Line profile study from diode laser spectroscopy in the CH4 2ν3 band perturbed by N2, O2, Ar, and He

We present a line profile study for two lines in the 2ν₃ band of CH₄ recorded with a frequency stabilized tunable diode laser spectrometer. The broadening and narrowing (Dicke effect) parameters of the R(0) line perturbed by N₂, O₂, and He are derived from a simultaneous fitting of spectra at pressures from 20 to 300 Torr by using the soft and hard collision models. These parameters are determined for the A and F components of the unresolved R(3) manifold perturbed by N₂, Ar, and He from the line profile analysis of spectra at pressures between 50 and 500 Torr. The line mixing effect between the two F components is also taken into account and the absorber speed dependent effect on broadening is estimated for N₂ and Ar.     

Dynamics of interaction of H2 and D2 with Ni(110) and Ni(110) surfaces

Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H₂ and D₂ on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s₀, irrespective of surface temperature T_s and mean kinetic energy normal to the surface 〈 E_⊥ 〉. The desorbing molecules exhibit a cos θ_e angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s₀ is only about 0.05 if both the gas and the surface are at room temperature. s₀ is again independent of T_s, but increases continuously with 〈 E⊥ 〉 up to a value of ～0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos⁵θ_e angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.     

Electron paramagnetic resonance of gamma irradiated [(CH3)4N]InCl4 and [(CH3)4N]2CdCl4 single crystals

Gamma irradiated [(CH₃)₄N]InCl₄ and [(CH₃)₄N]₂CdCl₄ single crystals were investigated by electron paramagnetic resonance at ambient temperature, and it has been found that both compounds indicate the existence of (CH₃)₃N⁺ radicals. The g factors were found to be isotropic, and the hyperfine constant for H atoms was measured as 2.86 mT and is isotropic for this radical in these substances. The hyperfine coupling constant of the N nucleus with the hole in (CH₃)₃N⁺ in [(CH₃)₄N]InCl₄ was found to be anisotropic with the A_zz=2.92, A_yy=1.62 and A_xx=1.40 mT. From these, it has been revealed that the C_3v-axis of (CH₃)₃N⁺ radical performs rotational or jumping reorientational motions around a fixed axis, in addition to the rotations of protons in CH₃ groups and the rotational motions of CH₃ groups around the C_3v-axis of the radical. The g, and the hyperfine coupling factors of the N nucleus were isotropic in (CH₃)₃N⁺ in [(CH₃)₄N]₂CdCl₄. This indicates the motional behaviour of the radical in this compound is as in a liquid. This isotropic behaviour of the hyperfine coupling constants was found to be same until the attainable lowest temperature of 113 K in our laboratory.     

Elucidating surface properties of dry-etched ZnO using H2/CH4 and H2/CH4/Ar plasma

This paper reports a study of reactive ion etching (RIE) of n-ZnO in H₂/CH₄ and H₂/CH₄/Ar gas mixtures. Variables in the experiment were gas flow ratios, radio-frequency (rf) plasma power, and total pressure. Structural and electrical parameters of the etched surfaces and films were determined. Both the highest surface roughness and highest etching rate of ZnO films were obtained with a maximum rf power of 300 W, but at different gas flow ratios and working pressures. These results were expected because increasing the rf power increased the bond-breaking efficiency of ZnO. The highest degree of surface roughness was a result of pure physical etching by H₂ gas without mixed CH₄ gas. The highest etching rate was obtained from physical etching of H₂/Ar species associated with chemical reaction of CH₄ species. Additionally, the H₂/CH₄/Ar plasma treatment drastically decreased the specific contact and sheet resistance of the ZnO films. These results indicated that etching the ZnO film had roughened the surface and reduced its resistivity to ohmic contact, supporting the application of a roughened transparent contact layer (TCL) in light-emitting diodes (LEDs).     

