ALBERT

Notes: The product-state-resolved dynamics of the reaction H+CO2→OH(2Π;ν,N,Ω,f)+CO have been explored in the gas phase at 298 K and center-of-mass collision energies of 2.5 and 1.8 eV (respectively, 241 and 174 kJ mol−1), using photon initiation coupled with Doppler-resolved laser-induced fluorescence detection. A broad range of quantum-state-resolved differential cross sections (DCSs) and correlated product kinetic energy distributions have been measured to explore their sensitivity to spin–orbit, Λ-doublet, rotational and vibrational state selection in the scattered OH. The new measurements reveal a rich dynamical picture. The channels leading to OH(Ω,N∼1) are remarkably sensitive to the choice of spin–orbit state: Those accessing the lower state, Ω=3/2, display near-symmetric forward–backward DCSs consistent with the intermediacy of a short-lived, rotating HOCO (X˜ 2A′) collision complex, but those accessing the excited spin–orbit state, Ω=1/2, are strongly focused backwards at the higher collision energy, indicating an alternative, near-direct microscopic pathway proceeding via an excited potential energy surface. The new results offer a new way of reconciling the conflicting results of earlier ultrafast kinetic studies. At the higher collision energy, the state-resolved DCSs for the channels leading to OH(Ω,N∼5–11) shift from forward–backward symmetric toward sideways–forward scattering, a behavior which resembles that found for the analogous reaction of fast H atoms with N2O. The correlated product kinetic energy distributions also bear a similarity to the H/N2O reaction; on average, 40% of the available energy is concentrated in rotation and/or vibration in the scattered CO, somewhat less than predicted by a phase space theory calculation. At the lower collision energy the discrepancy is much greater, and the fraction of internal excitation in the CO falls closer to 30%. All the results are consistent with a dynamical model involving short-lived collision complexes with mean lifetimes comparable with or somewhat shorter than their mean rotational periods. The analysis suggests a potential new stereodynamical strategy, "freeze-frame imaging," through which the "chemical shape" of the target CO2 molecule might be viewed via the measurement of product DCSs in the low temperature environment of a supersonic molecular beam. © 2000 American Institute of Physics.

Notes: Previous perturbed angular correlation (PAC) spectroscopy measurements on the donor indium in CdTe and its alloys have revealed several defect complexes. One defect characterized by two sets of quadrupole interaction parameters, νQ=83 MHz, η=0.08 and νQ=92 MHz, η=0.08, was observed in Hg0.8Cd0.2Te (x=0.2 MCT) and attributed to the substitutional indium–metal vacancy complex InM2+3+VM2+. A defect characterized by νQ=61±1 MHz and asymmetry parameter η between 0 and 0.19 was seen in CdTe and widely attributed to the same complex. Both of these assignments were based mainly on an observed relationship between complex formation and the loss of metal ions. In this article we present PAC measurements on 111In-doped x=0.45 MCT (Hg0.55Cd0.45Te). These measurements reveal defects having quadrupole interactions very similar to those seen previously in CdTe and in x=0.2 MCT. Two unique defect fractions f1 and f2, characterized by νQ1=60±3 MHz, η1≈0–0.2, and νQ2=87±4 MHz, η2≈0–0.15, were seen in x=0.45 MCT, in some cases simultaneously. The observation of both of these interactions in the same material—if they correspond to the defects seen in CdTe and x=0.21 MCT—precludes the possibility that they both correspond to precisely the same defect. We also observed a change in the relative fractions of these two defects with time at room temperature; the fraction f2 vanished over a period of a day, while f1 and f0 (the fraction of indium atoms in sites having cubic or higher symmetry) increased. While we cannot rule out the possibility of a slow electronic transition, at present we favor a model in which one of the interactions (probably the one near 60 MHz) corresponds to a complex in which indium is paired to a fast-diffusing monovalent metal ion like Ag+, Cu+, or Li+. © 1995 American Institute of Physics.

Notes: We present the first Raman-scattering studies of the behavior of the intrasubband plasmon mode of a two-dimensional electron gas which is undergoing lateral drift in an applied electric field. The data clearly show the expected Doppler shifts of the modes traveling up- and downstream, together with the expected dependence on the wave vector, but at higher drift velocities, the expected linear shift is distorted because of electron heating effects.

Notes: The relative densities of high-voltage stress-generated traps near the anode and cathode in 10-nm-thick silicon oxides have been measured after both positive and negative gate voltage stressing. The density of traps near the stress anode and cathode were approximately equal after negative gate voltage stressing. The density of traps near the stress cathode was higher than near the stress anode after positive gate voltage stressing. These results implied that impact ionization near the anode was not a significant generator of traps in 10-nm-thick oxides. © 1996 American Institute of Physics.

