Adsorption and solution of H2 and N2 by Ta and Nb

The condensation and desorption kinetics of H₂ and N₂ on polycrystalline wires of Ta and Nb are examined for solute concentrations between 0.1 and 100 monolayers. Accurate comparisons of adsorbate densities and sticking coefficients on Ta, Nb, Mo and W are obtained by making simultaneous measurements on all four substrates in the same vacuum system. Desorption exhibits second order kinetics for all substrates at all coverages. Desorption activation energies are higher on Ta and Nb than on W and Mo for both gases. Diffusion limited desorption is observed for H₂ at high heating rates. Solution of H₂ occurs at measurable rates (s ? 0.01) in Ta and Nb even at 78 °K while for N₂ no solution is observed for T < 600 °K.     

Adsorption and desorption of ammonia,hydrogen, and nitrogen on ruthenium (0001)

The adsorption of ammonia, hydrogen, and nitrogen on a Ru(0001) surface have been investigated by Auger electron spectroscopy, low-energy electron diffraction, and thermal flash desorption. The adsorption of ammonia on Ru(0001) can be divided into a low temperature mode (100 K) and a higher temperature mode (300–500 K). For a crystal temperature of 100 K the ammonia adsorbs into two weakly bound molecular γ states with s = 0.2. The ammonia desorbs as NH₃ molecules with desorption energies of 0.32 and 0.46 eV. At 300–500 K adsorption occurs via an activated process with a low sticking probability (s ? 2 × 10^?4).This adsorption is accompanied by dissociation and formation of an apparent (2 × 2) LEED pattern. Hydrogen adsorbs readily (s = 0.4) on Ru(0001) at 100 K and desorbs with 2nd order kinetics in the temperature range 350–450 K. Nitrogen does not appreciably adsorb on Ru(0001) even at 100 K; maximum nitrogen coverage obtained was estimated to be <2% of a monolayer. Changes in the ammonia flash desorption spectra after hydrogen preadsorption at 100 K will be discussed.     

Nitric oxide chemisorption on Re(0001) at 100 and 300 K: Evolution of the adsorbed layer during annealing from 100 to 700 K; XPS,UPS, TDS and work function measurements

At 100 K NO is molecularly adsorbed on Re(0001). Bridge bonded and linear species have been identified by XPS and UPS measurements. Moreover a weakly bonded species reversibly adsorbed at 100 K has been found, but not precisely identified. As the temperature of the surface is increased a complex transformation of the layer occurs: the weakly bonded molecules are probably transformed into a more strongly bonded state and desorb between 100 and 300 K. One part of the linear species desorbs between 300 and 500 K giving the α₂ molecular state, the other part dissociates and desorbs between 600 and 700 K giving the β₁ nitrogen molecules. In the same temperature range the bridge bonded molecules dissociate into nitrogen and oxygen atoms, but nitrogen desorbs into the gas phase between 700 and 1100 K as β₂ and β₃ states with a second order process. Oxygen is adsorbed as atoms and desorbs at higher temperature. If adsorption takes place at room temperature, NO is mainly dissociated and nitrogen desorbs as β₂ and β₃ states with a second order process.     

Hall effect and magnetoresistance of (SN)x films

Hall effect and electrical conductivity have been investigated between 77 K and 300 K and the magnetoresistance at 4.2 K for a number of (SN)_x films deposited at substrate temperatures between — 10 and 50°C. The small magnitude of the Hall mobility (? 1 cm² Vsec^?1 at 300 K) and its activated temperature dependence are interpreted in terms of a heterogenous model for (SN)_x films with thin depletion layers separating highly conductive islands. The hole concentration in these islands (p ≈ 10²¹ cm^?3, the microscopic mobility (μ ≈ 500 cm² Vsec^?1 at 4.2 K) and the temperatures dependence of μ are found to be close to values for (SN)_x crystals.     

