ALBERT

Notes: A fast low-cost device to detect the phase shift between the excitation and the response of a cantilever in tapping-mode atomic force microscopy is described. For cantilever signals with a good signal to noise ratio, as is commonly found, the device presented can replace a lock-in amplifier. The setup is based on indirect time measurements realized by a combination of commonly used analog and digital integrated circuits. Phase measurement can already be achieved within one cycle. Signal output rates up to 100 kHz allow the use of the phase shift as an auxiliary imaging channel. Cantilever frequencies may range from 6 to more than 500 kHz. The principle of the setup is illustrated together with technical data. Images of a hydrophobic–hydrophilic structured silicon surface obtained in air and of purple membrane obtained in fluid are presented. © 1999 American Institute of Physics.

Notes: In this letter we report quantum mechanical integral and differential cross sections for the title reactions as calculated on a new ab initio potential energy surface. The calculations, all carried out in the reagents arrangement channel employing negative imaginary potentials, were done within the coupled-states approximation. The final vibrational state-to-state differential and integral cross sections were compared with experiment. Altogether, a very encouraging agreement was obtained. © 1996 American Institute of Physics.

Notes: Quantum mechanical integral and differential cross sections have been calculated for the title reaction at the three collision energies studied in the 1985 molecular beam experiment of Lee and co-workers, using the new ab initio potential energy surface of Stark and Werner (preceding paper). Although the overall agreement between the calculated and experimental center-of-mass frame angular distributions is satisfactory, there are still some noticeable differences. In particular, the forward scattering of HF(v′=3) is more pronounced in the present calculations than it is in the experiment and the calculations also predict some forward scattering of HF(v′=2). A comparison with the quasiclassical trajectory results of Aoiz and co-workers on the same potential energy surface shows that the forward scattering is largely a quantum mechanical effect in both cases, being dominated by high orbital angular momenta in the tunneling region where the combined centrifugal and potential energy barrier prevents classical trajectories from reacting. The possible role of a reactive scattering resonance in contributing to the quantum mechanical forward scattering is also discussed in some detail. © 1996 American Institute of Physics.

Notes: A three dimensional potential energy surface for the F+H2→HF+H reaction has been computed using the internally contracted multireference configuration interaction (MRCI) method with complete active space self-consistent field (CASSCF) reference functions and a very large basis set. Calibration calculations have been performed using the triple-zeta plus polarization basis set employed in previous nine-electron full CI (FCI) calculations of Knowles, Stark, and Werner [Chem. Phys. Lett. 185, 555 (1991)]. While all variational MRCI wave functions yield considerably larger barrier heights than the FCI, excellent agreement with the FCI barrier height and the exothermicity was obtained when the Davidson correction was applied (MRCI+Q). The convergence of the barrier height and exothermicity, spectroscopic constants of the HF and H2 fragments, and the electron affinity of the fluorine atom with respect to the basis set has been carefully tested. Using the largest basis sets, which included 5d, 4f, 3g, and 2h functions on fluorine, a linear barrier height of 1.84 kcal/mol and an exothermicity of 31.77 kcal/mol (exp. 31.73 kcal/mol) was obtained. The true saddle point has a bent structure and the barrier height is predicted to be (1.45±0.25) kcal/mol. About 700 points on the three-dimensional potential energy surface have been computed using a slightly smaller basis set, which yield F–HH barrier heights of 1.92 kcal/mol (linear), 1.54 kcal/mol (bent), and an exothermicity of 31.3 kcal/mol. The barrier height for the H+FH→HF+H exchange reaction is predicted to be 41.2 kcal/mol. In the entrance channel cuts through the three potentials correlating with F(2P3/2,1/2)+H2(1Σ+g) have been computed, and the effect of spin–orbit coupling is investigated. It is found that the spin–orbit coupling increases the barrier height relative to the asymptotic F(2P3/2)+H2(1Σ+g) ground state by about 0.35 kcal/mol, leading to final estimates for the effective collinear and bent barriers of (2.18±0.25) kcal/mol and (1.80±0.25) kcal/mol, respectively. An accurate global analytical fit of the potential (without the effect of spin–orbit coupling) has been obtained using the method of Aguado and Paniagua. Our new ab initio potential is compared to various potentials used so far in dynamics calculations for the F+H2 reaction. © 1996 American Institute of Physics.

