Simulation of the thermal decomposition of the C9H10 methylcubane molecule

The thermal stability of the C₉H₁₀ methylcubane molecule was studied over the temperature range 1100–2100 K by molecular dynamics simulation with the tight binding potential. The temperature dependence of its lifetime to the decomposition moment was determined. The activation energy E _a = 1.7 ± 0.2 eV and frequency factor A = 10^{15.63 ± 0.53} s^?1 of the Arrhenius equation were found. Possible channels and final products of molecule decomposition were studied.     

Absorption spectra and oscillator strength of KBr:Pb

Absorption measurements on hydroxide-free KBr crystals containing various amounts of Pb²⁺ were made. The spectra were corrected for scattering and reflection losses and the B-, C- and D-bands were deconvoluted as three Gaussian bands. Band maxima were found: A -band at 4.15 ± 0.05 eV, 5-band at 5.1 ± 0.1 eV, C-band at 5.52 ± 0.07 eV, D¹-band at 5.79 ± 0.05 eV. The oscillator strengths for the A -, B- and C-bands were 0.12 ± 0.01, 0.04 ± 0.01 and 0.77 ± 0.02, respectively. The ƒ-sum for the three bands is 0.93. The nearness of the ƒ-sum to unity indicates that very little interaction between the impurity s² electrons and the host and supports the usual assumption that these bands originate from the same ground state.     

Luminescence and decay times of CsI:In+

Luminescence spectra of single crystals of CsI:In⁺ excited in the A(304 nm), B(288 nm), C(268 nm) and D(257 nm) absorption bands have been studied in the temperature range 4.2–300 K. Excitation in the A band at 4.2 K gives rise to the principal emission at 2.22 eV accompanied by a partly-overlapping weak band at 2.49 eV. An additional emission band at about 2.96 eV is observed on excitation in the B, C or D bands. Yet another emission band located at 2.67 eV is excited only in the D band. The relative intensities of the bands are very sensitive to excitation wavelength as well as to temperature. The origin of all these bands is assigned in terms of a model for the relaxed excited states (RES). All the luminescence spectra were resolved into an appropriate number of skew-Gaussian components. Moments analysis leads to a value of (1.35 ± 0.02) × 10¹³ rad s^-1 for the effective frequency (ω_eff) of lattice vibrations coupled to the RES. At the lowest temperature, the radiative decay times of each of the intracenter emission bands (2.22, 2.49 and 2.96 eV) show a slow decay ( ～ 10–100 μs) and a fast decay ( ～ 10–100 ns). The 2.96 eV band, which is assigned to an emission process which is the inverse of the D-band absorption, exhibits a single decay mode ( ～ 10 μs). The intrinsic radiative decay rates (k₁, k₂), the one-phonon transition rate (K) and the second-order spin-orbit splitting (D) for the RES responsible for the principal emission are: k₁ = (6.0±-0.3)×10³ s^-1, k₂ = (1.33±-0.06)×10⁵ s^-1, K = (2.4±-0.4)×10⁷ s^-1 and D = (13.8±-0.5) cm^-1.     

Crystalline fields,exchange interactions and spin reorientations in TmxHo1−xFe2 systems,studied by a Yb170 Mössbauer probe

Mössbauer studies of the 84.3 keV gamma ray of Yb¹⁷⁰ emitted from Yb in Tm_xHo_1?xFe₂ at various temperatures were performed. These yield the crystalline field parameters (A₄〈r⁴〉 = 36 ± 5 K, A₆〈r⁶〉 = -3 ± 2K, the exchange field (μ_BH_exch = 116 ± 4 K) and the Yb³⁺ free ion hyperfine constants (H_4f = 4100 ± 150 kOe, eqQ = 2400 ± 250 MHz and H (conduction electrons) = 350±100 kOe). Spin relaxation phenomena observed in TmFe₂ at low temperatures give a value of ～ 0.03 for |?(E_F)J_sf|. Spectra observed in Tm_0.2Ho_0.8Fe₂ in the spin reorientation transition region indicate that the transition is of second order.     

Mixed conduction in Cr-doped GaAs

Hall-effect and magnetoresistance measurements have been carried out in GaAs : Cr as functions of magnetic field strength (B = 0–18kG) and temperature (T = 125–420°K). Independent solutions for the mobilities, μ_n and μ_p, and the carrier concentrations, n and p, are obtained from the basic mixed-conductivity equations. These quantities, as well as the intrinsic carrier concentration, n_i are then calculated as a function of temperature for one sample, and subsequent analysis yields the following values in the range T = 360–420°K: an acceptor (presumably Cr) energy E_A = 0.69±0.02eV (from the valence band); the bandgap energy E_g = E_g0 + αT, with E_go = 1.48±0.02eV, $\text{α ? 3.2 × 10}{}^{\mathrm{?4}}\text{eV}\text{°}\text{K}$; $\text{μ}{}_{\mathrm{n}}\text{= 2700± 100}\text{cm}{}^2\text{V}\text{sec}$, decreasing slightly with temperature; = 350± 50 $\text{cm}{}^2\text{V sec}$; and an acceptor-to-donor concentration ratio, itN_A/N_D?8. The electron mobility appears to be limited by neutral impurity scattering, with N_A ? 2 × 10¹⁶cm^?3. Several other samples were also investigated but as a function of temperature only (at B = 0). At room temperature both positive (p-type) and negative (n-type) Hall coefficients were observed.     