Anomalous temperature dependence of the band gap in AgGaSe2 and AgInSe2

The lowest band gaps of AgGaSe₂ and AgInSe₂ single crystals in the temperature range from 90 to 300 K were determined from photoconductivity measurements. Below (above ≈ 120 K in AgInSe₂ and ≈ 125 K in AgGaSe₂ the temperature coefficient of the band gap is +5 × 10⁻⁴ eV/K (−1.5 × 10⁻⁴ eV/K) and +1.1 × 10⁻⁴ eV/K (−4.28 × 10⁻⁴ eV/K), respectively. The positive value is explained with the lattice dilation effect being the dominant mechanism for the band gap variation at the temperatures less than ≈ 120–125 K.     

Crystallographic,optical and magnetic properties of Eu2SiO4

The crystallographic properties of Eu₂SiO₄ are studied in terms of its isomorph Ca₂SiO₄. The recently discovered monoclinic room-temperature phase is ferroelastic and simultaneously ferromagnetic at low temperatures (T _e=5.40°K). The optical absorption and the dispersion properties have been measured in spectral intervals ranging from 0.5 to 3.6 eV and partly for temperatures between 300 and 500°K. This temperature range includes the ferroelastic-paraelastic phase-transition temperature (T _e=438°K). An anomaly of the dielectric constant atT _e suggests the presence of an unstable phase which would be ferroelectric. The Faraday rotation has been measured on either side of the absorption edge at 300 and 77°K. The recent results on crystal structure allow an explanation of the magnetic behaviour of the two ferromagnetic phases known up to now.     

The direct-indirect transition in In1−xGaxP

Data (300°K) are presented on laser quality liquid phase epitaxial (LPE) In_1?xGa_xP, grown on lattice-matched {100} GaAs_1?yP_y substrates, confirming a theoretical account presented by Altarelli that the direct-indirect transition occurs at x_c ≈ 0.74. Photoluminescence data show that crystals ranging in composition from x = 0.52 to x = 0.70, all of which operate as lasers at 77°K, exhibit the same decrease in luminescence intensity from 77 to 300°K, indicating that x_c > 0.70 at 300°K. The steep I–V characteristics (at 77 and 300°K) of Zn-diffused diodes prepared on crystals of composition x = 0.70 in contrast to the significantly more resistive behavior of x = 0.74 crystals is consistent also with the assignment x_c > 0.70. The Λ band edge (300°K), determined by the photoluminescence data points, intersects the X band edge (300°K) at x_c ≈ 0.74. A discussion, in terms of the problems inherent in In_1?xGa_xP crystals growth and quality, is presented to reconcile the difference in the crossover value (x_c ≈ 0.74) reported here on laser crystals and the lower value (x_c ≈ 0.63) of some other recent reports.     

Chemisorption of CO on tungsten (100): Combined flash desorption and electron stimulated desorption study. II

The chemisorption of CO on W(100) at ~ 100K has been studied using a combination of flash desorption and electron stimulated desorption (ESD) techniques. This is an extension of a similar study made for CO adsorption on W(100) at temperatures in the range 200–300K. As in the 200–300 K CO layer, both α₁-CO and α₂-CO are formed in addition to more strongly bound CO species upon adsorption at ~ 100K; the α-CO states yield CO⁺ and O⁺ respectively upon ESD. At low CO coverages, the α₁ and α₂-CO states are postulated to convert to β-CO or other strongly bound CO species upon heating. At higher CO coverages, α₁-CO converts to α₂-CO upon thermal desorption or electron stimulated desorption. There is evidence for the presence of other weakly-bound states in the low temperature CO layer having low surface concentration at saturation. The ESD behavior of the CO layer coadsorbed with hydrogen on W(100) is reported, and it is found that H(ads) suppresses either the concentration or the ionic cross section for α₁ and α₂-CO states.     

