Dynamics of Hydrogen Atoms Scattering from Surfaces

Hydrogen atoms interactions with single crystal surfaces are the simplest processes in surface science, which are of both practical and fundamental interest. Chemical reactions of hydrogen atoms with single crystal surfaces lead to adsorption. Despite the fact that it has been studied for decades, the dynamics of hydrogen atom adsorption is still not fully understood. Adsorption involves the impinging hydrogen atom losing its initial translational energy and dissipating the energy of the chemical bond formed with the surface. How hydrogen atoms dissipate their initial translational energy to adsorb on a surface is still an open question. In principle, a hydrogen atom may lose its initial translational energy to the vibrations of the surface atoms or to electron-hole pair excitations. To answer this question in detail, a state-of-the-art UHV machine was built to study atom-surface scattering. During my doctoral study, I conducted experiments on hydrogen atom scattering from single crystal surfaces, including Au(111) and Pt(111), Xe covered Au(111) and epitaxial graphene on Pt(111). High resolution scattering angle resolved translational energy loss distributions have been obtained. The goal of this research is to achieve a detailed understanding of the mechanisms of H atom interaction with different kinds of single crystal surfaces, and especially of the translational energy transfer between the atom and the surfaces, which is fundamentally important to the adsorption processes. To compare hydrogen atom translational energy loss to metals and insulators, I studied scatterings of hydrogen atoms from Au(111) and Xe layer on Au(111). Hydrogen atoms scattering from insulating Xe layer exhibit a small energy loss and a narrow translational energy distribution and can be understood using a binary collision model. In contrast, Hydrogen atoms scattering from Au(111) show a large energy loss and a broad translational energy distribution, indicating that a broad continuum of accepter states in the solid contribute to the translational energy loss. A MD simulation self-consistently including the non-adiabatic electronic excitations agrees with the experiments of hydrogen atom scattering on metal. In contrast, calculations neglecting the electronic excitations cannot capture the essence of the measurements, indicating hydrogen atoms scattering from metal is strongly non-adiabatic. Exchanging H atoms with D atoms only leads to minor change in the translational energy loss distribution. This is explained by a cancellation effect, where the phonon excitation is enhanced for D but the electron-hole pair excitation is reduced. To study the dynamics of chemically activated adsorption of hydrogen atoms, I did a series of experiments on hydrogen atoms scattering from epitaxial graphene, which has a barrier to C-H bond formation of several hundreds of meV. The scattered hydrogen atoms exhibit a bimodal distribution for translational energy and scattering angle. The fast component in the distribution originates from atoms scattered back without crossing the adsorption barrier and is near-elastic and near-specular. The slow component originates from atoms scattered back after crossing the adsorption barrier and exhibits large and rapid energy loss (energy loss power around 1019 W/mole atom). By monitoring the ratio between the slow and the fast component, we determine that the adsorption threshold is in the range of 0.41 eV to 0.48 eV for the H atom, and 0.43 eV to 0.47 eV for the D atom. Combined with DFT based AIMD calculations, we conclude that the fast component is H/D atom quasi-elastic scattering on a corrugated surface. The large and rapid energy loss of the slow component is caused by the formation of a transient reaction complex. Due to the π resonance structure of graphene, H atom colliding with one C atom will cause simultaneous displacements of the neighboring C atoms, leading to the formation of a transient reaction complex. Large portion of the energy loss is due to the inelastic scattering mechanism. Surface IVR provides insights of the phonon excitations in graphene

Long Term Spectral Evolution of Tidal Disruption Candidates Selected by Strong Coronal Lines

We present results of follow-up optical spectroscopic observations of seven rare, extreme coronal line emitting galaxies reported by Wang et al. (2012) with Multi-Mirror Telescope (MMT). Large variations in coronal lines are found in four objects, making them strong candidates of tidal disruption events (TDE). For the four TDE candidates, all the coronal lines with ionization status higher than [Fe VII] disappear within 5-9 years. The [Fe VII] faded by a factor of about five in one object (J0952+2143) within 4 years, whereas emerged in other two without them previously. A strong increment in the [O III] flux is observed, shifting the line ratios towards the loci of active galactic nucleus on the BPT diagrams. Surprisingly, we detect a non-canonical [O III]5007/[O III]4959 2 in two objects, indicating a large column density of O$^{2+}$ and thus probably optical thick gas. This also requires a very large ionization parameter and relatively soft ionizing spectral energy distribution (e.g. blackbody with $T < 5\times 10^4$ K). Our observations can be explained as echoing of a strong ultraviolet to soft X-ray flare caused by tidal disruption events, on molecular clouds in the inner parsecs of the galactic nuclei. Re-analyzing the SDSS spectra reveals double-peaked or strongly blue-shouldered broad lines in three of the objects, which disappeared in the MMT spectra in two objects, and faded by a factor of ten in 8 years in the remaining object with a decrease in both the line width and centroid offset. We interpret these broad lines as arising from decelerating biconical outflows. Our results demonstrate that the signatures of echoing can persist for as long as ten years, and can be used to probe the gas environment in the quiescent galactic nuclei.Comment: 30 Pages, 10 Figures, 2 Tables, Accepted for Publication in Ap

Performance of Spatial Diversity DCO-OFDM in a Weak Turbulence Underwater Visible Light Communication Channel

The performance of underwater visible light communication (UVLC) system is severely affected by absorption, scattering and turbulence. In this article, we study the performance of spectral efficient DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) in combination with the transceiver spatial diversity in turbulence channel. Based on the approximation of the weighted sum of lognormal random variables (RVs), we derived a theoretical exact bit error rate (BER) for DCO-OFDM systems with spatial diversity. The simulation results are compared with the analytical prediction, confirming the validity of the analysis. It is shown that spatial diversity can effectively reduce the turbulence-induced channel fading. The obtained results can be useful for designing, predicting, and evaluating the DCO-OFDM UVLC system in a weak oceanic turbulence condition

Long Fading Mid-Infrared Emission in Transient Coronal Line Emitters: Dust Echo of Tidal Disruption Flare

The sporadic accretion following the tidal disruption of a star by a super-massive black hole (TDE) leads to a bright UV and soft X-ray flare in the galactic nucleus. The gas and dust surrounding the black hole responses to such a flare with an echo in emission lines and infrared emission. In this paper, we report the detection of long fading mid-IR emission lasting up to 14 years after the flare in four TDE candidates with transient coronal lines using the WISE public data release. We estimate that the reprocessed mid-IR luminosities are in the range between $4\times 10^{42}$ and $2\times 10^{43}$ erg~s$^{-1}$ and dust temperature in the range of 570-800K when WISE first detected these sources three to five years after the flare. Both luminosity and dust temperature decreases with time. We interpret the mid-IR emission as the infrared echo of the tidal disruption flare. We estimate the UV luminosity at the peak flare to be 1 to 30 times $10^{44}$ erg s$^{-1}$ and for warm dust masses to be in the range of 0.05-1.3 Msun within a few parsecs. Our results suggest that the mid-infrared echo is a general signature of TDE in the gas-rich environment

ЧЕРВОНИЙ ХРЕСТ. КАТЕРИНОСЛАВСЬКИЙ ДОСВІД. (1908 – 1917)

В статті висвітлюється процес утворення та розвитку Катеринославського відділення Червоного Хреста та роль у цьому М.П. Урусова, який стояв на чолі дворянства Катеринославської губернії (1908 – 1917).This article introduces a process of creation and evolution Ekaterinoslav department of Red Cross and prince N.P. Urusov’s role in this process, which was a marshal of the nobility in Ekaterinoslav province (1908 - 1917)