New Optically Pumped FIR Laser Lines and Frequency Measurements from CH3OD

This work reports the discovery of 45 new optically pumped far-infrared (FIR) laser lines from CH₃OD in the range 42.6 m to 207.2 m. A highly efficient CO₂ laser was used as the pump laser for an Optically-pumped Fabry-Perot FIR laser. The frequencies of most of the new lines were measured in the range 1.7 to 6.4 THz. Twenty-eight CO₂ lines were used as pump lines; nineteen of these had frequencies lower than those previously used to pump CH₃OD in a FIR laser. Sixteen of the new FIR lines had absorption transitions which belong to the OD bending vibrational mode.     

New FIR laser lines and frequency measurements in CD3OD

Twenty-four new submillimeter laser lines in fully deuterated methyl alcohol (CD₃OD) in the wavelength range from 52 to 328 m have been obtained in a Fabry-Perot FIR resonator by optically pumping the methanol with a cw CO₂ laser. We have made accurate wavelength measurements and have determined the relative polarization of most of the known CD₃OD laser lines. The frequencies of 13 of the strongest lines were also measured.Contribution of the U.S. Government, not subject to copyright.     

Analysis and modeling of a high-temperature superconducting floating resonant strip in waveguide

A two-gap floating resonant strip is used for characterization of the high-temperature superconductor, YBa₂Cu₃O_7–. The method has the advantages of simplicity, no electrical contact, operation at various resonant frequencies, and of requiring only a small sample. An analysis was devised that allows for the accurate design of the strip dimensions to produce a desired resonant frequency. Experimental measurements in X and Kuband (8–18 GHz) agree well with the calculations. The sensitivities of the circuit to positional errors and size variations of the resonant strip were investigated. The surface resistance was measured and compared favorably with the theory.     

The influence of substrate material and annealing procedure on the properties of superconducting thin films

Thin layers YBa₂Cu₃O_7–x. are deposited by a laser ablation technique using a pulsed excimer laser operating at 308 nm. The influence of the substrate material and the annealing procedure on the superconducting behaviour of the 123 film and the reactions between the film and the substrate are studied by resistance, X-ray patterns and TEM measurements. The best results are obtained for deposition on (100) SrTiO₃ substrates. The resistance of the 1 m thick film shows a metallic behaviour, an onset in superconductivity at a temperature of 90 K, and has zero resistance at 86 K. The 123 material has a preferential oriented c-axis perpendicular to the surface plane.     

Stimulated emission of a laser photon by the excited states of a three-level atom

We study the stimulated emission spectra arising from the emission of a laser photon by two excited states of a three-level atom interacting with a laser field at low intensities. The lifetimes of the stimulated photons emitted by the two excited states are much longer than those emitted spontaneously, while the intensities of the induced peaks take negative values indicating that amplification occurs at low frequencies. The ratio of the intensities of the light emitted by the excited states |3 > and |2 > of the atom is proportional to (₃/₂)^1/2, where ₃ and ₂ are the radiative decay rates of the spontaneously emitted photons by the excited states |3 > and |2 > into the ground state |1 > of the atom, respectively. An absorption spectrum is induced into the ground state of the atom by the laser field. The competition between induced absorption and stimulated emission at low frequencies without population inversion is considered in the low-intensity limit of the laser field. It is shown that for values of ₃/₂ > 1 the relative intensity (height) of the induced peak takes positive values implying that the process of the induced absorption dominates. As the ratio ₃/₂ increases, the height of the induced peak decreases and vanishes for values of ₃/₂ < 400. For values of ₃/₂ > 400, the height of the induced peak becomes negative indicating that the process of the stimulated emission (amplification) is likely to occur at low frequencies. The computed spectra are graphically presented and discussed.Issued as NRCC No. 39088     

Single-plate far-infrared Fabry-Perot étalons with high-T c superconducting coatings

We report on single-plate far-infrared Fabry-Perot étalons with high-T _c superconducting coatings. The étalons consisted of plane-parallel silicon plates covered on both sides with (001)-oriented YBa₂Cu₃O_7- thin films. While the far-infrared transmission spectra showed a modulation for temperatures aboveT _c , sharp Fabry-Perot resonances were observed for low temperatures. In a frequency range around 2 THz we found values for the finesse of 10 to 20 and peak0transmissivities of 0.05 to 0.4 Due to the high transparency of Si, measuring of the transmissivity of étalons allows determination of the optical constants of the coatings.     

