ALBERT

Notes: Bound excited electronic states are observed in the anions of 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,5-dimethyl-N,N'-dicyanoquinodiimine (Me2–DCNQI), chloranil, and hexacyano-butadiene using electron photodetachment spectroscopy. Intensity dependence studies and two color experiments are consistent with a two-photon detachment mechanism. A mechanism is proposed to explain the observed photodetachment. The nature of the electronic state responsible for these excited states is discussed and other anions are suggested which should possess bound excited electronic states.

Notes: Magnetization measurements have been performed on epitaxial layers of Hg1−xMnxTe (MMT) using an alternating gradient field magnetometer (AGFM). The layers, which have a Mn concentration in the range 0.05〈x〈0.20, and thicknesses between 1 and 6 μm, are deposited by metalorganic vapor phase epitaxy (MOVPE) techniques. The MMT films are grown on GaAs substrates with CdTe and ZnTe buffer layers. Growth is achieved by the interdiffused multilayer process (IMP), where alternate layers of HgTe and MnTe are deposited, and then heated, to produce diffusion and, hence, Hg1−xMnxTe. The MnTe layers that are grown are roughly 1 nm thick, and so grow in the cubic structure, rather than the nickel arsenide structure observed in the bulk material.1 The magnetization of the majority of the samples is the expected paramagnetic behavior with values of χ close to those observed in the bulk material.2 However, a number of the samples have shown evidence for magnetic order at room temperature, and this is also observed in MnTe layers grown by MOVPE. The samples showing saturation of the magnetization also shown an anomalous behavior of the Hall voltage below 100 K and at fields up to 0.3 T. The anomalies in the HgMnTe samples are thought to be due to MnTe inclusions left over from the MOVPE growth process.© 1997 American Institute of Physics.

Notes: Two compositions of BaTiO3 positive temperature coefficient of resistance ceramics, prepared identically except for the fact that a small addition of Mn (0.04 at. %) was made to one of them, were studied. The samples were sintered simultaneously in air at 1400 °C for 1 h and then annealed at 1200 ° for 5 h, using a muffle furnace. Room-temperature dielectric measurements in the audio- and radio-frequency ranges confirmed that Mn has a negligible effect on the bulk resistance. Arrhenius plots of resistivity vs 1/[Tε'm(T)] were found to give straight lines for Tc〈T〈Tmax (where ε'm is the relative permittivity of the specimen measured at a constant frequency of 30 kHz, Tc is the ferroelectric transition temperature, Tmax is the temperature corresponding to the maximum in resistivity, and T is the absolute temperature), in accordance with the well-known Heywang model. The height of the potential barriers at different temperatures, as calculated from the slopes of these plots, were found to increase by about 40% (from ∼0.34 to ∼0.50 eV) by the addition of Mn. A small increase in the acceptor-state density at the grain surfaces, which was again obtained from these plots, was observed in Mn-doped specimens (3.9×1013 cm−2 as compared to 2.7×1013 for Mn-free specimens). It was also found that the inclusion of Mn had a negligible effect on ε'm above Tc.

Notes: Excited electronic states of the radical anion of 7,7,8,8- tetracyanoquinodimethane (TCNQDN) which are stable with respect to electron loss have been observed by electron photodetachment spectroscopy. An onset for photodetachment is seen at 1.2 eV, considerably below the known binding energy (2.8 eV). We propose a sequential two-photon detachment process, supported by intensity dependence studies. TCNQDN is representative of a general class of anions which we believe will have such bound states.

Notes: Electron photodetachment spectra have been measured in an ICR spectrometer for the enolate ions of acetone, cyclobutanone, cyclopentanone, cyclohexanone, cycloheptanone, methyl vinyl ketone, pinacolone, propionaldehyde, and 1,1,1-trifluoroacetone enolates. Electron affinities have been determined for acetone enolate radical 1.758±0.019, cyclobutanone enolate radical 1.801±0.008, cyclopentanone enolate radical 1.598±0.007, cyclohexanone enolate radical 1.526±0.010, cycloheptanone enolate radical 1.444+0.02/−0.002, tert-butyl methyl ketone (pinacolone) enolate radical 1.755+0.05/0.005, propionaldehyde enolate radical 1.621±0.006, and 1,1,1-trifluoroacetone enolate radical 2.625±0.010 eV. Autodetaching dipole-bound states are observed in some but not all of these spectra. The mechanism for autodetachment of these states is discussed and it is seen that the binding of an electron by a dipole is very sensitive to the motions of the dipole. The motions of the dipole can be predicted from the rotational motions of the molecule, allowing us to correlate the observation of dipole-bound states with the rotational motions of the dipole moment.