Proton magnetic resonance study of the phase transitions of [N(CH3)4]2BCl4 (B=Co, Cu, Zn, and Cd) single crystals

The proton spin-lattice relaxation rates in [N(CH₃)₄]₂BCl₄ (B=⁵⁹Co, ⁶³Cu, ⁶⁷Zn, and ¹¹³Cd) single crystals grown using the slow evaporation method were investigated over the temperature range 120-400 K. It was found that the relaxation processes of ¹H for all the [N(CH₃)₄]₂BCl₄ crystals can be described with single exponential functions. The changes in the ¹H relaxation behavior in the neighborhood of the phase transition temperatures are used to detect changes in the state of internal motion. From the ¹H spin-lattice relaxation rate measurements for [N(CH₃)₄]₂BCl₄ crystals, the activation energies were calculated for each phase. The large values of the activation energies indicate that the N(CH₃)₄ groups are significantly affected during the transitions. Although these [N(CH₃)₄]₂BCl₄ crystals all belong to the group of A₂BX₄-type crystals, their ¹H spin-lattice relaxation rates have different temperature dependences and indicate the occurrence of different molecular motions within the crystals. We additionally show for the first time that the differences in ¹H spin-lattice relaxation rates among the [N(CH₃)₄]₂BCl₄ (B=⁵⁹Co, ⁶³Cu, ⁶⁷Zn, and ¹¹³Cd) single crystals arise from differences in the electron structures of the metal ions within the series.     

High-resolution Fourier spectroscopy of 14N2 and 15N2 infrared emission spectrum: Analysis of the B′3Σu−-B3Πg system

The rotational analysis of some bands of the B′³Σ_u^?-B³Π_g system of nitrogen (¹⁴N₂ and ¹⁵N₂) excited in a microwave discharge and recorded under high resolution with a Fourier transform interferometer allowed the determination of molecular constants for the v = 0, 1, 2 levels in the B′³Σ_u^? state. Main equilibrium constants of the B′³Σ_u^? state for ¹⁴N₂ and ¹⁵N₂ are (in cm^?1) T_e = 5789.2178 (relative to B³Π_g (v = 0); ω_e = 1516.6239; ω_ex_e = 11.9721; B_e = 1.473 098; α_B = 0.01 664; ω_eⁱ = 1465.3549; ω_ex_eⁱ = 11.1783; B_eⁱ = 1.375 209; α_Bⁱ = 0.01 505.     

Comparison of CH4/H2 and C2H6/H2 inductively coupled plasma etching of ZnO

CH₄/H₂-based discharges are attractive for dry etching of single crystal ZnO because of their non-corrosive nature. We show that substitution of C₂H₆ for CH₄ increases the ZnO etch rate by approximately a factor of 2 both with and without any inert gas additive. The C₂H₆/H₂/Ar mixture provides a strong enhancement over pure Ar sputtering, in sharp contrast to the case of CH₄/H₂/Ar. The threshold ion energy for initiating etching is 42.4 eV for C₂H₆/H₂/Ar and 59.8 eV for CH₄/H₂/Ar. The etched surface morphologies were smooth, independent of the chemistry and the Zn/O ratio in the near-surface region was unchanged within experimental error after etching with both chemistries. The plasma etching improved the band-edge photoluminescence intensity and suppressed the deep level emission from the bulk ZnO under our conditions, due possibly to removal of surface contamination layer.     

Acetylene spectra with a tunable diode laser: (ν4 + ν5)0+−ν41fQ branches of 12C2H2 and 12C13CH2

The rotational structure of the Q branches of the (ν₄ + ν₅)⁰⁺?ν₄^1f bands of ¹²C₂H₂ and ¹²C¹³CH₂ at 13.7 μm has been observed in a natural sample of acetylene by using a tunable diode laser as a source in a high-resolution infrared grating spectrometer equipped with a precision grating drive. Altogether 23 lines from J = 6 to 28 for ¹²C₂H₂ and 15 lines from J = 6 to 20 for ¹²C¹³CH₂ have been identified. The observed full width at half maximum of the resolved lines of these Q branches is very close to the calculated Doppler width. Molecular constants ν₀ + B″, B′ ? B″ ? 2D″, D′ ? D″, and H′ ? H″ have been derived from the measured line positions of the rotational structure.     

Auger spectra of CO,C2H4, C2N2 and C6H6 adsorbed on Pt(111)

Electron excited carbon KVV Auger spectra of CO, C₂H₄, C₂N₂ and C₆H₆ adsorbed on Pt(111) are compared. By estimating the effective Coulomb interaction between the final-state holes it is possible to associate some features with transitions observed in free molecule spectra, but others must involve at least one electron with energy within the conduction band of the metal. Such “cross-transitions” are associated with strong 2π* character of filled states in the presence of a core hole in molecules such as CO.     

