An alternative to enhanced quantum diffusion in solid H2 below 0.3K

In samples of bulk solid H₂ having orthohydrogen concentration (x_o) below 3.1% that are cooled quickly at higher temperatures, it is found that the nmr lineshape and intensity are not time dependent at fixed temperature below 0.3K. Also, it is found that it is unnecessary to assume enhanced quantum diffusion rates to explain the temperature dependent lineshape changes observed, as they can be accounted for by changes in the transition rate between the rotational sublevels of unpaired oH₂ molecules. Measurements of the relative signal amplitudes of various parts of the NMR spectrum as a function of x_o are consistent with the assumption that the narrow line spectrum near the Larmor frequency is due to HD impurities.     

Phonon scattering by Ni3+ ions in Al2O3

Previous thermal conductivity measurements on Al₂O₃ + Ni single crystals have been extended down to 80 mK. The quantitative analysis shows that the lineshape of the low temperature resonant is determineded by random strains in the crystal, and gives a possible value for the ‘tunneling splitting’ 3Γ of 0.54 cm^?1.     

Anisotropy of the electron momentum distribution in Bi2Sr2CaCu2O8+δ superconductor studied by positron annihilation

The temperature dependent (30-300 K) Doppler broadening of the positron annihilated γ-radiation measurement has been investigated on single crystalline Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) high T_c superconducting samples along two different crystallographic orientations. It has been observed that throughout the temperature range the electron momentum distribution has a larger value along the crystallographic c-axis than in the a-b plane. The temperature dependent Doppler broadened positron annihilation γ-radiation lineshape analysis shows a step like increase of S-parameter at the temperature region 92-116 K.     

Low temperature thermodynamical properties of ErCu2Si2

ErCu₂Si₂ crystallises in the tetragonal ThCr₂Si₂-type crystal structure. In this paper results of magnetometric, electrical transport, specific heat as well as neutron diffraction are reported. Results of electrical resistivity and specific heat measurements performed at low temperature yield existence of magnetic ordering roughly at 1.3 K. These results are in concert with neutron diffraction measurements, which reveal simple antiferromagnetic ordering between 0.47 and 1.00 K. At temperatures ranging from 1.00 up to 1.50 K an additional incommensurate magnetic structure was observed. The propagation vector k=(0;0;0.074) was proposed to describe magnetic reflections within the amplitude modulated magnetic structure. Basing on specific heat studies the crystal field levels splitting scheme and magnetic entropy were calculated.     

X-ray and UV photoelectron spectra of the oxides NbO2, MoO2 and RuO2

HeI and Mg Kα_1,2 valence band photoelectron spectra of polycrystalline samples of NbO₂, MoO₂ and RuO₂ are reported. A marked increase is observed in the intensity of the metal 4d structure, relative to that due to oxygen 2p electrons, on changing from X-ray to UV excitation. The superior resolution of the 4d signals in the HeI spectra reveals the presence of the Fermi edge in the metallic oxides MoO₂ and RuO₂. In addition, the HeI spectrum of MoO₂ shows a splitting of the metal 4d signal, confirming established ideas concerning the electronic structure of such materials.     

N2- and O2-broadening coefficients in the ν2 and ν5 bands of CH3F at 183 and 298 K

The N₂- and O₂-broadening coefficients of 33 rovibrational lines in the ν₂ and ν₅ bands of ¹²CH₃F were measured at 183 K using a diode-laser spectrometer. The measurement of these coefficients was also realized at room temperature for 10 of these lines to determine their temperature dependence. The line parameters were obtained by fitting to the experimental profile the Voigt lineshape, and the Rautian and Galatry models taking into account the collisional narrowing. Calculations of the pressure-broadening coefficients were also performed for the same temperatures from a semiclassical model involving electrostatic, induction and dispersion interactions in the intermolecular potential. The calculated values reproduce well the experimental data for both temperatures and both perturbers and the theoretical temperature dependence of the broadening coefficients is in satisfactory agreement with that derived from the measurements.     