A study of the adsorption of oxygen on Ni(111) using auger electron spectroscopy: Chemical shifts and valence spectra

Auger electron spectra have been recorded when oxygen is adsorbed on a Ni(111) single crystal surface. For the coverage range θ < 1, an analysis of the plot of the peak to peak height (H) of the oxygen KVV (516 eV) transition versus the total number of molecules cm^2? impinging on the surface (molecular beam dosing) shows agreement with the kinetic mechanism proposed by Morgan and King [Surface Sci. 23 (1970) 259] for the adsorption of oxygen on polycrystalline nickel films. In this coverage range, no energy shifts of the nickel or oxygen Auger peaks were recorded.At coverages θ > 1 (standard dosing procedure) shifts in the valence spectra M_{2, 3}VV (61 eV) and L₃M_{2, 3}V (782 eV) of ?2.3 eV and ?1.8eV respectively are recorded at 1.4 × 10^?2 torr-sec. Up to these coverages no shift of the L₃VV transition (849 eV) is observed. A chemical shift of ?2.1 eV is recorded in the L₃M_{2, 3}M_{2, 3} Auger transition (716 eV) at 1.4 × 10^?2 torr-sec.In the coverage range θ > 1, shifts in the energy of the oxygen Auger peaks are observed. At 5.8 × 10^?3 torr-sec. the KVV (516 eV) and KL₁V (495.2 ± 0.3 eV) transitions show shifts of ?1.5 eV and ?(1.0 ±0.3) eV respectively. No shift up to this coverage is recorded in the KL₁L₁ (480.6 ± 0.3 eV) transition.     

Low temperature ESR of K2Pt(CN)4Br0.3. 3.2 H2O

Electron spin resonance has been observed in high purity single crystals of K₂Pt(CN)₄Br_0.3·3.2H₂O, (KCP), in the temperature range of 4.22–80 K. Two types of paramagnetic species are found to dominate the spectrum. One of these is an inhomogeneously broadened line showing no resolved hyperfine splitting and having g_∥ = 1.946 ± 0.003 and g⊥ = 2.340 ± 0.003. The second paramagnet is identified as a Cu²⁺ impurity center with g_∥ = 2.231 ± 0.003, g⊥ = 2.079 ± 0.002 and hyperfine tensor A_∥ = 467 MHz, A_⊥ = 70 MHz.     

Trapping probabilities and desorption energies of alkali atoms on a clean and an oxygen covered tungsten (110) surface

Alkali atoms were scattered with hyperthermal energies from a clean and an oxygen covered (θ ≈ 0.5 ML) W(110) surface. The trapping probability of K and Na atoms on oxygen covered W(110) has been measured as a function of incoming energy (0–30 eV) and incident angle. A considerable enhancement of trapping on the oxygen covered surface compared to a clean surface was observed. At energies above 25 eV there are still K and Na atoms being trapped by the oxygen covered surface. From the temperature dependence of the mean residence time τ of the initially trapped atoms the pre-exponential factor τ₀ and the desorption energy Q were derived using the relation: $\text{τ = τ}{}_0\text{exp}\text{(}\text{Q}\text{kT}{}_{\mathrm{<mtext>s</mtext>}}\text{)}$. On clean W(110) we obtained for Li: $\text{τ}{}_0\text{= (8 ±}\text{8}\text{4}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.78 ± 0.09) eV; for Na: τ₀ = (9 ± 3) × 10^?14 sec, Q = (2.55 ± 0.04) eV; and for K: τ₀ = (4 ± 1) × 10^?13 sec, Q = (2.05 ± 0.02) eV. Oxygen covered W(110) gave for Na: τ₀ = (7 ±3) × 10^?15 sec, Q = (2.88 ± 0.05) eV; and for K: $\text{τ}{}_0\text{= (1.3 ±}\text{0.9}\text{0.6}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.48 ±0.05) eV. The adsorption on clean W(110) has the features of a supermobile two-dimentional gas; on the oxygen covered W(110) adsorbed atoms have the partition function of a one-dimen-sional gas. The binding of the adatoms to the surface has a highly ionic character in the systems of the present experiment. An estimate is given for the screening length of the non-perfect conductor W(110):k_s^?1≈ 0.5 Å.     

