Ralph Klein

Adsorption of molecular nitrogen by the W(110) plane

Abstract Characteristics of the adsorption of nitrogen on the (110) plane of tungsten were determined by thermal desorption and work function measurements. The low temperature γ-N 2 state desorbs with first order kinetics and an activation energy of 6 kcal mole −1 . The absence of isotope mixing between 14 N 2 and 15 N 2 demonstrates γ-N 2 is adsorbed molecularly. Monolayer coverage shows a decrease of 0.19 eV in work function. A Topping model plot indicates the layer is immobile at 123 K.

Chemisorption and decomposition of nitric oxide on ruthenium

Abstract Nitric oxide, strongly chemisorbed on ruthenium, is desorbed almost completely as oxygen and nitrogen. Oxygen, nitrogen, and nitric oxide were observed singly on ruthenium with field emission microscopy. Thermal desorption spectroscopy from Ru(1010) showed that molecular nitrogen is only physisorbed but nitrogen from NO decomposition is strongly chemisorbed. Nitrogen from NO shows three binding states, the most strongly bound being present to only a small extent. NO shows three and two binding states when adsorbed at 120 K and 295 K respectively. Work function measurements gave Δφ = 1.3 eV for a monolayer of NO. NO is dissociatively adsorbed above 250 K but a lower temperature limit was not established. The decomposition of NO on Ru (10 1 0) under high vacuum conditions is catalytic in that no oxides of ruthenium were observed to form in the process.

Surface Lifetimes of Alkali Metals on Molybdenum

The surface lifetimes (τ¯) of all the alkali metals on a polycrystalline molybdenum surface in the temperature region 900–1350°K were measured. The temperature dependence followed an Arrhenius expression of the form τ¯° exp(l¯/kT). Desorption energies (l¯) were found to be 3.14, 2.60, 2.53, 2.31, and 2.10 eV for Li, Na, K, Rb, and Cs. The corresponding pre‐exponential factors (τ¯°) were 3× 10−16, 3×10−15, 2× 10−14, 1×10−13 and 3×10−13sec. From these data, ion and neutral desorption energies were calculated using a model employing a partial surface charge for the adsorbed particle and the Schottky relation. The validity of the Schottky relation for an adsorbed alkali was supported by analysis of recently available theory of partial charges. An explanation of the variation of the pre‐exponential factors is presented on the basis of relative mobility of the adsorbed species. A pre‐exponential factor was calculated using a partition function for an adsorbate with mixied localized and nonlocalized character. T...

HALOGENS ADSORBED ON MOLYBDENUM: THEIR SURFACE LIFETIMES AND DESORPTION KINETICS.

The surface lifetimes of fluorine, chlorine, bromine and iodine adsorbed on a molybdenum surface have been obtained from measurements of the halide ion desorption flux using the modulated molecular beam technique. Measurements were made at temperatures between 1350 and 1850 K and at surface coverages of less than 1010 halogen atoms cm−2. The temperature-lifetime data were fitted to an Arrhenius expression from which the binding energies l, and pre-exponential factors τ0, were obtained: F: l = 4.65eVandτ0 = 3 × 10−16 s ; Cl: l = 4.11eVandτ0 = 3 × 10−16 s ; Br: l = 3.70eVandτ0 = 2 × 10−15 s ; I: l = 3.15eVandτ0 = 1 × 10−16 s . It was concluded that the halogens are adsorbed as strongly bound surface compounds, MoX (where X is either F, Cl, Br, or I) from which gaseous ions and neutral atoms desorb at rates consistent with the Saha-Langmuir relationship. From that relationship and the above data, separate binding energies and pre-exponentials were calculated for ion and neutral desorption. The importance of the desorption of MoX was assessed.

Kinetics of D Atom Reactions with H2, CH4, and D2CO

The activation energies and steric factors of the abstraction reactions D+D2CO→D2+DCO and D+CH4→HD+CH3 have been determined by the technique of competing reactions. Mixtures of formaldehyde‐d2 and H2 as well as formaldehyde‐d2 and methane were photolyzed over a temperature interval in the region of 350°. Measurements of the HD/D2 product ratios as a function of temperature, initial compositions, and light intensity, give the activation energies and steric factors for D+D2CO→D2+DCO, the values E=4.8±0.6 kcal, P=0.04, and for D+CH4→HD+CH3, the values E=7.8±0.6 kcal, P=0.025. These are based on the values E=5.4 kcal and P=0.11 for the reaction D+H2→HD+H.