Interaction of O2, CO2, CO,C2H4 and C2H4O with Ag(110)

The interaction of O₂, CO₂, CO, C₂H₄ AND C₂H₄O with Ag(110) has been studied by low energy electron diffraction (LEED), temperature programmed desorption (TPD) and electron energy loss spectroscopy (EELS). For adsorbed oxygen the EELS and TPD signals are measured as a function of coverage (θ). Up to θ = 0.25 the EELS signal is proportional to coverage; above 0.25 evidence is found for dipole-dipole interaction as the EELS signal is no longer proportional to coverage. The TPD signal is not directly proportional to the oxygen coverage, which is explained by diffusion of part of the adsorbed oxygen into the bulk. Oxygen has been adsorbed both at pressures of less than 10^-4 Pa in an ultrahigh vacuum chamber and at pressures up to 10³ Pa in a preparation chamber. After desorption at 10³ Pa a new type of weakly bound subsurface oxygen is identified, which can be transferred to the surface by heating the crystal to 470 K. CO₂ is not adsorbed as such on clean silver at 300 K. However, it is adsorbed in the form of a carbonate ion if the surface is first exposed to oxygen. If the crystal is heated this complex decomposes into O_ad and CO₂ with an activation energy of 27 kcal/mol(1 kcal = 4.187 kJ). Up to an oxygen coverage of 0.25 one CO₂ molecule is adsorbed per two oxygen atoms on the surface. At higher oxygen coverages the amount of CO₂ adsorbed becomes smaller. CO readily reacts with O_ad at room temperature to form CO₂. This reaction has been used to measure the number of O atoms present on the surface at 300 K relative to the amount of CO₂ that is adsorbed at 300 K by the formation of a carbonate ion. Weakly bound subsurface oxygen does not react with CO at 300 K. Adsorption of C₂H₄O at 110 K is promoted by the presence of atomic oxygen. The activation energy for desorption of C₂H₄O from clean silver is ～ 9 kcal/mol, whereas on the oxygen-precovered surface two states are found with activation energies of 8.5 and 12.5 kcal/mol. The results are discussed in terms of the mechanism of ethylene epoxidation over unpromoted and unmoderated silver.     

N2-broadening coefficients in the ν3 band of CS2 at room and low temperatures

Using a tunable diode-laser spectrometer, N₂-broadening coefficients have been measured for 15 lines in the ν₃ band of C³²S₂ at room and low temperatures (298, 273.2, 248.2, 223.2, and 198.2 K). These lines with J values ranging from 2 to 62 are located in the spectral range 1519-1545 cm⁻¹. The collisional widths are obtained by fitting each measured spectral line with a Voigt and a Rautian lineshape model and for a few lines we also used a Galatry model. From these results, we have determined the n parameter of the temperature dependence of each broadening coefficient. A semiclassical calculation of these broadenings has been performed by considering in addition to the main electrostatic quadrupole-quadrupole interaction an anisotropic dispersion contribution leading to satisfactory results at all temperatures and providing the n temperature dependence parameter in good agreement with the experimental determination.     

N2-broadening coefficients in the ν4 band of CH4 at room temperature

Using a diode laser spectrometer, we have studied with a great accuracy the N₂-broadening coefficients in the ν₄ band of methane. The experiments were performed at room temperature for lines in the P- and R-branches. We have measured 39 lines in the spectral range 1237–1373 cm⁻¹ with J values between 1 and 12. Each line under study was recorded at four different nitrogen pressures, ranging from 20 to 91 mbar. The collisional half-widths were obtained by fitting individually a theoretical profile on the experimental profile of each line at each N₂-pressure. We fitted the usual Voigt profile, but also the Rautian and Galatry lineshape models which take into account the collisional narrowing due to the molecular confinement (Dicke effect). The Rautian and Galatry fits are always better adjusted on the experimental profiles. For some lines, when the overlapping could not be disregarded, a fit of the blended profiles was performed using the same lineshape models. The collisional broadening coefficients obtained with Galatry and Rautian models are nearly equal and always higher than those derived from Voigt profile. Finally, we compare our results with previous determinations realized for several absorption bands.     

