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We present a comprehensive study of photoionization of OH radicals from the ground and first excited electronic states. Cross sections and asymmetry parameters for the valence orbitals are reported for photon energies from near the threshold to 60 eV. The calculations have been performed with the $R$-matrix method. Two different continuum basis types, namely, the Gaussian and $B$-spline bases, are used to assess the quality of the description of the continuum. The results show both types of continuum bases provide a similar quality description in the near threshold resonant region, but the latter can predict more reliable photoionization observables in higher-energy regions. Our calculations are compared with available experimental and theoretical results, and the discussions are provided.

The problem of photoionization of atomic hydrogen in a white-dwarf-strength magnetic field is revisited to understand the existing discrepancies in the positive-energy spectra obtained by a variety of theoretical approaches reported in the literature. Oscillator strengths for photoionization are calculated with the adiabatic-basis-expansion method developed by Mota-Furtado and O'Mahony [Phys. Rev. A 76, 053405 (2007)]. A comparative study is performed between the adiabatic-basis-expansion method and our previously developed coupled-channel theory [L. B. Zhao, O. Zatsarinny, and K. Bartschat, Phys. Rev. A 94, 033422 (2016)]. A detailed analysis of the positive-energy spectra obtained here and those from other theoretical approaches shows that the adiabatic-basis-expansion method can produce more accurate positive-energy spectra than other reported approaches for low field strengths.

Experiments reveal that unusual surface behavior at the terminations of a recently discovered magnetic topological insulator depends on the interplay between different building blocks within the material.

The low-energy electron collisions with cyanamide molecule are investigated by using the UK molecular $R$-matrix codes for electron energies ranging from 0.01 eV to 10 eV. Three models including static-exchange, static-exchange plus polarization, and close-coupling (CC) approximations are employed to reveal the dynamic interaction. Elastic (integrated and differential), momentum-transfer, and excitation cross sections from the ground state to the three low-lying electron excited states have been presented. Two shape resonances, two core-excited resonances, and two Feshbach resonances are detected in the CC approximation. The role of active space in the target and scattering problem including the resonances is discussed. The precise resonance parameters are found to be sensitive to the treatment of polarization effects employed. These resonances may be responsible for the fragments observed in a recent experiment of the dissociative electron attachments to cyanamide. Since the cyanamide molecule has a large permanent dipole moment, a Born closure procedure is used to account for the contribution of partial waves higher than $l=4$ to obtain converged cross sections.

We present a comprehensive study of the photodetachment of the negative gallium ion and elastic electron scattering from neutral Ga for photon and electron energies ranging from threshold to 12 eV. The calculations are carried out with the $B$-spline $R$-matrix method. A multiconfiguration Hartree-Fock method with nonorthogonal term-dependent orbitals is employed to generate accurate initial- and final-state wave functions. The close-coupling expansions include the $4s{\phantom{\rule{0.16em}{0ex}}}^{2}4pnl(kl)$ bound and continuum states of Ga and the $4s$-excited autoionizing states $4s4{p}^{2}$. The calculated photodetachment and elastic cross sections exhibit prominent resonance features. In order to clarify the origin of these resonances, the contributions of the major ionization channels to the partial cross sections are analyzed in detail.

We present a comprehensive study of electron collisions with boron atoms by using the $B$-spline $R$-matrix method for electron energies ranging from threshold to 100 eV. Elastic, excitation, and ionization cross sections were obtained for all transitions between the lowest 11 states of boron. A multiconfiguration Hartree-Fock method with nonorthogonal term-dependent orbitals was employed to generate accurate wave functions for the target states. Close-coupling expansions including 13, 51, and 999 physical and pseudo-target states of boron were used to check the sensitivity of the results to changes in the theoretical model. The cross-section dataset obtained from the large-scale calculations is expected to be sufficiently accurate and comprehensive for most current modeling applications involving neutral boron.

We show with systematic photoemission spectroscopy and scanning tunneling spectroscopy data that a spin helical surface state appears on the (110) surface of noncentrosymmetric, three-dimensional HgTe. The topological surface state in HgTe exhibits sharp, linear dispersion without ${k}_{z}$ variation, as well as clear, left-right imbalanced spin polarization and circular dichroism. Chemical gating by alkali metal deposition on the surface causes the unexpected opening and/or increase of a surface insulating gap without changing its topological property. Such an unusual behavior we uncover in three-dimensional HgTe sheds light on a convenient control of the Fermi surface and quantum transport in a topological insulator.

The novel pseudogap state has been systematically studied on flat-top In, Al, and Pb islands/films grown on $\mathrm{Si}(111)$ surfaces with scanning tunneling microscopy. Our observations and analysis suggest that the interplay between electron-phonon interaction and quantum confinement, the dynamic Coulomb blockade, and the orthodox Coulomb blockade are, respectively, the dominant mechanism of the formation of such pseudogap states in the flat-top metal islands at different island size scales. Our results can help to settle the dispute on the origins of the pseudogap state in such systems.

Combining scanning tunneling microscopy and first-principles calculations, we have shown that single Co atoms adsorbed on a $\mathrm{Si}(111)\text{\ensuremath{-}}(7\ifmmode\times\else\texttimes\fi{}7)$ surface have eight different configurations that possess different spin magnetic moments. Despite the large adsorption energy, we have demonstrated that both the position and the spin state of single adsorbed Co atoms can be well controlled through the vertical atomic manipulation and the ``tip-touch'' operation that converts Co atoms among various adsorption configurations. Our approach to construct atomic-scale magnetic structures on a semiconductor surface can provide a new pathway to realize spin-based devices, including a scalable solid-state quantum computer.