Notes: We report on quantitative, spatially resolved density, temperature, and velocity measurements on ground-state atomic hydrogen in an expanding thermal Ar–H plasma using two-photon excitation laser-induced fluorescence (LIF). The method's diagnostic value for application in this plasma is assessed by identifying and evaluating the possibly disturbing factors on the interpretation of the LIF signal in terms of density, temperature, and velocity. In order to obtain quantitative density numbers, the LIF setup is calibrated for H measurements using two different methods. A commonly applied calibration method, in which the LIF signal from a, by titration, known amount of H generated by a flow-tube reactor is used as a reference, is compared to a rather new calibration method, in which the H density in the plasma jet is derived from a measurement of the two-photon LIF signal generated from krypton at a well-known pressure, using a known Kr to H detection sensitivity ratio. The two methods yield nearly the same result, which validates the new H density calibration. Gauging the new "rare gas method" by the "flow-tube reactor method," we find a krypton to hydrogen two-photon excitation cross section ratio σKr(2)/σH(2) of 0.56, close to the reported value of 0.62. Since the H density calibration via two-photon LIF of krypton is experimentally far more easy than the one using a flow-tube reactor, it is foreseen that the "rare gas method" will become the method of choice in two-photon LIF experiments. The current two-photon LIF detection limit for H in the Ar–H plasma jet is 1015 m−3. The accuracy of the density measurements depends on the accuracy of the calibration, which is currently limited to 33%. The reproducibility depends on the signal-to-noise (S/N) ratio in the LIF measurements and is orders of magnitude better. The accuracy in the temperature determination also depends on the S/N ratio of the LIF signal and on the ratio between the Doppler-width of the transition and the linewidth of the excitation laser. Due to the small H mass, the current linewidth of the UV laser radiation is never the accuracy limiting factor in the H temperature determination, even not at room temperature. Quantitative velocity numbers are obtained by measuring the Doppler shift in the H two-photon excitation spectrum. Both the radial and axial velocity components are obtained by applying a perpendicular and an antiparallel excitation configuration, respectively. The required laser frequency calibration is accomplished by simultaneously recording the I2 absorption spectrum with the fundamental frequency component of the laser system. This method, which is well-established in spectroscopic applications, enables us to achieve a relative accuracy in the transition frequency measurement below 10−6, corresponding to an accuracy in the velocity of approximately 200 m/s. This accuracy is nearly laser linewidth limited. © 2002 American Institute of Physics.

Notes: Positive temperature coefficient of resistance BaTiO3 specimens containing different donor dopant concentrations of Ho ranging from 0.05 to 1.8 at. % were investigated. The intergranular barrier layer capacitance per unit area, C'L, measured at a constant frequency of 30 kHz at both 40 and 160 °C was found to be proportional to the donor concentration up to 0.55 at. %, but then began to decrease as the donor concentration was increased beyond this. This indicated that both the density of acceptor states at the grain surfaces, Ns, and the relative permittivity εL of the material within the barrier layer were not affected by donor impurity concentrations below 0.55 at. % Ho. However, above this level of Ho concentration, the decrease in C'L appears to be related mainly to an increase in the value of Ns although it is possible that there were changes in εL. Initially both the maximum resistance and the room-temperature resistance (normalized per grain boundary per unit area), ρ'max and ρcold, respectively, were found to decrease sharply with donor concentration towards a broad minimum between ∼0.5 and ∼1.5 at. %, followed thereafter by a gradual increase. The temperature Tmax at which ρ'max occurred was also affected by the donor concentration; initially Tmax was found to increase with donor concentration followed by a reduction forming a broad maximum between about the same donor concentration limits corresponding to the minima in ρcold and ρ'max. These results are interpreted in terms of the well-established Heywang model.

Notes: Homoepitaxial layers of CdTe ranging from 0.5 to 5 μm in thickness have been grown on twin free, single-crystal {111}A oriented CdTe substrates by metalorganic vapor phase epitaxy over the temperature range 360–440 °C. The as-grown surfaces were invariably very smooth resulting in reflection high energy electron diffraction (RHEED) patterns characterized by streaks of diffracted intensity lying perpendicular to the shadow edge of the sample. After a light chemical etch, the 〈110〉 zone axis RHEED patterns exhibited an arrangement of double spots which corresponds to a twinned fcc structure. Furthermore, this twinned structure was observed in all layers grown on substrates of this specific orientation.