High-density plasma etching of indium-zinc oxide films in Ar/Cl2 and Ar/CH4/H2 chemistries

The dry etching characteristics of transparent and conductive indium-zinc oxide (IZO) films have been investigated using an inductively coupled high-density plasma. While the Cl₂-based plasma mixture showed little enhancement over physical sputtering in a pure argon atmosphere, the CH₄/H₂/Ar chemistry produced an increase of the IZO etch rate. On the other hand, the surface morphology of IZO films after etching in Ar and Ar/Cl₂ discharges is smooth, whereas that after etching in CH₄/H₂/Ar presents particle-like features resulting from the preferential desorption of In- and O-containing products. Etching in CH₄/H₂/Ar also produces formation of a Zn-rich surface layer, whose thickness (∼40 nm) is well-above the expected range of incident ions in the material (∼1 nm). Such alteration of the IZO layer after etching in CH₄/H₂/Ar plasmas is expected to have a significant impact on the transparent electrode properties in optoelectronic device fabrication.     

Magnetodielectric coupling in MnCr2O4 spinel

We have investigated the magnetic and dielectric properties of polycrystalline samples of the spinel MnCr₂O₄. Below the ferrimagnetic ordering temperature at T_N∼43 K, both magnetization and dielectric measurements show signatures of the onset of a conical structure at T_s∼17 K and a lock-in temperature at T_f∼14 K. These values are similar to those previously reported for single-crystal samples, where the spiral structure is short-range ordered (SRO) at low temperatures. The application of magnetic field suppresses the dielectric anomaly at T_f indicating that the coherence length of the ordering increases. MnCr₂O₄ exhibits a symmetrical magnetodielectric response between T_f and T_s that scales with the square of the magnetization. This suggests that the magnetodielectric coupling originates from the P²M² term in the free energy expansion. The magnetodielectric response becomes asymmetric with respect to field below T_f.     

Neutron scattering investigation of the magnetic ground state of PrO2

We report on a series of neutron experiments on stoichiometric PrO₂. The material, which has the cubic CaF₂ structure, orders antiferromagnetically at 14 K with a type I magnetic structure as found in UO₂. The ordered moment is 0.6 ± 0.1 μ_B/Pr atom. No lattice distortion has been observed below T_N with a high-resolution neutron-diffraction experiment. Neutron inelastic scattering at the Intense Pulsed Neutron Source, Argonne National Laboratory, has been used to determine the crystal-field splitting of 130 ± 5 meV between the Γ₈ ground state and the Γ₇ excited state of the $\text{J =}\text{5}\text{2}$ multiplet. This represents the first direct observation of a crystal-field splitting in the rare-earth or actinide dioxides, and the largest such electronic splitting ever observed by neutron scattering. We propose that the ground state of PrO₂ is the Γ₈ quartet with the degeneracy lifted by a dynamic Jahn-Teller effect. The A₄〈r⁴〉 crystal field potential term for PrO₂ is - (57 ± 3) meV, and the significance of this determination for other oxides is discussed.     

Mechanical properties and hardness of new carbon-rich superhard C11N4 from first-principles investigations

The structural, mechanical properties and hardness of the new carbon-rich material C₁₁N₄ are studied by first-principles total energy calculations based on the density-functional theory. We use the empirical equations of state (EOS) to investigate the lattice properties and bulk modulus. It is found that the calculated lattice constants and bulk modulus are in good agreement with previous calculations. And the full set elastic constants are calculated using the stress-strain method. The Voigt-Reuss-Hill approximation is used to evaluate the mechanical moduli. The elastic constants show that the two phases of C₁₁N₄ are mechanically stable. The tetragonal-C₁₁N₄ (α-C₁₁N₄) exhibits larger mechanical moduli than the orthorhombic-C₁₁N₄ (β-C₁₁N₄). The mechanical anisotropy is calculated of several different anisotropic indexes and factors, such as universal anisotropic index (A^U), the percent anisotropy (A_G and A_B) and shear anisotropic factors (A₁, A₂ and A₃). Furthermore, the hardness of α-C₁₁N₄ and β-C₁₁N₄ are evaluated according to the intrinsic hardness calculation theory. α-C₁₁N₄ is predicted to be a superhard material with the Vickers hardness of 67.17 GPa, which is slightly higher than that of the cubic boron nitride. And the β-C₁₁N₄ is also a superhard material with the calculated Vickers hardness of 45.63 GPa. C₁₁N₄ can be considered as candidate superhard compounds.     