Using scanning tunneling microscopy supplemented with first-principles calculations, we examine all the thermally activated atomistic processes of Ag-n (n = 1-26) constructed atom-by-atom on a Si(111)-(7 x 7) substrate, and we exploit such cluster dynamical information to further determine the energetic stability (or magicity) of the clusters. By generalizing the traditional concept of cluster magicity solely based on cluster association/dissociation to also include various complex collective cluster motions, we identify the existence of two classes of magic clusters. The most stable class, Ag-10 and Ag-25, is defined by geometrical shell closure; the less stable class of Ag-n (n = 3, 5, 13, 16, 19) is associated with lower kinetic barriers against internal restructuring of, or atom detachment from, their respective clusters of neighboring sizes. Our detailed analysis also reveals that the substrate effect, rather than the number of bonds within the clusters, dominates the cluster stabilities. The conceptual advances gained in the present study are broadly applicable to many related cluster systems in contact with external media, and they are expected to be instrumental in tuning the dynamical behaviors of clusters in surface catalysis, nanoplasmonics, and other technological areas.

Scanning tunneling spectroscopy measurements of Pb islands on Si(111) at high energy resolution reveal a novel pseudogap, or a pseudopeak in special cases, around the Fermi level in addition to the usual quantum well states. These gap or peak features persist to temperatures as high as approximately 80 K and are uniquely related to the quantum well nanostructure of the Pb islands. A systematic analysis indicates that electron-phonon scattering is responsible for the observed electronic structure.

Using scanning tunneling microscopy, we studied the diffusion of single Ag atoms within the Si(111)-(7 x 7) unit cell. A striking difference was observed in the time-dependent tunneling current spectra for Ag atoms moving in the unfaulted and faulted half unit cells, with a dual-time characteristic in the former but a single time in the latter. Our observations demonstrate the importance of the stacking fault in affecting the interaction between Ag atoms and the Si(111)-(7 x 7) surface and can be understood in terms of an asymmetric interplay between intrabasin and interbasin diffusion.

Using scanning tunneling spectroscopy, we have investigated the local electronic property change upon single Co atom adsorption on Pb nanoislands and/or films grown on a $\text{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ surface. The quantum well states formed on the clean Pb film were found to be locally destroyed by the adsorbed single Co atom. Moreover, the differential conductance $dI/dV$ curves, exhibiting an asymmetric line shape with a dip and a hump, respectively, below and above the Fermi level, demonstrated a Kondo effect from the adsorbed Co atom on the Pb film. The similar line shapes of the $dI/dV$ spectra and the similar Kondo temperatures for Co atoms adsorbed on Pb island areas of different Pb thicknesses further showed that the different densities of states at the Fermi level originated from the quantum well states do not play a significant role, possibly due to the destruction of the quantum well states upon the Co atom adsorption.

We report a new kind of experimental realization of a molecular rectifier, which is based on a single azafullerene ${\mathrm{C}}_{59}\mathrm{N}$ molecule in a double-barrier tunnel junction via the single electron tunneling effect. An obvious rectifying effect is observed. The positive onset voltage is about 0.5--0.7 V, while the negative onset voltage is about 1.6--1.8 V. Theoretical analyses show that the half-occupied molecular orbital of the ${\mathrm{C}}_{59}\mathrm{N}$ molecule and the asymmetric shift of the molecular Fermi level when the molecule is charged are responsible for the molecular rectification.

Using scanning tunneling microscopy and first principles calculations, the adsorption sites of single Cu, Ag, and Au atoms on the $\mathrm{Si}(111)\mathrm{\text{\ensuremath{-}}}(7\ifmmode\times\else\texttimes\fi{}7)$ surface have been systematically investigated and identified. Despite their monovalence, these atoms were found to adsorb at high coordination sites, seeking to saturate the maximum number of dangling bonds. The stable adsorption sites were further observed to be distinctly different in the faulted and unfaulted half unit cells, showing an asymmetry that has never been observed for many other adsorbates.

By confining a diffusion atom in a nanometer region using surface potential heterogeneity, we have successfully employed a time-dependent tunneling spectroscopy to quantitatively study its random motion. A hopping rate in the range of $1--{10}^{4}\text{ }\text{ }\mathrm{H}\mathrm{z}$, $\ensuremath{\sim}3$ orders of magnitude faster than those accessible by the existing diffusion methods based on scanning tunneling microscopy, was demonstrated for single Cu atoms diffusing in the faulted half unit cell of $\mathrm{S}\mathrm{i}(111)\mathrm{\text{\ensuremath{-}}}(7\ifmmode\times\else\texttimes\fi{}7)$. Our technique is potentially useful to detect fast diffusion processes such as H quantum diffusion at atomic scale.

The single-electron tunneling behavior of crystalline palladium nanoparticles with narrowly distributed core sizes ranging from 1.6 to 4 nm is studied by scanning tunneling microscopy/spectroscopy. The current-voltage $(I\text{\ensuremath{-}}V)$ characteristics of Pd nanoparticles exhibit size-dependent fine features, which are assigned to the discreteness of energy states of the ultrasmall Pd particles. It is found that the peak widths, as well as the intrapeak spacings in differential conductance $dI∕dV$ spectra increase with the decrease of the size of Pd nanoparticles. Our analysis shows that the dwell time of the weak tunnel junction may not be a major contribution to the peak widths due to the large resistance of the tunnel junction of about ${10}^{7}\ensuremath{\Omega}$. The possible effect of residual charge is also excluded. An explanation of the size-dependent behaviors of the peak width and the intrapeak spacing is attributed to the clustered electronic structures around the Fermi level due to certain size-dependent dynamic effects.