Microwave response to a magnetic phase transition in a molecular magnet based on [Mn12O12( Me CO2)16(H2O)4] clusters and tetramethyltetrathiafulvalene molecules

The electron paramagnetic resonance spectra of a molecular magnet synthesized from [Mn₁₂O₁₂(MeCO₂)₁₆(H₂O)₄] high-spin clusters and tetramethyltetrathiafulvalene donor organic molecules are investigated. It is revealed that, in the temperature range 5–7 K, there appears an additional signal against the background of the ferromagnetic resonance spectrum. The additional signal consists of a large number of lines and has a resonance nature, because it is observed in the well-reproduced narrow ranges of the constant magnetic field of the spectrometer (300–650 and 850–1300 Oe) at a frequency of 9.4152 GHz. The appearance of the additional signal at a temperature of 5 K is attended by a multiple increase in the magnetic susceptibility of the sample. There is a correlation between the spectral characteristics of the additional signal (such as the average amplitude, the spectral power, etc.) and the parameters of the ferromagnetic resonance spectrum (the resonance field, the linewidth, and the integrated intensity). The assumption is made that the additional signal is associated with the magnetic-field-induced transitions between spin states of Mn₁₂ high-spin clusters for different orientations of crystallites with respect to the external magnetic field. Original Russian Text ? R.B. Morgunov, V.L. Berdinskiĭ, A.I. Dmitriev, Y. Tanimoto, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 5, pp. 945–950.     

EPR lineshape study of the incommensurate phase in γ-irradiated K2SeO4

The temperature dependence of the SeO^4-₄ EPR frequencies and the asymmetric broadening of the EPR lines in the incommensurate phase of K₂SeO₄ can be explained by an incommensurate spatial modulation of the g tensors which corresponds to the “broad” phase soliton limit. A comparison between the experimental and calculated lineshape shows a ≈1% volume fraction of commensurate regions in the middle of the incommensurate phase at 110 K.     

X-ray absorption spectroscopic study of mixed valence systems EuCu2Si2, YbCu2Si2 and Sm4Bi3

X-ray absorption spectroscopy technique is employed to determine the valence of the rare earth ions in EuCu₂Si₂, YbCu₂Si₂ and Sm₄Bi₃. In each case, two absorption peaks corresponding to two different valence states of respective rare earth ions have been observed. Low temperature (77 K) study of EuCu₂Si₂ indicates distinct change in the relative intensities of the absorption peaks compared to those registered at room temperature (300 K). It is inferred from the change in the relative intensities that the population of Eu²⁺ in EuCu₂Si₂ decreases at liquid nitrogen temperature compared to Eu³⁺. Conclusions drawn from these results agree well with those reported by others using different experimental techniques. In Sm₄Bi₃, Sm²⁺ and Sm³⁺ are found to occur in the ratio of 3:1.     

Magnetic ordering in NdRh2Si2 and ErRH2Si2

Neutron diffraction and magnetization study of polycrystalline NdRh₂Si₂ and ErRh₂Si₂ was performed in the temperature range from 4.2 to 293 K. Both compounds are of ThCr₂Si₂ type crystal structure and exhibit antiferromagnetic ordering below T_N = 53 K and T_N = 12.8 K respectively. The magnetic structure wave vector is τ = [0, 0, 1].     

Magnetic properties of NdCo2Ge2, ErCo2Ge2 and PrFe2Ge2 compounds

Magnetometric and neutron diffraction studies of polycrystalline NdCo₂GE₂, ErCo₂Ge₂ and PrFe₂Ge₂ compounds were carried out in the temperature range between 4.2 and 300 K. All samples are antiferromagnetic with Néel temperature 26.5, ～ 4.2 and 13 K, respectively. The RECo₂Ge₂ compounds have collinear antiferromagnetic order of +?+? type. For PrFe₂Ge₂ a sinusoidal magnetic structure is observed. Magnetic moment is localized on RE atoms only and is equal to that of RE³⁺ free ion value. In ErCo₂Ge₂ the magnetic moment of Er atoms is perpendicular to the c-axis, whereas for remaining compounds it is parallel to the c-axis.     

Low and room temperature measurements and calculations of O2-broadening coefficients in the ν3 band of CS2

O₂-broadening coefficients have been measured for 16 lines in the P and R branches of the fundamental ν₃ band of ¹²C³²S₂ at room and low temperatures (298.0, 273.2, 248.2, 223.2, and 198.2 K), using a tunable diode laser spectrometer and a low temperature cell. These lines from P(62) and R(64) are located in the spectral range 1519-1547 cm⁻¹. The collisional half-widths are obtained by fitting each observed profile with the Voigt and Rautian lineshape models. The broadening coefficients have also been calculated at all experimental temperatures using a semiclassical calculation performed by considering in addition to the electrostatic quadrupole-quadrupole interaction, a simple anisotropic contribution. Finally, from all the results, the parameter n of the temperature dependence of the broadening coefficients has been determined both experimentally and theoretically.     