Structural transformations and magnetic properties of Fe60Pt15B25 and Fe60Pt25B15 nanocomposite alloys

Structural and magnetic properties of two rapidly solidified and post-annealed Fe₆₀Pt₁₅B₂₅ and Fe₆₀Pt₂₅B₁₅ alloys are compared. The as-quenched Fe₆₀Pt₁₅B₂₅ ribbon was fully amorphous whereas in the Fe₆₀Pt₂₅B₁₅ alloy the amorphous phase coexists with an fcc FePt disordered solid solution. Differential scanning calorimetry curves of both alloys reveal a single exothermal peak with onset temperatures of 873 and 847 K for Fe₆₀Pt₁₅B₂₅ and Fe₆₀Pt₂₅B₁₅, respectively. Magnetically hard, tetragonal ordered L1₀ FePt and magnetically soft Fe₂B nanocrystalline phases were formed due to the annealing of the alloys, as indicated by X-ray diffraction and Mössbauer spectroscopy measurements. Two-phase behavior was detected in the temperature dependence of magnetization of the annealed samples. A magnetic hardening was observed for all annealed ribbons. Magnetic properties of the annealed alloys, studied by hysteresis loop measurements, were related to the differences in the relative fractions of the hard and soft magnetic phases calculated from Mössbauer spectra. The alloy with 25 at% Pt exhibits better hard magnetic properties (H_c=437 kA/m, M_r/M_s=0.74) than the alloy with smaller Pt content (H_c=270 kA/m, M_r/M_s=0.73) mainly due to the larger abundance of the ordered tetragonal FePt phase.     

A static SIMS/TPD study of the kinetics of methoxy formation and decomposition on O/Pt(111)

The kinetics and mechanism of the decomposition of methanol (CH₃OD) on oxygen-covered Pt(111) were studied using static secondary ion mass spectrometry (SIMS) and temperature programmed desorption (TPD). The initial sticking coefficient and the saturation first layer coverage of CH₃OD are unity and 0.36 ML, respectively. The maximum amounts decomposed in TPD on O/Pt(111) and clean Pt(111) are 0.19 and 0.047 ML, respectively. At low methanol coverages (< 0.05 ML) on O/Pt(111) the only reaction products were CO₂, H₂O and D₂O, whereas at saturation CO, H₂O, D₂O and H₂ were observed. The decomposed amount did not saturate at or before the onset of molecular methanol desorption, but increeased monotonically until saturation of the first layer. No oxygen exchange was observed between CH₃OD and preadsorbed ¹⁸O. A decomposition mechanism involving methoxy and hydroxyl type species is proposed. Methanol coverages as low as 0.002 ML could be detected with SIMS. The CH₃⁺ ion was the most intense ion in the positive SIMS spectrum of both methanol and methoxy. Larger ion clusters such as (CH₃OD)_n⁺ (n = 2, 3) developed successively at specific multilayer coverages. At low coverages on O/Pt(111), methoxy formation occurs above 125 K and its decomposition becomes detectable above 150 K during temperature programming. In the isothermal mode, the SIMS CH₃⁺ ion was used to follow the kinetics. Over the temperature range 120–145 K, the second order Arrhenius rate parameters for methoxy formation are E = 5.5±0.7 kcal/mol and A = 1.5×10^−7±0.6 cm²/s·molecule for an initial methanol coverage of 0.05 ML. Methoxy decomposition was studied in the temperature range 150–165 K and at an initial coverage of 0.015 ML. The first order kinetic parameters, E = 11.4±0.5 kcal/mol and A = 5.3×10^13±1 s⁻¹ were derived. Advantages and limitations of using SIMS as a tool for kinetic studies are discussed.     