Temperature dependence of oxidation rate in clean Ge(111)

The temperature dependence of the sticking coefficient of oxygen on a clean Ge(111) surface has been investigated over a wide temperature range from 300 to 1100 °K using three methods. In the interval 300–600 °K a flash technique was used, the desorbed germanium oxide being detected by the time of flight mass-spectrometer. In the range from 500 to 1000 °K the sticking coefficient was measured from the pumping speed of oxygen by the sample surface, and in the range from 800 to 1100 °K the temperature dependence of the etching speed by oxygen was determined.The measured temperature dependence of the sticking coefficient is complex. It increases between 300 and 400 °K, remaining virtually constant from 400 to 500 °K with a new increase in the range from 500 to 1000 °K. A rapid fall in the sticking coefficient was observed at temperatures above 1000 °K.The dependence of the adsorption coverage on exposure has also been obtained for sample temperatures of 300, 350, 400 and 500 and 600 °K. The form of the adsorption curves differs considerably from a theoretical one based on a decrease in the sticking coefficient with coverage given by s = s₀(1 ? θ)². At 600 °K the sticking coefficient decreases more slowly than predicted by this equation. On the contrary, at 300 °K it begins to decrease rapidly at low coverages less than 0.1 of a monolayer.To explain the results it is assumed that oxygen molecules adsorb on the surface structural defects. At 300 °K such defects may be in the form of steps or other morphological disturbances on the surface, and above 500 °K they are probably equilibrium thermal defects, for example, surface vacancies.     

Adsorption and interaction of CO and NO on Pt(410): I. TPD studies

NO and CO adsorption and the NO/CO reaction on Pt(410) are studied by TPD. NO is found to dissociate on Pt(410) at 120 K, but it reacts to form N₂O at higher exposures. The N₂O desorbs in two peaks at 135 and 150 K. CO adsorbs molecularly, and desorbs in 5 peaks at 550, 500, 450, 380 and about 130 K. CO is also found to dissociate upon heating, leaving a carbon residue on the surface which changes the TPD spectra. The NO/CO reaction shows a surface explosion at 360 K. These results provide additional evidence that Pt(410) has unusual reactivity, as predicted by Banholzer, Park, Mak and Masel, Surface Sci. 128 (1983) 176.     

Adsorption and reaction of nitric oxide and oxygen on Rh(111)

Adsorption of NO and O₂ on Rh(111) has been studied by TPD and XPS. Both gases adsorb molecularly at 120 K. At low coverages (θ_NO < 0.3) NO dissociates completely upon heating to form N₂ and O₂ which have peak desorption temperatures at 710 and 1310 K., respectively. At higher NO coverages NO desorbs at 455 K and a new N₂ state obeying first order kinetics appears at 470 K. At saturation, 55% of the adsorbed NO decomposes. Preadsorbed oxygen inhibits NO decomposition and produces new N₂ and NO desorption states, both at 400 K. The saturation coverage of NO on Rh(111) is approximately 0.67 of the surface atom density. Oxygen on Rh(111) has two strongly bound states with peak temperatures of 840 and 1125 K with a saturation coverage ratio of 1:2. Desorption parameters for the 1125 peak vary strongly with coverage and, assuming second-order kinetics, yield an activation energy of $\text{85 ± 5}\text{kcal}\text{mol}$ and a pre-exponential factor of 2.0 cm² s^?1 in the limit of zero coverage. A molecular state desorbing at 150 K and the 840 K state fill concurrently. The saturation coverage of atomic oxygen on Rh(111) is approximately 0.83 times the surface atom density. The behavior of NO on Rh and Pt low index planes is compared.     

Novel electron spin resonance spectra of a tetrathiafulvalene salt of copper chloride, (TTF) 2CuCl2

An ESR study has been carried out on electrically conducting (TTF)₂CuCl₂ crystals. The peak-to-peak width W of the ESR spectrum exhibited an unusual angular dependence with respect to the angle θ between the TTF-stack direction and the external field: the W showed a sharp minimum (4.9 G at 300 K) at θ = 0°, a maximum (7.5 G) at 60° (corresponding to 3cos²θ ∼ 1), and another minimum (6.9 G) at 90°. This angular depence has been explained by assuming an anisotropic motional narrowing that may arise from a spin correlation in one-dimensional electroconductive lattices. The temperature dependence of the W showed no anomaly around the temperature of metal-semiconductor transition: the spin relaxation is substantially unchanged by the phase transition.     