Dynamic properties of P4O6S and P4O7: P spin–echo and P MAS–NMR investigations

The rotational dynamics of P₄O₆S and P₄O₇ in the solid state were studied by means of ³¹P NMR spectra of spinning and static powder samples in the temperature range of 153–295 K and 295–388 K, respectively. All spectra were simulated to confirm the type of the motion and to extract the time scales as a function of the temperature. Good agreement between experimental and theoretical data was obtained on the basis of a three-site jump model. For P₄O₆S, the activation energy and the pre-exponential factor derived from the lineshape simulations amount to 51(2) kJ/mol and 6(3)·10¹⁵ s⁻¹. For P₄O₇, the spectral analysis yields an activation energy of 67(1) kJ/mol and a pre-exponential factor of 6(2)·10¹⁴ s⁻¹. The dynamic behavior was checked independently by lineshape analyses under both MAS and static conditions. Activation energies are consistent within the errors for the lineshape analyses. Additionally, we have analyzed spin–lattice relaxation measurements, which show the correct trends for the activation energies.     

The 923-cm−1 band of CF235Cl2 (freon 12), studied by infrared-microwave double resonance

The constants of the 923-cm^?1 band (ν₆) of CF₂³⁵Cl₂ were accurately determined by the application of infrared-microwave double resonance using CO₂ and N₂O lasers. The frequencies of 32 ground-state and 34 (v₆ = 1) state rotational transitions and 14 infrared transitions were measured. The infrared transition frequencies were generally obtained with an accuracy of ± 20 MHz, but in some cases observation of two-photon Lamb-dips allowed the accuracy to be improved to ± 5 MHz. Many double-resonance signals displayed a predominantly “dispersion-type” lineshape and it has been shown that the phase of the observed signals gives information over the relative disposition of the energy levels involved.     

H2-broadening coefficients in the ν3 band of CH3D at low temperatures

Using a diode-laser spectrometer, we have measured H₂-broadening coefficients of CH₃D at low temperatures (153.5, 183.5, and 223.5 K) for four lines in the ν₃ band. The collisional widths are obtained by fitting each absorption line with three lineshape models: the Voigt, Rautian, and Galatry profiles. The broadening coefficients are also calculated on the basis of a semiclassical model of interacting linear molecules by considering an atom-atom Lennard-Jones potential in addition to electrostatic contributions. By comparing the broadening coefficients at room and low temperatures the temperature dependence of these broadenings has been determined both experimentally and theoretically.     

Self-broadening coefficients in the ν2 and ν5 bands of CH3F at 183 and 298 K

The self-broadening coefficients of 33 rovibrational lines in the ν₂ and ν₅ bands of ¹²CH₃F were measured at a sample temperature of 183 K using a diode-laser spectrometer. We have also realized the measurement of these coefficients at room temperature for 10 of these lines in order to determine their temperature dependence. These results were obtained by fitting to the experimental profile the Voigt lineshape and the Rautian and Galatry models taking into account the collisional narrowing. Calculations of the self-broadening coefficients were also performed for the same temperatures from a semiclassical model involving only electrostatic interactions in the intermolecular potential. The calculated values are significantly larger than the experimental data for both temperatures but the J-dependences of the self-broadenings are well reproduced. Moreover, the theoretical temperature dependence of these coefficients is in good agreement with that derived from the measurements.     

Esca investigations of some NiO/SiO2 and NiO—Al2 O3 /SiO2 catalysts

The ESCA spectra of a series of NiO/SiO₂ and NiO—Al₂ O₃/SiO₂ catalysts are reported, together with those of some reference compounds. The positions and shapes of the lines, in conjunction with a quantitative surface analysis from relative intensities, allow the identification of different surface phases, e.g. an NiO-like phase in the impregnated catalysts with very low catalytic activity and an Ni talc-like phase in the precipitated catalysts which have higher activity. The addition of Al₂O₃ has a great influence on the surface structure (formation of alumosilicate).