numbers: 34.80.Ht I. INTRODUCTION Low-energy free electrons are widely considered to play an important role in the radiation-induced chemistry of bio-molecules [3] and organic chemistry in the interstellar medium [4]. Dissociative electron attachment (DEA) is one of the primary fundamental interactions that drives free electron chemistry, and attracts considerable inter- est, not only for the need to understand electron-induced molecular breakup and negative ion production in nature, but also in systems of technological interest [5]. Both gaseous methanol and its ice are ubiquitous in interstellar clouds and comets, but the mechanisms of methanol synthesis in these systems are not well under- stood, although low-energy electrons are likely to play an important role [6, 7]. Free-electron driven chemistry also provides many reaction pathways leading to more complex molecules of which methanol may be an inter- mediate [4]. On Earth, methanol has widely been pro- posed as a potential large-scale alternative to fossil fuels, as it can be produced both agriculturally and syntheti- cally. Today, industrial production of methanol is typi- cally achieved by reaction of synthesis gas (mixtures of H 2 , CO 2 and CO) in the presence of a Cu/Zn/Al 2 O 3 cat- alyst at high temperatures and pressures [8]. A similar scheme has been proposed for storage of CO 2 and elec- trical energy [9] and it is possible that DEA or the time- reverse process, associative detachment, play important roles in these industrial applications. Several experimental studies have been conducted on DEA to methanol in the past, including measurements of the translational kinetic energy release (KER) [1, 10] and velocity slice imaging [11] of the dissociation prod- ucts. K¨ uhn et al. [10] found evidence of hydrogen scram- bling in the production of OH − in the 10.5 eV resonance in their measurements of DEA relative cross sections and DSSlaughter@lbl.gov KER spectra in methanol over electron energies spanning 0 eV to 17 eV. Curtis and Walker [1] extended the study to the H − ion channel, making comparisons between the three Feshbach resonances in methanol and the Fesh- bach resonances in the relatively well-understood water molecule,[1, 11–20] in addition to comparing the DEA spectra to the corresponding parent Rydberg states in both VUV photo-absorption spectra and near-threshold electron energy loss spectra. They identified each of the first two Feshbach resonances to be due to promotion of an electron from either of the highest lying occupied or- bitals, (7a ) 2 or (2a ) 2 , of methanol in its ground state, having planar (C s ) symmetry, and simultaneous elec- tron attachment to the 3s Rydberg orbital; the 6.5 eV and 8 eV resonances being 2 A [(2a ) 1 (3s) 2 ] and 2 A [(7a ) 1 (3s) 2 ], respectively. More recently, Ib˘anescu et al. [21] provided further insight on these resonances by comparing their high- precision measurements of DEA ion yields, photoelectron spectra and vibrational excitation cross-sections from several alcohols, characterizing a previously unknown shape resonance at around 3 eV. Ib˘anescu and Allan [22] followed this with a time-dependent density functional theory description of the dynamics of the first two Ry- dberg states in methanol, predicting that the dynamics of dissociation of the 1 2 A Feshbach resonance in the methanol anion would follow the dynamics of the parent Rydberg excited state [23] of the molecule. Prediction and measurement of the dynamics of the methanol anion dissociation processes are formidable challenges from both theoretical and experimental per- spectives. In general, detailed measurement and analysis of the dynamics of DEA to polyatomic molecules have, until recently, been limited to small molecules containing only a few atoms. Our understanding of multidimen- sional dynamics resulting from DEA are hindered by the few elements of symmetry and many degrees of freedom that exist in larger polyatomics. Methanol is one of the simplest systems that possesses relatively weak planar symmetry and many vibrational modes, presenting an ac

We describe an experimental approach to image the three-dimensional (3D) momentum distribution of the negative ions arising from dissociative electron attachment (DEA). The experimental apparatus employs a low energy pulsed electron gun, an effusive gas source and a 4π solid-angle ion momentum imaging spectrometer consisting of a pulsed ion extraction field, an electrostatic lens, and a time- and position-sensitive detector. The time-of-flight and impact position of each negative ion are measured event by event in order to image the full 3D ion momentum sphere. The system performance is tested by measuring the anion momentum distributions from two DEA resonances, namely H(-) from H(2)O(-) ((2)B(1)) and O(-) from O(2)(-) ((2)Π(u)). The results are compared with existing experimental and theoretical data.

