Effect of beam offset on the characteristics of copper/304stainless steel electron beam welding

This is a review of atomic pair distribution function (PDF) analysis as applied to the study of molecular materials. The PDF method is a powerful approach to study short- and intermediate-range order in materials on the nanoscale. It may be obtained from total scattering measurements using X-rays, neutrons, or electrons, and it provides structural details when defects, disorder, or structural ambiguities obscure their elucidation directly in reciprocal space. While its uses in the study of inorganic crystals, glasses, and nanomaterials have been recently highlighted, significant progress has also been made in its application to molecular materials such as carbons, pharmaceuticals, polymers, liquids, coordination compounds, composites, and more. Here, an overview of applications toward a wide variety of molecular compounds (organic and inorganic) and systems with molecular components is presented. We then present pedagogical descriptions and tips for further implementation. Successful utilization of the method requires an interdisciplinary consolidation of material preparation, high quality scattering experimentation, data processing, model formulation, and attentive scrutiny of the results. It is hoped that this article will provide a useful reference to practitioners for PDF applications in a wide realm of molecular sciences, and help new practitioners to get started with this technique.

Structural Analysis of Molecular Materials Using the Pair Distribution Function.

The Landau--Zener (LZ) type classical-trajectory surface-hopping algorithm is applied to the nonadiabatic nuclear dynamics of the ammonia cation after photoionization of the ground-state neutral molecule to the excited states of the cation. The algorithm employs the recently proposed formula for nonadiabatic LZ transition probabilities derived from the adiabatic potential energy surfaces. The evolution of the populations of the ground state and the two lowest excited adiabatic states is calculated up to 200 fs. The results agree well with quantum simulations available for the first 100 fs based on the same potential energy surfaces. Four different time scales are detected for the nuclear dynamics: Ultrafast Jahn--Teller dynamics between the excited states on a 5 fs time scale; fast transitions between the excited state and the ground state within a time scale of 20 fs; relatively slow partial conversion of a first-excited-state population to the ground state within a time scale of 100 fs; and nearly constant populations after roughly 120 fs due to a dynamical equilibrium between all three states. The latter provides a possible explanation of the experimental evidence that ammonia cation is nonfluorescent.

Nonadiabatic nuclear dynamics of the ammonia cation studied by surface hopping classical trajectory calculations

LINE INTENSITIES AND MOLECULAR OPACITIES OF THE FeH F ^4Delta~i-X ^4Delta~i TRANSITION

Simulations of nonresonant ultrafast x-ray scattering from a molecular wave packet in ${\mathrm{H}}_{2}$ are used to examine and classify the components that contribute to the total scattering signal. The elastic component, which can be used to determine the structural dynamics of the molecule, is also found to carry a strong signature of an adiabatic electron transfer that occurs in the simulated molecule. The inelastic component, frequently assumed to be constant, is found to change with the geometry of the molecule. Finally, a coherent mixed component due to interferences between different inelastic transitions is identified and shown to provide a direct probe of transient electronic coherences.

/pdf/electronic-coherence-in-ultrafast-x-ray-scattering-from-knovpcmeyx.pdf

Electronic Coherence in Ultrafast X-Ray Scattering from Molecular Wave Packets

Vibronically resolved squared form factors of ${a}^{\ensuremath{'}\ensuremath{'}}{\phantom{\rule{0.16em}{0ex}}}^{1}{{\ensuremath{\Sigma}}_{g}}^{+}$, $b{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Pi}}_{u}$, ${b}^{\ensuremath{'}}{\phantom{\rule{0.16em}{0ex}}}^{1}{{\ensuremath{\Sigma}}_{u}}^{+}$, ${c}_{3}{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Pi}}_{u}$, ${c}_{4}^{\ensuremath{'}}{\phantom{\rule{0.16em}{0ex}}}^{1}{{\ensuremath{\Sigma}}_{u}}^{+}$, and ${o}_{3}{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Pi}}_{u}$ of molecular nitrogen are determined by inelastic x-ray scattering with a high resolution of 70 meV. Through a comparison with the previous results measured by electron energy loss spectroscopy, it is found that the first Born approximation is satisfied at an incident electron energy of 300 eV in the small momentum transfer region for a dipole-allowed transition of $b{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Pi}}_{u}$, but the first Born approximation is not reached at an impact energy of 500 eV for the dipole-forbidden transition of ${a}^{\ensuremath{'}\ensuremath{'}}{\phantom{\rule{0.16em}{0ex}}}^{1}{{\ensuremath{\Sigma}}_{g}}^{+}$. It is also observed from the present experimental results that the momentum transfer dependence behavior of the excitations to $b{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Pi}}_{u}({\ensuremath{\nu}}^{\ensuremath{'}}=3,4)$ is different from that of the excitations to $b{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Pi}}_{u}({\ensuremath{\nu}}^{\ensuremath{'}}=1,2)$, which can be attributed to the perturbation of ${c}_{3}{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Pi}}_{u}$ to $b{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Pi}}_{u}$. The present results provide experimental benchmark data of inelastic squared form factors of molecular nitrogen.