Measurements of intensities and nitrogen-broadened linewidths in the CO fundamental at low temperatures

Intensities and nitrogen-broadened half-widths of lines R(0), R(8) and R(16) in the fundamental band of ¹²C¹⁶O have been measured at 83°K, 100°K, 150°K, 200°K and 298°K. The intensities of several other lines in the P- and R-branches of the band have also been measured at 298°K. The absolute intensity derived from the line intensity data using the Herman-Wallis formula is S^°_v = 273 ± 10 cm^-2atm^-1 at S.T.P. A separate measurement employing the Wilson-Wells-Penner-Weber method has yielded S^°_v = 277 ± 4 cm^-2 atm^-1 at S.T.P. Both of these values are within 6 per cent of most of the previously published direct measurements of this parameter. The values for the line intensities reported earlier by other authors are lower by nearly 16 per cent.     

Resonant neutron capture in 40Ca

The neutron capture cross section of ⁴⁰Ca has been measured with ≈ 0.2 % energy resolution below E_n = 300 keV. Resonance parameters have been extracted for many new p- and d-wave resonances. Gamma-ray spectra were also measured following capture in one doublet and two resolved resonances below 50 keV. Strong feeding of low-lying p-wave levels was observed in all cases. Calculations showed that valence transitions were inadequate to account for the observed dominance of these transitions and a further mechanism is required. The average resonance parameters obtained from the data are as follows: 〈D〉 = 37 + 4keV, 10⁴S₁ = 0.16 ± 0.05, 10⁴S₂ = 2.0 ± 0.7. The average radiative widths and standard deviations of their distributions were found to be strongly l-dependent as follows: 〈Γ_γ〉_s = 1.5 ± 0.9 eV, 〈Γ_γ〉_p = 0.36 ± 0.09 eV and 〈Γ_γ〉_d = 0.7 ± 0.4 eV.     

Charge transfer over localized states in a TlS single crystal

It is revealed that TlS single crystals exhibit a variable range hopping conduction along a normal to their natural layers at temperatures T ≤ 230 K in a dc electric field and a nonactivated hopping conduction at low temperatures in strong electric fields. Estimates are made for the density of states near the Fermi level (N _F = 2.8 × 10²⁰ eV^?1 cm^?3 and their energy spread (ΔW = 0.02 eV), the localization radius (a = 33 Å), the average jump distance in the region of activated (R _av(T) = 40 Å) and nonactivated (R _av(F) = 78 Å) hopping conduction, and also the drop in the charge carrier potential energy along the jump distance in an electric field F: eFR = 0.006 and 0.009 eV at F = 7.50 × 10³ and 1.25 × 10⁴ V/cm, respectively.     

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.     

X-ray spectroscopy in the EC nucleus

The probabilities P_KK of double K-shell vacancy production per K electron capture decay and per K internal conversion of ¹⁰⁹Cd and ²⁰⁷Bi have been determined by means of the double- and triple-coincidence experiments using Kα X-ray and K internal conversion. For ¹⁰⁹Cd we find P_KK(EC) = (4.2 ± 0.5) × 10⁻⁵ and P_KK(IC) = (4.32 ± 0.46) × 10⁻⁵, and for ²⁰⁷Bi, P_KK(EC) = (2.54 ± 0.50) × 10⁻⁵. The observed X-ray energy shifts of the hypersatellite Ag (Kα₁^H) X-ray and the hypersatellite Pb (Kα₁^H) X-ray lines are 545±15 eV and 1238±45 eV, respectively.     

Content and Isotope Ratios of Noble Gases in Congelation Ice of Lake Vostok

Isotope ratios of noble gases (He, Ne, Ar) were studied in samples collected by degassing of cores of water frozen over a glacier of Lake Vostok. The gases were collected into glass retorts during three days of degassing of cores, which have just been extracted from the borehole. Within the error, the isotope ³He/⁴He ratios of 0.28 ± 0.08 R_A (R_A = 1.38 × 10^–6 is the ratio for air) correspond to those from [1]. The ⁴He/²⁰Ne and ⁴⁰Ar/³⁶Ar ratios (12.4 ± 4.6 R_A and 1.0074 ± 0.0023 R_A, respectively) exceed their contents in air (⁴He/²⁰Ne_A = 0.29; ⁴⁰Ar/³⁶Ar_A = 298.6) and may indicate some contribution of terrigenous gas to the gaseous balance of the lake, as well as the high content of ancient ground waters in the lake. The ³He/⁴He ratio of 0.28 R_A means low mantle ³He flux typical of continental platforms far from active rift zones.     