Preparation,structural characterization and conductivity of LiZr2(PO4)3

Amorphous LiZr₂(PO₄)₃ has been prepared at room temperature starting from aqueous solutions of ZrOCl₂, H₃PO₄, and LiOH and then crystallized by heating at temperatures between 600 and 900°C. The material obtained at 900°C has been characterized by X-ray powder diffractometry, DSC analysis, and ac conductivity. It is monoclinic from 20 up to about 300°C and orthorhombic at higher temperatures. A change in the activation energy for conduction (from 0.79 to 0.43 eV) and a weak endothermic effect (0.9–1.7 cal/g) are associated with the phase transition. The ac conductivity of sintered pellets is, on average, 7×10⁻⁴ S cm⁻¹ at 300°C.     

The adsorption of CO,O2, and H2 on Pt(100)-(5×20)

The adsorption/desorption characteristics of CO, O₂, and H₂ on the Pt(100)-(5 × 20) surface were examined using flash desorption spectroscopy. Subsequent to adsorption at 300 K, CO desorbed from the (5×20) surface in three peaks with binding energies of 28, 31.6 and 33 kcal gmol^?1. These states formed differently from those following adsorption on the Pt(100)-(1 × 1) surface, suggesting structural effects on adsorption. Oxygen could be readily adsorbed on the (5×20) surface at temperatures above 500 K and high O₂ fluxes up to coverages of $\text{2}\text{3}$ of a monolayer with a net sticking probability to ssaturation of ? 10^?3. Oxygen adsorption reconstructed the (5 × 20) surface, and several ordered LEED patterns were observed. Upon heating, oxygen desorbed from the surface in two peaks at 676 and 709 K; the lower temperature peak exhibited atrractive lateral interactions evidenced by autocatalytic desorption kinetics. Hydrogen was also found to reconstruct the (5 × 20) surface to the (1 × 1) structure, provided adsorption was performed at 200 K. For all three species, CO, O₂, and H₂, the surface returned to the (5 × 20) structure only after the adsorbates were completely desorbed from the surface.     

Decomposition of CO2 on (100) W

The binding states and condensation kinetics of CO₂ and its decomposition products CO and O₂ on (100) W are examined by flash desorption mass spectrometry. Carbon dioxide desorbs almost entirely as CO and oxygen at all coverages. At saturation there are three major states of CO. These correspond to the high temperature states of CO alone although all peaks are shifted and amounts are altered slightly.     

COMBINED EFFECTS OF SUBCOOLING AND SURFACE ORIENTATION ON POOL BOILING OF HFE-7100 FROM A SIMULATED ELECTRONIC CHIP

The effects of orientation and subcooling on pool boiling of the HFE-7100 dielectric liquid near atmospheric pressure (0.085 MPa) from a 10 × 10 mm smooth copper surface are investigated experimentally. Results are obtained for inclination angles θ = 0° (upward-facing), 30°, 60°, 90°, 120°, 150°, and 180° (downward-facing) and liquid subcoolings ΔT_sub = 0, 10, 20, and 30 K. Increasing θ decreases the saturation nucleate boiling heat flux at high surface superheats (ΔT_sat > 20 K), but increases it only slightly at lower surface superheats. The critical heat flux (CHF) decreases slowly with increasing θ from 0° to 90°, and then deceases faster with increasing θ to 180°. CHF increases linearly with increased subcooling, but the rate increases from 0.016 K^?1 at 0° to 0.048 K^?1 at 180°. At θ = 0° and ΔT_sub = 30 K, CHF is ～ 36 W/cm² and 24.45 W/cm² for saturation boiling, while at θ = 180° CHF = 10.85 W/cm² at ΔT_sub = 30 K and only 4.30 W/cm² at saturation. The developed correlation for CHF of HFE-7100, as a function of θ and ΔT_sub, is within ±10% of the present data. The recorded still photographs of the boiling surface in the experiments illustrate the effects of liquid subcooling and surface orientation at different nucleate boiling heat fluxes and surface superheats on vapor bubble accumulation and/or induced mixing at the surface.     