Field‐Emission Study of Carbon Monoxide on Tantalum

Adsorbed carbon monoxide shows three states of binding on tantalum. The weakest of these is desorbed at temperatures above 125°K with no observable surface migration. The bonding with the surface is of the van der Waals type. This is in contrast with the second state, which is desorbed above 650°K. The third state is dissociated before desorption to give the oxygen‐on‐tantalum pattern. Surface migration involving the second adsorbed state occurs with an activation energy of 38 kcal. The work function of a carbon monoxide‐covered tantalum surface, obtained by spreading the carbon monoxide on a shadowed tip at 40°K, is 0.8 eV greater than that of the corresponding clean tantalum. Fowler—Nordheim plots of clean and shadowed tips show that at least under these conditions, the infinite‐field extrapolation of logi/V2 is proportional to the log of the emitting areas.

Nitrogen on rhenium observed with the field emission microscope

Abstract The adsorption of nitrogen on rhenium was observed with a field emission microscope. Nitrogen decreases the work function of rhenium but also decreases the pre-exponential term of the Fowler-Nordheim expression. The result is a small decrease in emission with nitrogen adsorption. Surface migration could not be directly observed because desorption precedes it. There are two chemisorbed binding states associated with nitrogen on rhenium. The first is little stronger than physisorbed but accounts for most of the nitrogen in a saturated layer at 50 °K. The second is inferred from small pattern changes above 750 °K. The weakly bound state probably consists of a number of states with a distribution of desorption energies.

Standard molar enthalpies of formation of alloys in the pseudobinary system Zr(AlxCo1 − x)2 at the temperature 298.15 K

Abstract The standard molar enthalpies of formation ΔfHm° of alloys with compositions in the pseudobinary system Zr(AlxCo1 − x)2Zr have been determined by solution calorimetry in a mixture of hydrofluoric and nitric acids. The values of ΔfHm°(298.15 K) obtained are as follows: −(113±11) kJ mol−1 for x=0; −(123±8) kJ mol−1 for x= 1 12 ; −(120±8) kJ mol−1 for x= 1 6 ; −(130±7) for x= 1 4 ; and −(147±10) kJ mol−1 for x= 1 2 . Uncertainties correspond to twice the standard deviation of the mean. The following empirical relation was found to be applicable over the range of compositions studied: ΔfHm°{Zr(AlxCo1 − x)2} ≈ x·ΔfHm°(ZrAl2)+(1 − x)·ΔfHm°(ZrCo2).

A field emission study of carbon monoxide on hcp metals rhenium and ruthenium

Abstract The surface migration, work function increments, and desorption of carbon monoxide on two hcp metals, rhenium and ruthenium, were observed with field emission techniques. Carbon monoxide is weakly bound to ruthenium and more strongly bound to rhenium. For a complete monolayer, the work function increment is 1.3 eV for ruthenium and 0.8 eV for rhenium. Surface migration commences at a readily observable rate at about 125 K on ruthenium. The migration is characterized by a sharp boundary that becomes boundary free in the later stages. Because desorption precedes surface migration for CO on rhenium, the onset of migration is ill defined. Two well defined binding states α and α, are seen for CO on rhenium.

Field‐Emission Observations of Carbon on Tantalum

The carbon—tantalum system, observed with field emission, shows the temperature effects on the solubility and precipitation of the carbide phase. Carbon, once deposited on tantalum, cannot be removed by high‐temperature treatment, as in the case of tungsten. With low‐carbon contamination the 334 planes appear as dark areas, just as for carbon on tungsten. At temperatures in the region of 950°K, platelets, presumably Ta2C, form and migrate to the [111] zones of the emitter single‐crystal tip. The energy of activation of this surface migration process is 54 kcal.