Emergence of Subsurface Oxygen on Rh(111).

Oxygen atoms on transition metal surfaces are highly mobile under the demanding pressures and temperatures typically employed for heterogeneously catalyzed oxidation reactions. This mobility allows for rapid surface diffusion of oxygen atoms, as well as absorption into the subsurface and reemergence to the surface, resulting in variable reactivity. Subsurface oxygen atoms play a unique role in the chemistry of oxidized metal catalysts, yet little is known about how subsurface oxygen is formed or returns to the surface. Furthermore, if oxygen diffusion between the surface and subsurface is mediated by defects, there will be localized changes in the surface chemistry due to the elevated oxygen concentration near the emergence sites. We observed that oxygen atoms emerge preferentially along the boundary between surface phases and that subsurface oxygen is depleted before the surface oxide decomposes.

Chiral Selective Chemistry Induced by Natural Selection of Spin-Polarized Electrons.

The search to understand the origin of homochirality in nature has been ongoing since the time of Pasteur. Previous work has shown that DNA can act as a spin filter for low-energy electrons and that spin-polarized secondary electrons produced by X-ray irradiation of a magnetic substrate can induce chiral selective chemistry. In the present work it is demonstrated that secondary electrons from a substrate that are transmitted through a chiral overlayer cause enantiomeric selective chemistry in an adsorbed adlayer. We determine the quantum yields (QYs) for dissociation of (R)- or (S)-epichlorohydrin adsorbed on a chiral self-assembled layer of DNA on gold and on bare gold (for control). The results show that there is a significant difference in the QYs between the two enantiomers when adsorbed on DNA, but none when they are adsorbed on bare Au. We propose that the effect results from natural spin filtering effects cause by the chiral monolayer.

Spin-polarized electron induced asymmetric reactions in chiral molecules.

Understanding the origin of chirality in nature has been an active area of research since the time of Pasteur. In this chapter we examine one possible route by which this asymmetry could have arisen, namely chiral-specific chemistry induced by spin-polarized electrons. The various sources of spin-polarized electrons (parity violation, photoemission, and secondary processes) are discussed. Experiments aimed at exploring these interactions are reviewed starting with those based on the Vester-Ulbricht hypothesis through recent studies of spin polarized secondary electrons from a magnetic substrate. We will conclude with a discussion of possible new avenues of research that could impact this area.

Thermal and electron-induced decomposition of 2-butanol on Pt(111).

The adsorption, thermal evolution, and electron irradiation of 2-butanol on Pt(111) were investigated with reflection absorption infrared spectroscopy (RAIRS). A simulated vibrational spectrum of a single 2-butanol molecule was calculated using density functional theory to facilitate vibrational assignments. Exposures of 0.2 Langmuir (L) and lower result in both isolated 2-butanol molecules with minimal lateral interactions and hydrogen-bonded clusters. The thermal evolution following a 4.0 L exposure shows that the hydrogen-bonded multilayer desorbs around 170 K, leaving a 2-butanol monolayer where hydrogen bonding still exists. At 190 K, a new feature at 1699 cm(-1) is attributed to the formation of butanone. Irradiation with 750 or 100 eV electrons leads to 2-butanol desorption and partial conversion to butanone, as indicated by the appearance of a peak at 1709 cm(-1).

Time-resolved synchrotron radiation excited optical luminescence: light-emission properties of silicon-based nanostructures.

The recent advances in the study of light emission from matter induced by synchrotron radiation: X-ray excited optical luminescence (XEOL) in the energy domain and time-resolved X-ray excited optical luminescence (TRXEOL) are described. The development of these element (absorption edge) selective, synchrotron X-ray photons in, optical photons out techniques with time gating coincide with advances in third-generation, insertion device based, synchrotron light sources. Electron bunches circulating in a storage ring emit very bright, widely energy tunable, short light pulses (<100 ps), which are used as the excitation source for investigation of light-emitting materials. Luminescence from silicon nanostructures (porous silicon, silicon nanowires, and Si-CdSe heterostructures) is used to illustrate the applicability of these techniques and their great potential in future applications.

Getting to the core of the problem: origin of the luminescence from (Mg,Zn)O heterostructured nanowires.

We have measured the time-resolved, X-ray excited optical luminescence spectra from two types of MgxZn(1-x)O core-shell, heterostructured nanowires: type I, with a small x, wurtzite core, encased in a larger x, wurtzite sheath; and type II, with a wurtzite core (x approximately 0), encased in a rock-salt sheath (x>0.62). By monitoring the X-ray energy dependence of the various luminescence peaks, we have determined the local environment of the sites where these peaks originate.