The effect of Cu substitution on the A1g mode of La0.7Sr0.3MnO3 manganites

We report on the first Raman data of Cu substituted La_1−ySr_yMn_1−xCu_xO₃ (0≤x≤0.10 and 0.17≤y≤0.3, accordingly in order to have the same Mn⁴⁺/[Mn⁴⁺+Mn³⁺] ratio), collected in the frequency range 100-900 cm⁻¹ and at room temperature, with parallel (e_i∥e_s) and crossed (e_i⊥e_s) polarizations of the incident (e_i) and scattered (e_s) light. Spectra were fitted with a Drude-Lorentz model, and peaks at 190-220 and 430 cm⁻¹, together with two broad structures centered at near 500 and 670 cm⁻¹, have been found. We also have observed that the A_1g mode is substantially shifted with increasing Cu substitution. The A_1g phonon shift is a linear function of the tolerance factor t and the rhombohedral angle α_r, thus following the structural changes of the MnO₆ octahedra in the system.     

PAM analysis of the microwave spectra of thioacetic acid,CH3COSH and CH3COSD

The microwave spectra of two isotopic species of thioacetic acid, CH₃COSH and CH₃COSD, have been studied. Using the principal axis method (PAM), including terms through n = 6 in the perturbation series and the denominator correction, the spectra were analyzed and 45 lines for CH₃COSH and 40 lines for CH₃COSD were assigned. The parameters obtained by the least-squares analysis are A = 9913.29 ± 0.56 MHz, B = 4923.11 ± 0.23 MHz, C = 3354.60 ± 0.24 MHz, θ = 57.080 ± 0.030°, s = 6.2980 ± 0.0012, and I_α = 3.198 ± 0.020 amu$\text{A}\text{?}{}^2$ for CH₃COSH, and A = 9662.80 ± 0.78 MHz, B = 4810.74 ± 0.26 MHz, C = 3273.92 ± 0.18 MHz, θ = 55.097 ± 0.024°, s = 5.9742 ± 0.0016, and I_α = 3.171 ± 0.020 amu$\text{A}\text{?}{}^2$ for CH₃COSD. The barrier to internal rotation of the methyl group is V₃ = 222.6 ± 1.4 cal/mole for CH₃COSH and V₃ = 212.9 ± 1.4 cal/mole for CH₃COSD. The Stark effect measurements of A species transitions for CH₃COSH led to the dipole moment μ = 1.821 ± 0.013 D with the components μ_a = 0.191 ± 0.010 D and μ_b = 1.811 ± 0.013 D.     

Near-infrared absorption of MgF2:Fe2+

The multiphonon sideband of the spin allowed transition 6T_2g:A₁ ? 2〉 → 6E_g:B₃ ? 2〉 within the 3d⁶ orbital of Fe²⁺ in D_2h symmetry was analyzed with respect to the one phonon absorption and density of states of the coupled phonon modes. Temperature dependence of the zero-phonon line shows, that the axial anisotropy of the S = 2 spin multiplet in the excited B₃ state is about 0.9 cm^-1.     

Gas-surface scattering from W(100): Kinetic energy dependence of the effective corrugation

Angular and velocity distributions have been obtained for the scattering of argon and N₂ from a W(100) surface for incidence energies, E_i, in the range 0.03–5.5 eV, and surface temperatures, T_s, from 90 to 1700 K. For E_i < 0.1 eV, we find broad angular distributions (> 60° FWHM) which are relatively insensitive to T_s and velocity distributions which are inconsistent with parallel momentum conservation, indicating a relatively corrugated interaction potential. Increasing E_i first causes a rapid narrowing in these distributions, but as E_i exceeds ~ 2 eV, they broaden again, as the effective corrugation again becomes large.     

Multispectrum fits for line mixing in the ν3 band Q branch of methane

First-order line-mixing coefficients and model relaxation matrix element scaling factors have been obtained for allowed transitions in the ν₃ band Q branch of CH₄ broadened by H₂, He, N₂, O₂, Ar, and CH₄. The broadening, shifting, Dicke-narrowing, and line-mixing parameters are determined by simultaneous least-squares fitting of spectra at pressures from 0.014 to 66.66 kPa recorded with a high-resolution difference-frequency laser. These results confirm, improve, and extend a previous analysis of the lower pressure (?13.3 kPa) data [A.S. Pine, J. Chem. Phys. 97 (1992) 773] which yielded averaged coefficients of individually fit spectra where adjacent broadened lines are still partially resolved.