Transition of a homogeneous type II superconductor cylinder under the influence of axial current

We have calculated the magnetic field distribution and the resistance of a homogeneous type II superconductor cylinder at transition under the influence of axial current in zero external magnetic field. In this calculation we have expressed the magnetic field dependence on flow resistivity _f by two or more potential functions and have shown that the accuracy of this approximation is sufficient. For some values of the lower critical field and various temperatures, the magnetic field distribution and the resistance of the cylinder are calculated as a function of the applied electrical field and/or the applied current. The temperature dependence of the electric field at which, the normal state appears on the surface of the cylinder is also given.     

Molecular origins of surface poisoning during CO oxidation over RuO2(1 1 0)

Metal oxides are of interest as environmental oxidation catalysts, but practical applications are often limited by poorly understood surface poisoning processes. RuO₂ is active for CO oxidation under UHV conditions but is deactivated by some surface poisoning processes at ambient pressures. In this work, we report kinetic models of surface poisoning during CO oxidation over RuO₂(1 1 0), based on data obtained from plane-wave, supercell DFT calculations. While a surface carbonate is stable at low O₂ pressures and high CO₂ exposures, it is not stable under catalytic conditions. A surface bicarbonate is more stable and deactivates the RuO₂ surface over a wide range of CO and oxygen pressures in the presence of trace amounts of water.     

Measurement of the boundary frequency of the inertial interval of capillary wave turbulence at the surface of liquid hydrogen

The boundary frequency was experimentally measured for the upper edge of an inertial interval corresponding to the Kolmogorov spectrum for energy distribution over the oscillation frequencies at the surface of liquid hydrogen. It is shown that the dependence of boundary frequency ω_b on the wave amplitude η_p at the pump frequency ω_p is well described by the power law ω_b～η _p ^4/3 ω _p ^23/9 .     

Interaction of H2O,CO, and H2 with Pt(111) and Pt(111)+K surfaces

The potential energy and surface dipole were calculated as a function of the geometry of the coadsorbed systems using the cluster method and theoretical oscillation frequencies and work function changes were compared with experiment. It was found that the K fills unoccupied Pt 5d states and reduces the local polarizability of the metal. The H₂O molecule binds to the K atom, such that the H atoms point towards the surface inducing an increase in the work function. For the CO molecule a charge transfer (KCO) through the surface stabilizes the bond and induces a change of adsorption place (on-topbridge). The K increases the tendency to H₂ dissociation because of the local decrease of the work function. Zero-point energy effects add important dynamical features to the electronic H₂- surface interaction. Three examples for the Pt(111)-H₂ system are presented: (i) A virtual attractive potential well produced by the softening of the H-H bond near the surface, (ii) a virtual potential barrier for dissociation due to the hindering of molecular rotations at the surface, and (iii) a change in the apparent surface temperature in H₂ desorption processes.     

Effect of trapped electrons in ultrathin SiO2 on the two-state inversion capacitance at varied frequencies of metal-oxide-semiconductor capacitor

Two-state inversion capacitances of a metal-oxide-semiconductor capacitor (MOSCAP) at varied AC frequencies after negative/positive constant voltage stress (negative/positive CVS) treatments are investigated. When the device was biased into inversion, a low/high inversion-capacitance state (set state/reset state) was achieved after the negative/positive CVS treatments with/without a few trapped electrons in the ultrathin SiO₂ layer. The inversion capacitances of set states were frequency independent, whereas those of reset states increased with the decreasing frequencies. It is different from the general characteristics of an MOSCAP whose inversion capacitances disperse at low frequencies. For this observed finding of the two-state inversion capacitances at varied frequencies, a mechanism of trapped-electrons-induced screening effect on the inversion electrons is proposed. The number of the trapped electrons in the SiO₂ layer affects the number of the inversion electrons, and thus dominates the values of the inversion capacitances. Besides, simulation curves of the inversion capacitances of set states are demonstrated. They are fitted well with the experimental data utilizing the mechanism we proposed. This work investigates further into the influence of the trapped electrons in the ultrathin SiO₂ layer on the inversion capacitance response.     