Squared form factors of vibronic excitations in 12-13.3 eV of nitrogen studied by high-resolution inelastic x-ray scattering

In the framework of time-dependent density-functional theory, we obtain high-order harmonics of photon energies up to $10\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}{U}_{p}$ from bilayer crystals with an interlayer spacing $d=70\phantom{\rule{0.16em}{0ex}}\AA{}$. At grazing incidence, a clear double-plateau structure is observed in the harmonic spectrum. The photon energy of the second plateau far beyond atomiclike harmonics can be well explained by the inclusion of backscattering of ionized electrons. Ab initio simulations reveal that the cutoff of the second plateau is continuously extended with an increasing $d$. Our classical calculations predict that the maximum electronic kinetic energy is linearly dependent on $d$ over a wide range. Moreover, the harmonic yield in the second plateau is significantly enhanced by increases in the wavelength of the driving laser. Owing to the confined spreading of the electronic wave packet, a beneficial wavelength scaling of ${\ensuremath{\lambda}}^{2.85}$ is obtained. This study therefore establishes an efficient way of producing high-energy photon source based on layered nanostructures.

Higher harmonic generation from bilayer nanostructures assisted by electron backscattering

The high-resolution x-ray-scattering technique is used to study the elastic scattering of atoms and molecules in the gas phase. The elastic squared form factor, which is the square of the Fourier transformation of the electron density distribution in position space and reveals the pure electronic structure of atoms and molecules in the ground state, of molecular hydrogen is measured at an incident photon energy of about 9889 eV and an energy resolution of about 70 meV. Although it is generally thought that the x-ray-scattering technique is identical to high-energy electron scattering, at least for elastic scattering these two techniques have an apparent difference, i.e., the pure electronic structure of a molecule in the ground state can be determined by x-ray scattering while it cannot be obtained by the high-energy electron impact method due to the interference between the scattering of separate nuclei and of the electrons in the target. The present experimental results match the theoretical calculations very well, which demonstrates that high-resolution x-ray scattering is a powerful tool to study the electronic structure of atoms and molecules in the ground state.

Determination of the electronic structure of atoms and molecules in the ground state: Measurement of molecular hydrogen by high-resolution x-ray scattering

Molecular Opacity Calculations for Lithium Hydride at Low Temperature

Coulomb-explosion imaging is a broadly employed technique to reconstruct the geometry of molecules from the direct multibody breakups of its ions. However, we reveal that this technique fails for a large class of systems, such as (${\mathrm{CO})}_{2}{}^{3+}$ and ${\mathrm{ArCO}}^{3+}$, since the events of the ``direct breakup channel'' are not the real direct breakups but the rapid sequential breakups with a short-lived and ultrafast-rotational fragment. Using Ar-CO as a prototype, we have investigated theoretically its Coulomb-explosion process. We find that due to the interfield between the metastable ${\mathrm{CO}}^{2+}$ and ${\mathrm{Ar}}^{+}$, ${\mathrm{ArCO}}^{3+}$ changes from its initial T shape into the more stable linear shape within 100 fs; such a rotation of ${\mathrm{CO}}^{2+}$ is hundreds of times faster than the field free case. Further, the advanced three-body surface hopping simulations indicate that the angle ${\ensuremath{\angle}}_{(\mathrm{Ar},\mathrm{CO})}$ has rotated from an initial ${95}^{\ensuremath{\circ}}$ to $110.6{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$ on average before the Coulomb explosion, which agrees well with the one (${115}^{\ensuremath{\circ}}$) detected by Gong et al. [Phys. Rev. A 88, 013422 (2013)]. Furthermore, we point out that correct geometry of ArCO can be obtained from the three-body breakups with high kinetic-energy release ($g20$ eV).

Yi-Geng Peng

Papers

Squared form factors of vibronic excitations in 12-13.3 eV of nitrogen studied by high-resolution inelastic x-ray scattering

Higher harmonic generation from bilayer nanostructures assisted by electron backscattering

Determination of the electronic structure of atoms and molecules in the ground state: Measurement of molecular hydrogen by high-resolution x-ray scattering

Molecular Opacity Calculations for Lithium Hydride at Low Temperature

Breakdown of the Coulomb-explosion imaging technique induced by the ultrafast rotation of fragments