We have used cold target recoil ion momentum spectroscopy to study the continuum correlation between the photoelectron of core-photoionized neon and the subsequent Auger electron. We observe a strong angular correlation between the two electrons. Classical trajectory Monte Carlo calculations agree quite well with the photoelectron energy distribution that is shifted due to the potential change associated with Auger decay. However, a striking discrepancy results in the distribution of the relative angle between Auger and photoelectron. The classical model predicts a shift in photoelectron flux away from the Auger emission direction, and the data strikingly reveal that the flux is lost rather than diverted, indicating that the two-step interpretation of photoionization followed by Auger emission is insufficient to fully describe the core-photoionization process.

We investigate photo-double-ionization of ${\mathrm{H}}_{2}$ by circular polarized photons at $h\ensuremath{\nu}=160\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The previously observed two-center interference pattern in the angular distribution of the electron sum momentum is shown to depend strongly on the internuclear distance and the summed electron momenta.

We investigate single-photon double ionization of H(2) by 130 to 240 eV circularly polarized photons. We find a double slitlike interference pattern in the sum momentum of both electrons in the molecular frame which survives integration over all other degrees of freedom. The difference momentum and the individual electron momentum distributions do not show such a robust interference pattern. We show that this interference results from a non-Heitler-London fraction of the H(2) ground state where both electrons are at the same atomic center.

The wave nature of particles is rarely observed, in part because of their very short de Broglie wavelengths in most situations. However, even with wavelengths close to the size of their surroundings, the particles couple to their environment (for example, by gravity, Coulomb interaction, or thermal radiation). These couplings shift the wave phases, often in an uncontrolled way, and the resulting decoherence, or loss of phase integrity, is thought to be a main cause of the transition from quantum to classical behavior. How much interaction is needed to induce this transition? Here we show that a photoelectron and two protons form a minimum particle/slit system and that a single additional electron constitutes a minimum environment. Interference fringes observed in the angular distribution of a single electron are lost through its Coulomb interaction with a second electron, though the correlated momenta of the entangled electron pair continue to exhibit quantum interference., This work was supported by the Deutsche Forschungsgemeinschaft and by the Office of Basic Energy Sciences, Division of Chemical Sciences of the U. S. Department of Energy under contract DE-AC03-76SF00098.

With a femtosecond laser pulse we rapidly ionize potassium atoms (K{sup 0}) in the gas phase, generating potassium ions (K{sup +}), and monitor the altered energy-level scheme with a subsequent hard x-ray pulse. Removal of the potassium 4s valence electron increases the binding energies of both the valence and the 1s core levels, and induces an ultrafast change of the 1s-4p x-ray transition energy by about 2.8 eV. We simultaneously observe a 50% increase in oscillator strength of K{sup +} over K{sup 0} for that transition.

We have measured the charge-state distributions of argon and potassium after ionization by photons with energies near the K-shell ionization threshold. Despite the similarity in core electron configurations, the two atoms show remarkable differences in the resulting distribution of ion charge states. The valence electron in potassium is rarely a spectator during core relaxation, and its presence enhances the loss of electrons excited into Rydberg levels or strongly reduces the recapture of slow photoelectrons during postcollision interaction.

We report the results of our combined experimental and theoretical investigation on the dynamics of dissociative electron attachment to the triatomic molecule CO2. The experiments employ a 4π momentum spectrometer to collect kinematically complete data for this 2-body dissociation reaction. The angular dependence of the attachment entrance amplitudes, calculated by ab initio methods, connect the laboratory frame to the molecular frame. Preliminary results from our ongoing investigations of DEA to larger molecules, including formic acid and the nucleobases uracil and and thymine, will also be presented.

In the collision of electrons with molecules and molecular ions, excitation and dissociation are dominated by resonant processes, where the electron becomes temporarily trapped, changing the forces felt by the nuclei. We have carried out calculations on the resonant process leading to dissociative attachment and dissociative recombination. We separate the problem into two steps. First, the resonance parameters are obtained from accurate electron scattering calculations using the Complex Kohn variational method. Then these parameters are used as input to the dynamics calculations. We will illustrate the method with two examples, dissociative attachment in CO2 and dissociative recombination in O+2.