Chemisorption of H2 on W(100): Absolute sticking probability,coverage, and electron stimulated desorption

Measurements of both the absolute sticking probability near normal incidence and the coverage of H₂ adsorbed on W(100) at ~ 300K have been made using a precision gas dosing system; a known fraction of the molecules entering the vacuum chamber struck the sample crystal before reaching a mass spectrometer detector. The initial sticking probability S₀ for H₂/W(100) is 0.51 ± 0.03; the hydrogen coverage extrapolated to S = 0 is 2.0 × 10¹⁵ atoms cm^?2. The initial sticking probability S₀ for D₂/W(100) is 0.57 ± 0.03; the isotope effect for sticking probability is smaller than previously reported. Electron stimulated desorption (ESD) studies reveal that the low coverage β₂ hydrogen state on W(100) yields H⁺ ions upon bombardment by 100 eV electrons; the ion desorption cross section is ~ 1.8 × 10^?23 cm². The H⁺ ion cross section at saturation hydrogen coverage when the β₁ state is fully populated is ? 10^?25 cm². An isotope effect in electron stimulated desorption of H⁺ and D⁺ has been found. The H⁺ ion yield is ? 100 × greater than the D⁺ ion yield, in agreement with theory.     

The CO coadsorption and reactions of sulfur,hydrogen and oxygen on clean and sulfided Mo(100) and on MoS2(0001) crystal faces

The chemisorption and reactivity of O₂ and H₂ with the sulfided Mo(100) surface and the basal (0001) plane of MoS₂ have been studied by means of Thermal Desorption Spectroscopy (TDS), Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). These studies have been carried out at both low (10^?8–10^?5Torr) and high (1 atm) pressures of O₂ and H₂. Sulfur desorbs from Mo(100) both as an atom and as a diatomic molecule. Sulfur adsorbed on Mo(100) blocks sites of hydrogen adsorption without noticeably changing the hydrogen desorption energies. TDS of ¹⁸O coadsorbed with sulfur on the Mo(100) surface produced the desorption of SO at 1150 K, and of S, S₂ and O, but not SO₂. A pressure of 1 × 10^?7 Torr of O₂ was sufficient to remove sulfur from Mo(100) at temperatures over 1100 K. The basal plane of MoS₂ was unreactive in the presence of 1 atm of O₂ at temperatures of 520 K. Sputtering of the MoS₂ produced a marked uptake of oxygen and the removal of sulfur under the same conditions.     

Thermal evolution of acetylene and ethylene on Pd(111)

The thermal evolution of acetylene and ethylene and their deuterated counterparts on a palladium (111) surface has been studied by high-resolution electron energy loss spectroscopy in the temperature range 150–500 K. Analysis of the vibrational spectra indicates that chemisorbed acetylene evolves at 300 K in the presence of surface hydrogen to mainly ethylidyne, CCH₃, and a small amount of residual acetylene. Spectra obtained with and without preadsorbed hydrogen provide evidence for a 〉C CH₂ intermediate in the reaction. Chemisorbed ethylene also evolves to ethylidyne after heating from 150 to 300 K but much of the ethylene desorbs. The high temperature (400–500 K) behavior of C₂H₂ and C₂H₄ involves formation of a CH species. Although a small amount of the CH species may be formed from the dehydrogenation of ethylidyne, it is found that carbon-carbon bond scission of acetylene near 400 K is the dominant mechanism in CH formation.     

Measurement of intensities of multiplets in the 2ν3-band of methane at low temperatures

The integrated intensities of the multiplets P(1)–P(10), R(0)–R(9), and of the Q-branch in the 2ν₃-band of ¹²CH₄ have been measured at 102°K, 152°K, 202°K, 251°K, and 300°K. Comparison of our data with theoretical line strengths confirms, at all of the temperatures mentioned, the intensity anomalies observed by Margolis⁽⁵⁾ for lines in this band. The integrated intensity of the 2ν₃-band is found to be S_v = (1·76±-0·04)(300/T (°K)) cm^?2 atm^?1.