Effect of chemical etching on the Cu/Ni metallization of poly (ether ether ketone)/carbon fiber composites

Poly(ether ether ketone)/carbon fiber composites (PEEK/Cf) were chemical etched by Cr₂O₃/H₂SO₄ solution, electroless plated with copper and then electroplated with nickel. The effects of chemical etching time and temperature on the adhesive strength between PEEK/Cf and Cu/Ni layers were studied by thermal shock method. The electrical resistance of some samples was measured. X-ray photoelectron spectroscopy (XPS) was used to analyze the surface composition and functional groups. Scanning electron microscopy (SEM) was performed to observe the surface morphology of the composite, the chemical etched sample, the plated sample and the peeled metal layer. The results indicated that CO bond increased after chemical etching. With the increasing of etching temperature and time, more and more cracks and partially exposed carbon fibers appeared at the surface of PEEK/Cf composites, and the adhesive strength increased consequently. When the composites were etched at 60 °C for 25 min and at 70-80 °C for more than 15 min, the Cu/Ni metallization layer could withstand four thermal shock cycles without bubbling, and the electrical resistivity of the metal layer of these samples increased with the increasing of etching temperature and time.     

Influence of the acoustic phonon scattering anisotropy on the distribution function and the electronic transport inn-Si

The excess noise of dc current-carrying thin metal wires in gases of different kind and pressure has been investigated. The noise depends on the kind of the gas. Therefore its origin must be in the gas or in the boundary between the metal and the gas. By means of a simple theory the power spectrumW _s of the random heat current _s flowing into the gas (surface source) can be calculated from the observed noise voltage. At frequencies above 100 HzW _s is given by a Nyquist formula, at lower frequencies an additional term 2 · 10^–3 · ₀ ² /N_hf is superimposed (₀ = dc heat current,f=frequency). The numberN _h characterizes this Hooge-contribution. A lot ofN _h -values for different metals, gases, pressures and heating currents have been measured.N _h is smaller for light gases and small wire diameters. For heavy gasesN _h is about ten times larger. A possible qualitative explanation is given. At higher pressures and heating currents thef ^–1 dependence turns over into af ^–3 one. This may be due to gas convection.Dedicated to Prof. Dr. H.E. Müser on the occasion of his 60th birthday     

Optically pumped submillimeter laser lines from CH2F2 and CD2Cl2 using a12C18O2 CW laser

Thirty-nine new submillimetre laser lines in CH₂F₂ and twelve in CD₂Cl₂ have been obtained in a Fabry-Perot FIR resonator by optically pumping with a CW¹²C¹⁸O₂ laser. The wavelength range obtained for CH₂F₂ is 126m to 1091m and for CD₂Cl₂ 212m to 774m. The wavelength measurements are accurate to within 5.10^–3. The relative polarisations of the pump laser and the FIR laser output were also determined. Tentative assignments of the IR and FIR transitions were made using existing microwave data.     

RF properties of superconducting vanadium

Q values of a superconducting vanadium cavity in theT E ₀₁₁ mode at 9.6 GHz were measured to determine the residual surface resistanceR ₀ and the reduced energy gap 2(0)/kT _c with respect to different surface treatments such as electropolishing, oxipolishing and anodizing. The residual surface resistance was found to be lowest for the oxipolished cavity, corresponding to a residualQ of 2×10⁸ at 1.2 K. Independent of the surface treatment the reduced energy-gap value amounted to 3.52±0.1. The influence of different vanadium oxides on the rf properties of superconducting vanadium was investigated.     