Notes: The interaction of molecular sulfur with the GaAs(100) surface has been investigated by reflectance anisotropy spectroscopy (RAS). The use of arsenic-capped GaAs(100) surfaces provided a means to study the interaction of sulfur with both the arsenic-rich (2×4) and gallium-rich (4×1) surface reconstructions. A sulfur flux was generated in ultra-high-vacuum (UHV) by the thermal decomposition of silver sulfide. The room-temperature deposition of a 1–2 monolayer saturation coverage of sulfur on both arsenic- and gallium-rich surfaces produces a (1×1) low-energy electron-diffraction pattern. This surface displays a strongly anisotropic optical response with RAS spectra which contain two clear features at 3.5 and 5.2 eV. These features become more pronounced as the sulfur-covered surface is annealed up to 500 °C where the surface displays a (2×1) reconstruction. The origin of these spectral features is believed to be due to optical transitions related to the sulfur dimers which are formed on this surface. The extent of the reduction in surface band bending induced by the sulfur coverage can be monitored by measuring the magnitude of the linear electro-optic feature which appears prominently in the RAS spectra of high-doped GaAs samples. © 1995 American Institute of Physics.

Notes: Picosecond excitation-probe measurements using a far-infrared free-electron laser (CLIO) have revealed large nonlinearities for indium antimonide at 4.7 μm. A theoretical model is described to determine the cw third-order nonlinear susceptibility and the interband relaxation time of the semiconductor which were found to be −8.6×10−11 m2 V−2 and 0.3 ns, respectively. Furthermore, the observation of the associated coherent transient grating effects allows us to obtain a coherence time of the laser system (2.5 ps) and the χ(3) of the transient grating which was found to be −7.2×10−13 m2 V−2. The measurements were performed at room temperature on undoped bulk InSb. © 1995 American Institute of Physics.

Notes: Room-temperature photoreflectance measurements of a GaAs/Al0.2Ga0.8As single-quantum well structure showed well defined Franz–Keldysh oscillations in the neighborhood of the GaAs and Al0.2Ga0.8As band-edge energies. That experiment investigated the origin of the Franz–Keldysh oscillations by sequential etching and photoreflectance analysis of the grown layers and showed that the phase of the Franz–Keldysh oscillations shifted as the upper Al0.2Ga0.8As barrier was etched, with eventual phase reversal when roughly half of the upper barrier was removed. Here, these phase shifts are determined accurately using a novel Kramers–Kronig approach and they are interpreted in terms of optical interference effects using both a simple two-ray model and a multiple-reflection treatment incorporating a calculation of the Seraphin coefficients. The results also enable the thickness of the layers removed to be determined. © 1995 American Institute of Physics.

Notes: A technique for engineering micron and submicron scale structures from magnetic films of transition metals has been developed using a combination of electron- and ion-beam lithography enabling high-quality arrays of submicron magnetic Fe wires to be fabricated. This process can be used to fabricate novel devices from a variety of metal combinations which would not be possible by the usual liftoff metallization method. The structure and magnetic properties are reported of an epitaxial 25 nm Fe(001)/GaAs(001) film and the wire gratings which are fabricated from it. The width of the wires in the grating is 0.5 μm for all structures studied, but the separation of each wire is varied in the range 0.5 to 16 μm. An artificially induced shape anisotropy field of around 1 kG, consistent with a magnetostatic calculation, was observed for all separations studied. The field dependence of the magneto-optic Kerr effect and magnetoresistance (MR) data is consistent with a twisted magnetization configuration across the width of the sample beneath saturation for transverse applied fields. In this case, the detailed form of the field dependence of the MR is strikingly modified from that observed in the continuous film and is consistent with coherent rotation of the magnetization.

Notes: Experimental results are presented for the physical origins of room-temperature photoreflectance features of a AlGaAs/GaAs single-quantum well structure. The spectra exhibit well-defined Franz–Keldysh oscillations which overlap with photoreflectance features due to the quantum well and complicate the determination of the energies of the transitions within the quantum well. The origin of the Franz–Keldysh oscillations are determined using wet chemical etching to selectively remove grown layers down to the substrate. The resulting spectra are presented as a function of etch depth which allows the magnitude of the built-in electric fields to be determined and reveals the location within the quantum well structure where the Franz–Keldysh oscillations originate. © 1995 American Institute of Physics.

Notes: Direct optical coupling to the plasmon modes of a two-dimensional electron gas in a stratified semiconductor system requires an overlaid grating coupler. Plasmons can be excited with wave vectors equal to integer multiples of the grating wave vector, and the periodic screening by the grating splits these into two modes, symmetric and antisymmetric with respect to the grating profile. We present calculations of the dispersions and optical coupling strengths for several orders of both modes in a typical structure, and show that the splitting and coupling strengths of the higher order modes oscillate with varying grating mark fraction. These effects are discussed in terms of the oscillating field strengths and charge density profiles.