Milton D. Scheer

Kinetics of Cs+ Desorption from Tungsten

The mean adsorption lifetime τ3 of Cs+ on tungsten has been measured in the 1000°—1200°K temperature range, using a pulsed beam technique. Under conditions of low surface coverage and with either Cs or CsI as beam materials τ3=(1.0±0.5)×10−12exp[(23 600±500)/T]sec was obtained. The heat of desorption can be calculated as the energy required to remove an isolated Cs+ ion from the surface of an electrical conductor. The presence of an adsorbed contaminating layer, arising from residual vacuum gases, decreased the Cs+—W binding energy by 0.5 eV and increased the pre‐exponential factor by about two orders of magnitude. Anomalous results were obtained when CsCl was used for a beam material, suggesting a reaction between the surface tungsten atoms and atomic chlorine.

Entropies, Heats of Sublimation, and Dissociation Energies of the Cesium Halides

The vapor pressures of the crystalline cesium halides have been measured in the 700–900°K temperature range. The salt vapor effusing from a Knudsen effusion cell was accurately collimated into a molecular beam with a well‐defined cross‐sectional area. Beam intensities were determined from the positive ion currents produced by dissociation of the halide and ionization to Cs+ on a tungsten surface at 1700°K. From the experimental vapor pressures and heats of sublimation, the crystal entropies, dissociation energies, and heats of sublimation at 298°K were computed using available spectroscopic and thermal data.

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...

Kinetics of Desorption. II. Cs+ and Ba+ from Rhenium

The mean desorption lifetimes (τ3) of Cs+ and Ba+ on an atomically clean rhenium surface have been measured. Under conditions of low surface coverage τ3=(1.9±0.9)×10−13exp(23 300±500T)sec was obtained for Cs+ in the temperature range 950<T<1080°K. This is in excellent agreement with the data obtained previously for the Cs+–W system. For Ba+ on rhenium τ3=(0.6±0.4)×10−13exp(54 700±2400T)sec was obtained in the temperature range 2100<T<2310°K. The Cs+–Re, like the Cs+–W bonding, can be attributed to the electrostatic image energy of a Cs+ ion on an electrically conducting surface. The Ba+–Re interaction, on the other hand, must include a large exchange force in addition to the electrostatic image force because of the unpaired valence electron remaining in the univalent barium positive ion. The presence of an adsorbed layer of residual vacuum gases decreased the Cs+–Re binding energy by 0.4 eV and increased the pre‐exponential factor by two orders of magnitude. These results are almost identical with that re...

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.

V→R, T energy transfer processes in vibrationally excited pentafluorobenzene using a Hg tracer absorption method

Abstract A new technique based upon the Doppler and Lorentz broadening of the isotopic and hyperfine Hg multiplet lines near 254 nm was used to monitor the translational equilibration of vibrationally excited pentafluorobenzene (PFB). Excitation was achieved with a pulsed CO 2 infrared laser focused into a cell containing PFB and a trace amount of Hg. Rates of V→ R, T energy transfer were found to be linearly dependent on both the excitation energy and the pressure of PFB. Excitation energies were independently determined by the Hg absorption technique and by measuring the change in absorption by the PFB at 254 nm. For PFB ∗ -PFB ∗ collisions, the average energy transferred per collision divided by the average excitation energy of the colliding pair, 〈Δ〉/ 〈 E 〉, is0.0133±0.0016.

Measurement of collisional V→T, R energy transfer in vibrationally excited SF6 using Doppler broadening of the Hg(6 1S0–6 3P1) multiplet

A new method for measuring energy transfer between colliding molecules has been developed. It is based upon the temperature dependence of the isotopic and hyperfine line widths of the Hg multiplet in the neighborhood of 254 nm. The light source was a Hg resonance lamp whose line profiles could be precisely controlled by varying the Hg pressure. Multiphoton absorption at 944 cm−1 was used to excite SF6 vibrationally during a 250 ns CO2 laser pulse. The subsequent rate of equipartition of this excess vibrational energy with the translational and rotational degrees of freedom was determined by measuring the initial slope of the increase in the absorption of 254 nm radiation by a trace of Hg vapor. It was found that pressures above 2 Torr were required to involve all of the SF6 molecules in the absorption of the laser radiation. The V→T, R energy transferred per SF*6 –SF6 collision was found to be proportional to the 3/2 power of the excitation energy between 1500 and 5000 cm−1 and gradually changed to a firs...

POSITIVE AND NEGATIVE SELF-SURFACE IONIZATION OF TUNGSTEN AND RHENIUM.

The self‐surface ionization of tungsten and rhenium has been investigated with a specially designed mass spectrometer. It has been found that both positive and negative singly charged atomic ions sublime from these metal surfaces in the 1900°—2600°K temperature range. The assumption that these processes can be described by a generalized Saha—Langmuir equation has been shown to be valid. The energies required for positive‐ion sublimation were found to be 12.1 and 10.7 eV for tungsten and rhenium, respectively. The negative‐ion sublimation energy for rhenium was determined as 12.6 eV and that for tungsten is given by approximately the same value. At 2300°K, the ratio of positive to negative ions evaporating from these surfaces was found to be about 40 for tungsten and about 20 000 for rhenium. These results are consistent with the following estimates for the electron affinities: A(W)=0.50±0.30 eV and A(Re)=0.15±0.10 eV.

Electron Affinity of Lithium

The positive and negative surface ionization of a lithium atom beam on a thin, heated, molybdenum ribbon was studied using a specially designed mass spectrometer. At a constant incident beam flux, Li+ yields were determined as a function of surface temperature in the 1400°–2100°K range. Using 5.39 eV for the first lithium ionization potential (I), this data yielded an average positive ion work function (φ+) of 4.38 eV with a standard deviation of 0.01 eV. The [Li+/Li−] ratio was then determined at beam intensities and surface temperatures made sufficiently large to give measurable Li− yields. From the above values for I, φ+, T, and [Li+/Li−], it was found that A (Li) = φ− − (3.35 ± 0.05) eV. The quantity φ− is an average negative ion work function. This could not be measured with any precision because of the very small Li− yields. A recent determination of the single‐plane work functions of molybdenum shows a variation from 4.0 eV for the (116) plane to 5.0 eV for a (110) surface. Since the negative i...

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.