Pressure dependence of the Shubnikov-de Haas oscillation spectrum of \beta'-(BEDT-TTF)4(NH4)[ Cr(C2 O4)3] .DMF

The Shubnikov-de Haas (SdH) oscillation spectra of the -(BEDT-TTF)₄(NH₄)[ Cr(C₂O₄)₃] .DMF organic metal have been studied in pulsed magnetic fields of up to either 36 T at ambient pressure or 50 T under hydrostatic pressures of up to 1 GPa. The ambient pressure SdH oscillation spectra can be accounted for by up to six fundamental frequencies which points to a rather complex Fermi surface (FS). A noticeable pressure-induced modification of the FS topology is evidenced since the number of frequencies observed in the spectra progressively decreases as the pressure increases. Above 0.8 GPa, only three compensated orbits are observed, as it is the case for several other isostructural salts of the same family at ambient pressure. Contrary to other organic metals, of which the FS can be regarded as a network of orbits, no frequency combinations are observed for the studied salt, likely due to high magnetic breakdown gap values or (and) high disorder level evidenced by Dingle temperatures as large as ≃7 K.     

Connection of Residual Losses of a Microwave Energy in Superconducting State of YBa2Cu3O7?δ Films on Sapphire Substrates with Their Resistance in a Normal State

In the work, the results of measurements of temperature dependence of the surface resistance Rs(T) of the YBa₂Cu₃O_– epitaxial films deposited on sapphire substrates have been presented. The films were obtained using the method of impulse laser deposition. The measurements of surface impedance Rs(T) have been carried out at a frequency of 135 GHz. It is shown that Rs(T) agrees with existing model, considering heterogeneity in the YBa₂Cu₃O_7– epitaxial films as the normal spherical domains. Analysis has been carried out for the set of specimens which were obtained at different technological regimes and have different critical parameters and geometry.     

Effect of phase structures on the formation rate of hydroxyl radicals on the surface of TiO2

To study the relationship between the phase structures of TiO₂ and the photoinduced hydroxyl radicals (OH), TiO₂ nanocrystallines were synthesized by a hydrolysis-precipitate method using tetrabutylorthotitanate (TBOT) as precursor, and then calcined at 450, 600, 700, 800 and 900 °C for 2 h, respectively. The calcined samples were characterized by X-ray diffraction and N₂ sorption. The formation rate of OH on the surface of UV-illuminated TiO₂ was detected by the photoluminescence (PL) technique using terephthalic acid as a probe molecule. The results show that with increasing calcined temperatures, the amorphous (Am) TiO₂ precursor begins to turn into anatase (A) at 450 °C and rutile (R) phase appears at 600 °C, which is completely turned into the rutile phase at 900 °C. The BET specific surface areas of the catalyst decrease as the calcined temperatures increase. TiO₂ sample calcined at 600 °C, with a mixed phase of anatase and rutile, shows the highestOH formation rate, and the order of the OH formation rate is as follows: A+R>A>R>Am. Phase structures of TiO₂ play a more important role than specific surface areas in the OH formation rate. Two phase structure of anatase and rutile with a proper ratio is beneficial to the OH formation due to decrease of the combination rate of photo-generated electrons and holes. Our experimental result implies that the mixed phase of anatase and rutile can markedly enhance the photocatalytic activity of TiO₂.     

De Haas-van Alphen effect in the organic superconductor κ-(BEDT-TTF)2I3

In the two-dimensional organic superconductor -(BEDT-TTF)₂I₃ de Haas-van Alphen oscillations were observed at magnetic fields above 5 T and temperatures between 0.4 and 2 K. We found two dHvA frequencies at 3.846 kT and 0.570 kT, which correspond to the cross-sectional areas of the Fermi surface expected from a tight-binding calculation. From the temperature dependence of the oscillation amplitudes the effective mass belonging to the larger orbit was found to be 3.80m_o. Precise measurements of the angular dependence of the dHvA frequency show no deviation from that expected for a cylindrical Fermi surface. The angular dependence of the amplitude including spin splitting zeroes can essentially be described by a two-dimensional Fermi surface. Certain systematical deviations, however, hint for a slight corrugation.