Tetravalent vanadium in CaF2 and SrF2 crystals: An EPR study

An EPR study of tetravalent vanadium centers created by room temperature X-irradiation in CaF₂ and SrF₂ is presented. The production efficiency of these centers is enhanced by previous annealing of the samples at 1000 K in air. The symmetry of V⁴⁺ ions is tetragonal and its EPR spectrum can be described by an axial spin Hamiltonian including a Zeeman and hyperfine term with $\text{S =}\text{1}\text{2}$ and $\text{I =}\text{7}\text{2}$ (corresponding to ⁵¹V nuclei). The following values for the spin Hamiltonian parameters are obtained g_∥ = 1.947 ± 0.002, g_⊥ = 1.935 ± 0.005, A_∥ = 500 ± 5 MHz, A_⊥ = 150 ± 10 MHz in the case of SrF₂ and g_∥ = 1.945 ± 0.002, A_∥ = 505 ± 5 MHz and A_⊥ < 200 MHz, in the case of CaF₂. A model for the center including an interstitial O^2? ion is tentatively proposed.     

Microscopic magnetic properties of nonstoichiometric V2O3+x—NMR and inelastic spin-flip neutron scattering measurements

The local magnetic properties of the V sites in the nonstoichiometric V₂O_3+x (0 ? x <0.08) have been examined by nuclear magnetic resonance and inelastic spin-flip neutron scattering techniques. The samples with x = 0.01 and 0.02 show a paramagnetic metal (PM)-antiferromagnetic insulator (AFI) transition. In the AFI phase, two distinct ⁵¹V NMR signals with hyperfine fields H_n = 184.9±0.5 kOe and 71±1 kOe were observed at 1.8 K, which were assigned as due to V³⁺ and V³⁺ sites, respectively. On the other hand, the samples with x = 0.04 and 0.06 were metallic down to 1.4K, and showed a paramagnetic (PM)-antiferromagnetic (AFM) transition at about 10 K. In these samples, a ⁵¹V NMR signal with H_n = 58±2 k0e and one with 〈H_n〉 = 9kOe were observed at 1.8 K, which were assigned as due to V³⁺-like sites and the matrix V sites, respectively. These results are entirely consistent with those obtained from the neutron experiment. We propose that in the metallic phase (0.04 ? x < 0.08) the minority V⁴⁺-like sites are magnetically localized in the delocalized V matrix and may be responsible for the antiferromagnetic long range order below 10 K.     

Electrical conductivity,absorption and luminescence measurements in cubic boron nitride

Electrical and optical measurements were carried out on tiny crystals of cubic boron nitride. The dark current i_D was found to change exponentially with T, with activation energies in the range 0·2–0·4 eV. A red electroluminescence, of intensity i_EL. proportional to i_D was observed. Upon illumination at low temperatures a photocurrent i_p proportional to the square root of the excitation intensity appeared. It varied exponentially with T, with an activation energy of 0.05 ± 0.01 eV. The crystals exhibited a red thermoluminescence with several unresolved peaks covering the temperature range 100–400K, and having activation energies in the range 0·15–0·40 eV.     

The exciton absorption in deformed germanium

Absorption spectra at 77° K near the direct (κ = 0) exciton transition are reported for deformed and undeformed single-crystal films of n-type Ge oriented on (111); Elliott's theory is applied. The optical width of the forbidden band for this transition is found as E_{g 0} = (0.8821 ±±0.0002) eV, while the exciton binding energy is found as E_ex(0) = = (0.0016±0.0003) eV for undeformed Ge at 77 ° K. The mean temperature coefficient of E_g for κ = 0 in the range 77 °–297 ° K is (dE_g/ /dT)_p =?3.50 · 10^?4 eV/deg. The effects of thermoelastic deformation on the exciton spectrum give (dE_g/dT)_d = (?1.5±0.1) · 10^?4 eV/deg. The half-width σ ≈ 5 · 10^?4 eV of the exciton peak gives the exciton lifetime as gt ≥ 10^?12 sec.     

Untersuchung der vier ersten angeregten Zustände des6Li-Kernes durch Elektronenstreuung

Inelastic electron scattering cross sections were measured for energies below 60 MeV and momentum transfersq between 0.2 and 0.6 fm^?1. Ground state radiative widths Γ _γ ⁰ and transition radiiR _tr were deduced. 2.18 MeV: Γ _γ ⁰ (E2)=(4.40±0.34) · 10^?4 eV,R _tr=(4.28±0.39)fm; 3.56MeV: Γ _γ ⁰ (M1)=(8.31±0.36)eV,R _tr=(2.90±0.10)fm; 4.27 MeV: Γ _γ ⁰ (E2)=(5.4±2.8) · 10^?3 eV,R _tr=(3.4±1.2) fm. The excitation of the 5.37 MeV level shows a transverse angular dependence.