ALBERT

Notes: Ultrathin amorphous SiO2 films are radio frequency sputter deposited onto thin film gold electrodes and incorporated as the insulating barrier in metal/insulator/metal tunnel junctions of the type Au/SiO2/Pb. Inelastic electron tunneling spectra (IETS) recorded at 4.2 K reveal characteristic zero bias anomalies associated with rapid variations in the Pb density of states on either side of the superconducting gap which shows unequivocally that tunneling is the primary conduction mechanism through the junctions at this temperature. Full IET spectra reported are essentially identical to those previously recorded for similar SiO2 films on aluminum. Atomic force and scanning tunneling microscope images of the SiO2 and underlying gold films are also presented; these data confirm that the SiO2 films are sufficiently uniform and continuous for IETS.

Notes: These experiments on bromopropionyl chloride investigate a system in which the barrier to C–Br fission on the lowest 1A‘ potential energy surface is formed from a weakly avoided electronic configuration crossing, so that nonadiabatic recrossing of the barrier to C–Br fission dramatically reduces the branching to C–Br fission. The results, when compared with earlier branching ratio measurements on bromoacetyl chloride, show that the additional intervening CH2 spacer in bromopropionyl chloride reduces the splitting between the adiabatic potential energy surfaces at the barrier to C–Br fission, further suppressing C–Br fission by over an order of magnitude. The experiment measures the photofragment velocity and angular distributions from the 248 nm photodissociation of Br(CH2)2COCl, determining the branching ratio between the competing primary C–Br and C–Cl fission pathways and detecting a minor C–C bond fission pathway. While the primary C–Cl:C–Br fission branching ratio is 1:2, the distribution of relative kinetic energies imparted to the C–Br fission fragments show that essentially no C–Br fission results from promoting the molecule to the lowest 1A‘ potential energy surface via the 1[n(O),π*(C=O)] transition; C–Br fission only results from an overlapping electronic transition. The results differ markedly from the predictions of statistical transition state theories which rely on the Born–Oppenheimer approximation.While such models predict that, given comparable preexponential factors, the reaction pathway with the lowest energetic barrier on the 1A‘ surface, C–Br fission, should dominate, the experimental measurements show C–Cl bond fission dominates by a ratio of C–Cl:C–Br=1.0:〈0.05 upon excitation of the 1[n(O),π*(C=O)] transition. We compare this result to earlier work on bromoacetyl chloride, which evidences a less dramatic reduction in the C–Br fission pathway (C–Cl:C–Br=1.0:0.4) upon excitation of the same transition. We discuss a model in which increasing the distance between the C–Br and C=O chromophores decreases the electronic configuration interaction matrix elements which mix and split the 1n(O)π*(C=O) and np(Br)σ*(C–Br) configurations at the barrier to C–Br bond fission in bromopropionyl chloride. The smaller splitting between the adiabats at the barrier to C–Br fission increases the probability of nonadiabatic recrossing of the barrier, nearly completely suppressing C–Br bond fission in bromopropionyl chloride. Preliminary ab initio calculations of the adiabatic barrier heights and the electronic configuration interaction matrix elements which split the adiabats at the barrier to C–Br and C–Cl fission in both bromopropionyl chloride and bromoacetyl chloride support the interpretation of the experimental results. We end by identifying a class of reactions, those allowed by overall electronic symmetry but Woodward–Hoffmann forbidden, in which nonadiabatic recrossing of the reaction barrier should markedly reduce the rate constant, both for ground state and excited state surfaces.

