John D. McKinley

Mass‐Spectrometric Investigation of the High‐Temperature Reaction between Nickel and Chlorine

The volatile products of the reaction between a molecular beam of room‐temperature chlorine and a polycrystalline nickel surface have been identified; their formation rates have been measured by a mass‐spectrometric technique at surface temperatures between 1000° and 1600°K and beam fluxes equivalent to pressures of 10—7 to 10—5 mm Hg. The surface coverage is estimated to be less than 0.1% under these conditions. NiCl and NiCl2 are formed on the nickel surface; the former is observed above 1450°, the latter below 1100°; and both are found at intermediate temperatures. About 70% of the incident chlorine is consumed by the interaction with nickel over the pressure and temperature ranges covered. The removal of chlorine and the production of NiCl and NiCl2 are first order in chlorine pressure. A mechanism which accounts satisfactorily for the results involves dissociative adsorption of chlorine to form two NiCls on the surface; these either evaporate, or disproportionate to form NiCl2 which evaporates.

Spectra of Matrix‐Isolated NiF2 and NiCl2

The infrared (400–4000‐cm−1) and ultraviolet (5000–2200‐A) absorption spectra of NiF2 and NiCl2 molecules isolated in an Ar matrix at 14°K have been observed. Sharp systems of absorptions near 780 cm−1 for NiF2 and 520 cm−1 for NiCl2 can be assigned to the antisymmetric stretching fundamental of the isolated molecules. Most of the vibrational splittings of the more abundant isotopic species have been resolved. Absorptions due to dimers and to other as yet uncharacterized species also appear. Evidence supports the hypothesis that NiF2 and NiCl2 are linear. No ultraviolet absorptions have been observed for NiF2, but the intense absorption previously observed for NiCl2 between 3650 and 2750 A has been found to have extensive vibrational structure, probably contributed by the symmetric stretching mode of the upper state.

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.

The adsorption kinetics of nitrogen on rhenium

The adsorption of room temperature nitrogen on atomically clean polycrystalline rhenium has been studied. The sticking coefficients at zero coverage were found to be 0.060, 0.009, and 0.007 at rhenium surface temperatures of 205, 300, and 373 °K respectively. These values are between one and two orders of magnitude smaller than those in the nitrogentungsten system. The dependence of the sticking coefficient upon surface coverage and temperature could be accounted for by assuming an intermediate state weakly bound to an adsorption site. These sites were assumed to be uniformly distributed over the surface. Their concentration was found to be less than 110 of the number of substrate-rhenium atoms/cm2. The binding energies of the intermediate state and the permanently adsorbed state were in the ratio of about 1:20 for both the nitrogen-rhenium and nitrogen-tungsten systems. In view of this similarity, it is difficult to account for the large difference in sticking probabilities unless one assumes that tungsten absorbs energy from the intermediate binding state much more efficiently than does the rhenium.

Mass‐Spectrometric Investigation of the Nickel—Fluorine Surface Reaction

Mass‐spectrometric measurements have been made of the volatile products of the reaction between nickel and bromine at nickel temperatures between 300° and 1600°K, and at bromine pressures between 10—7 and 10—4 mm Hg. Bromine dissociates on the surface at a temperature‐dependent rate; the fraction dissociated is much less than that corresponding to equilibration of the incident bromine with the hot nickel surface. NiBr2 and NiBr are formed in surface reactions at rates proportional to the bromine pressure. The former product is formed between 600° and 1200°K with a maximum in the rate at 900°; the latter is formed above 1200° at a rate increasing with nickel temperature.

Mass‐Spectrometric Investigation of the Yttrium—Chlorine Surface Reaction

Volatile products of the surface reaction between chlorine at room temperature and heated polycrystalline yttrium were identified and their formation rates were measured mass spectrometrically. At surface temperatures between 300° and 1700°K (yttrium melting point 1750°K) and chlorine pressures between 10—5 and 5×10—4 Torr, Cl, YCl2, and Cl2 were observed. No gaseous YCl or YCl3, was detected. The results are consistent with the presence of a strongly chemisorbed layer of chlorine atoms, stable over the entire temperature range, on which Cl2 adsorbs as Cl atoms. These either evaporate or react with YCl to form volatile YCl2. An activation energy of 55 kcal/mole for YCl2 evaporation is apparently associated with dissociative adsorption of chlorine on the chemisorbed Cl layer.