Laser generation of narrow-band and directed ultrasound

An aluminium hemicylindrical sample has been irradiated with an array of laser lines, with each line acting as a source of acoustic waves. Detection of the generated ultrasonic waves was performed using both a wide-band stabilized Michelson interferometer and a 20 MHz piezoelectric transducer. Experimental and theoretical results are presented which reveal that the use of a spatially modulated laser source produces significant narrow-banding of the detected ultrasound, compared with a single point or single line source case. Additionally, for a given line spacing, ultrasound of a particular frequency can be directed. Owing to the nature of the acoustic signals generated by each individual array element, superposition of several signals does not result in any energy directivity similar to that encountered in phase electromagnetic array antennas. While time or frequency feature enhancement may be obtained in a desired direction, in most cases the far field energy directivity pattern is simply the incoherent sum of the energy directivity of each array element.     

UPS measurements of molecular energy level of condensed gases

There have been numerous attempts to use ultraviolet photoemission spcctroscopy (UPS) to monitor the chemical states of adsorbed gas molecules on metal surfaces. To interpret the data correctly, one has to determine the effect of photoemission on the measured energy levels of the molecule. We have measured the UPS spectra of seven gases (C₆H₆, C₅H₅N, CH₃OH, C₂H₅OH, H₂CO, H₂O, NH₃) condensed on a LN₂ cooled MoS₂ substrate at hv = 21.2 eV. The inertness of the MoS₂ substrate assures that no strong chemical bonding exists between the substrate and adsorbed molecules. For each gas, the spectrum of the condensed phase is similar to the corresponding spectrum of the gas phase except all the energy levels are shifted up by the same amount. This shift ranges from 1 to 1.65 eV for the gases studied. The energy shift is attributed to the dielectric screening of the hole produced during the optical excitation.     

Negative affinity 3–5 photocathodes: Their physics and technology

Negative electron affinity (NEA) photocathodes are defined by the relationship between the potential barrier at the surface and the bottom of the conduction band in the bulk of the material. If the bottom of the conduction band liesabove the potential barrier at the surface, the device is said to have a negative electron affinity. In practice this condition is obtained by heavyp-doping of the semiconductor (to encourage downward band bending at the surface) and by adding a thin film (several atomic layers) of cesium richcesium oxide on the clean semiconductor surface. The physics, development, fabrication, and applications of the NEA cathode are reviewed. The threshold of response of a NEA photocathode is set by the semiconductor bandgap. By alloying to form ternary or quaternary 3–5 compounds (3–5 compounds are formed from elements of the 3rd and 5th columns of the periodic table), the bandgap (and thus the threshold) can be placed at any desired photon energy within certain limits. The most important limit is that at about 1.1 eV which is the lowest limit achieved for NEA cathodes. This limit is set by the point at which the bandgap of the 3–5 material becomes less than the surface potential barrier. Fundamental work aimed at understanding the 3–5: cesium oxide “interfacial” barrier which sets this limitation is briefly discussed. Because of the “interfacial” barrier, the quantum yield of NEA cathodes decreases as the threshold of response moves to lower photon energy. Field assisted photocathodes provide a means of extending the threshold of response beyond 1.1 eV. Two different approaches to field assisted photocathodes and recent achievements are discussed. A major advancement has been the achievement of semi-transparent NEA photocathodes by sealing GaAs to glass. This makes possible practical NEA image tubes. The thermionic emission from 3-5 NEA cathodes can be orders of magnitude lower than that from conventional photocathodes. The reasons for this are discussed. Yield and dark current data are given on 3-5 NEA cathodes in operating photomultipliers. Work supported in part by the Advanced Research Projects Agency of the Department of Defense and monitored by Night Vision Laboratory, U.S Army Electronics Command, under Contract No. DAAK 02-74-C-0069.     

Catalyst Type and Concentration Dependence in Catalytic Transfer Hydrogenolysis of α,β-Unsaturated Carbonyls and Nitriles Via Ammonium Formate

The catalytic reduction of a variety of α, β-unsaturated compounds into saturated analogs in the presence of other reducible moieties is described using ammonium formate as a hydrogen source. The rate dependence on the concentration of Pd-C catalyst as well as on 5% Pd-BaSO4 and Ra-Ni are also characterized.     

THz spectroscopic investigation of 2,4-dinitrotoluene

We have investigated the terahertz (THz) spectrum of 2,4-DNT by using Fourier transform infrared spectroscopy in the 0.2–19.5 THz region. We also examined low-frequency intermolecular or phonon modes between 0.2 and 1.8 THz via THz time-domain spectroscopy. The extracted absorption coefficient and refractive index of an intermolecular band at 1.08 THz are 110 cm⁻¹ and 1.67, respectively. Density functional theory (DFT) was applied to obtain structure and vibrational frequencies of 2,4-DNT. The calculated results are in agreement with the experimental data. Observed vibrational frequencies have been interpreted using DFT. Two intermolecular or phonon modes were identified at 1.08 and 2.52 THz.     

Assessment of head-space gas-liquid chromatography for the rapid detection of growth in blood cultures

Blood for transfusion was inoculated with between 10(0) and 10(2) colony-forming units (CFU) per ml of each of 59 microbial isolates and added to cooked meat broth. At intervals up to 72 h incubation, the cultures were examined by conventional visual inspection and automated head-space gas-liquid chromatography (HS-GLC). Forty-six isolates including all those examined of Staphylococcus aureus, Streptococcus pyogenes, S. pneumoniae, S. faecalis, S. milleri, S. mitior, S. mitis, S. salivarius, S. sanguis, Escherichia coli, Klebsiella pneumoniae, K. oxytoca, Proteus mirabilis, Morganella morganii, Serratia sp., Enterobacter cloacae, Bacterioides fragilis, Clostridium perfringens, Candida albicans, C. krusei and Torulopsis glabrata, and three isolates of Staphylococcus epidermidis, were detected by HS-GLC. HS-GLC failed to detect the growth of eleven isolates including all those of Pseudomonas aeruginosa, Acinetobacter calcoaceticus. Haemophilus influenzae, Corynebacterium sp. and two isolates of S. epidermidis. The growth of all 59 isolates were detected by visual inspection. No significant difference was found between HS-GLC analysis and visual inspection in the speed of detection of bacterial isolates. All the yeast isolates were detected by HS-GLC after 24 h incubation, indicating that it may be possible to detect fungemias earlier by HS-GLC analysis than by other methods.     

Temperature and Solvent Dependent Catalytic Transfer Hydrogenolysis in Aromatic Aldehydes and Ketones Via Ammonium Formate

Temperature control and solvent specification are used to reduce aromatic aldehydes and ketones to intermediate alcohols rather than methylene derivatives using HCO₂NH₄ as a catalytic hydrogen transfer agent. A mechanism for the catalytic reduction is proposed.