Notes: These experiments use molecular photodissociation of CH3SH to probe the dynamics and the influence of nonadiabatic coupling in the transition state region of the CH3+SH→CH3S+H reaction. Photoexcitation at 222 and 248 nm in the first of two absorption bands accesses the lower of the two coupled potential energy surfaces near the saddle point of the excited state reaction coordinate. Measurement of the resulting photofragments' velocities and angular distributions determine the branching between the CH3+SH and the CH3S+H exit channels. At all wavelengths within the first absorption band, we observe preferential fission of the stronger S–H bond over the weaker C–S bond. Fission of the C–S bond occurs only to a small degree at 222 nm and is not observable at 248 nm. Comparison with our earlier data at 193 nm, corresponding to excitation to the upper bound adiabat which is nonadiabatically coupled to the lower dissociative surface reached at 222 nm, shows that the branching ratio between C–S bond fission and S–H bond fission is a factor of eight larger at 193 nm.To probe the forces in the Franck–Condon region, we also measure the photoemission spectrum from dissociating CH3SH excited at 222 nm and compare it to the previous measurement at 193 nm. The 222 nm spectrum evidences emission into the S–H stretch and methyl stretch vibrations but not into C–S stretching modes, consistent with the dominance of S–H fission on the lower adiabat, while the 193 nm emission spectrum, reassigned here, has only a progression in the C–S stretch. The comparison of the spectra suggests a model in which stretching along the C–S coordinate on the bound upper state occurs as the amplitude couples nonadiabatically to the lower dissociative surface, allowing the molecule to access the region near the saddle point on the lower surface at extended C–S bond lengths. This results in better overlap with the C–S fission exit channel and thus an increased branching to C–S bond fission over that observed upon direct excitation to the lower dissociative surface at 222 nm. To further advance the experimental conclusions, we present collaborative calculations of the potential energy surfaces using the effective valence-shell Hamiltonian method developed by Freed and co-workers.

Notes: A simple, low-cost pulse nuclear magnetic resonance (NMR) spectrometer is described. There are two key features of the instrument: (1) all radio frequency phase shifting is done digitally with a new-generation frequency synthesizer and (2) the pulse programmer is based on a high-speed 32-bit parallel interface card in a desktop computer. The spectrometer is well suited for single-frequency channel experiments such as nuclear quadrupole resonance and deuterium quadrupole echo NMR experiments. Because the rf phase shifting is done digitally, the spectrometer operates easily over a wide frequency range, here 10–300 MHz. The pulse programmer and other software functions are written in LabView, a graphical programming language.

Notes: Charge-carrier mobilities have been measured for the first time in Cd0.8Zn0.2Te single crystals using time-of-flight measurements of charge carriers produced by short (10 ns) light pulses from a frequency-doubled Nd:YAG laser (λ=532 nm). The electron mobility displayed a T−1.1 dependence on the absolute temperature T in the range 200–320 K, with a room-temperature mobility of 1350 cm2/V s. The hole mobility displayed a T−2.0 dependence in the same temperature range, with a room-temperature mobility of 120 cm2/V s. Cd0.8Zn0.2Te appears to be a very favorable material for a room-temperature electronic nuclear radiation detector.

Notes: A thin film, produced by carbon ion implantation and outdiffusion at the temperature of irradiation, has been confirmed to be diamond using micro-Raman spectroscopy and electron diffraction. The Raman spectra contained a definitive diamond peak shifted slightly from that of natural diamond. This shift may be due to residual strain and is consistent with the broad linewidth. Fragments of the film over a micron in size were examined with transmission electron microscopy and found to be untwinned, single crystals. The film had been produced on a polycrystalline copper substrate. The carbon ions were at an ion energy of 120 keV and the irradiation was carried out to a high dose.

Notes: Emission of a 100 mW cw beam into a nearly diffraction limited near-circular spot by a grating surface emitting (GSE) semiconductor laser, with radiance of 5.3×106 W/sr cm2, is described. The distributed out-coupled master oscillator power amplifier (DOC-MOPA) devices comprise active grating outcoupled power amplifiers and monolithic distributed feedback laser oscillators. With single wavelength (930 nm) operation demonstrated to 260 mW, they produce narrow linewidth (10 MHz) beams and near-ideal far-field patterns. The DOC-MOPA promises to become an efficient, compact source of high-quality ≥1 W cw light. Such fully collimated high power cw beams demonstrate the potential of GSE lasers to serve in systems requiring higher power than available from conventional edge-emitting technology.