Notes: Antarctic Submillimeter Telescope and Remote Observatory (AST/RO), a 1.7 m diameter telescope for astronomy and aeronomy studies at wavelengths between 200 and 3000 μm, was installed at the South Pole during the 1994–95 Austral summer. The optical design is Gregorian, offset in both azimuth and elevation, with the exit pupil at the chopping tertiary mirror: this arrangement provides for consistent illumination of the primary mirror even when the beam is thrown one degree or more on the sky. Aberrations are minimized by the choice of secondary mirror offset angle. Alignment is accomplished by mechanical means. There is a Coudé focus in a warm, spacious receiver room and also a Nasmyth focus. Both the elevation and azimuth axes are driven by two pinion gears with opposed torques to eliminate backlash. The encoders are unusually robust but have high friction, necessitating a stiff coupling. The azimuth limit switch scheme permits 1.5 rotations, but the switches will operate under extreme conditions with no single point of failure. The instrument is now operational with four heterodyne receivers and three acousto-optical spectrometers. © 1997 American Institute of Physics.

Notes: Stabilization and control of a high-pressure glow discharge by means of a microhollow cathode discharge has been demonstrated. The microhollow cathode discharge, which is sustained between two closely spaced electrodes with openings of approximately 100 μm diam, serves as plasma cathode for the high-pressure glow. Small variations in the microhollow cathode discharge voltage generate large variations in the microhollow cathode discharge current and consequently in the glow discharge current. In this mode of operation the electrical characteristic of this system of coupled discharges resembles that of a vacuum triode. Using the microhollow cathode discharge as plasma cathode it was possible to generate stable, direct current discharges in argon up to atmospheric pressure, with estimated electron densities in the range from 1011 to 1012 cm−3. The recently demonstrated parallel operation of these discharges indicates the potential of this technique for the generation of large volume plasmas at high gas pressure through superposition of individual glow discharges. © 1999 American Institute of Physics.

Notes: Tapping-mode atomic force microscopy (TM-AFM) is a powerful tool to study soft biological samples. Higher eigenmodes of the vibrating cantilever offer enhanced signal and smaller time constants increasing the sensitivity of the tapping probe as compared to conventional TM-AFM. The first five eigenmodes of a v-shaped silicon cantilever were investigated with respect to their suitability for imaging. Stable imaging was possible in the first and third modes. Phase imaging in the third mode was extremely sensitive to surface inhomogeneities and surface contamination particles not visible in standard TM-AFM. © 1999 American Institute of Physics.

Notes: Direct current glow discharges have been operated in atmospheric air by using 100 μm microhollow cathode discharges as plasma cathodes. The glow discharges were operated at currents of up to 22 mA, corresponding to current densities of 3.8 A/cm2 and at average electric fields of 1.2 kV/cm. Electron densities in the glow are in the range from 1012 to 1013 cm−3. Varying the current of the microhollow cathode discharge allows us to control the current in the atmospheric pressure glow discharge. Large volume atmospheric pressure air plasmas can be generated by operating microhollow cathode discharges in parallel. © 1999 American Institute of Physics.

Notes: We compare the use of free-space electro-optic sampling (FSEOS) with photoconducting antennas to detect terahertz (THz) radiation in the range of 0.1–3 THz. For the same average THz power and low-frequency modulation, signal-to-noise ratio and sensitivity are better with antenna detection at frequencies smaller than 3 THz. When the modulation frequency is increased to more than 1 MHz in FSEOS, both detection schemes have comparable performance. Using a singular-electric-field THz emitter, we demonstrate the feasibility of a THz imaging system using real-time delay scanning in FSEOS and only 20 mW of laser power. © 1998 American Institute of Physics.

Notes: The effect of high fields produced during space–charge neutralization of an electron beam is demonstrated by production of relativistic electrons (≈0.6 MeV) from a simple, small hollow cathode source operated at 20 kV. Acceleration occurs during a well-characterized, repeatable, glow-type transient plasma phase. The accelerating electron beam is produced within the device, and the total number of (approximately-greater-than)0.5 MeV electrons is estimated to be ≤1010. A model correlating time dependence of the transient plasma with energetic electron production is discussed. © 1996 American Institute of Physics.