eV For Na Research Articles

Interaction of K, Na, Li and Ti with silicon (111) and (100) surfaces; surface ionization and kinetics of desorption

Atomic beams of K, Na, Li and Tl are used to determine the work function and desorption energies for Si(111) and (100) surfaces. The (111) surface appears to go through a surface transition at ca. 1100K. Below 1000K, the (111) surface appears stable with a surface ionization work function φ + = 4.80 ± 0.05 eV. The desorption of K and Na from this surface has both a first- and second-order component with activation energies for desorption of E a1 = 3.15 ± 0.15 eV, E a2 = 2.53 ± 0.10 eV for K, and E a1 = 3.47 ± 0.15 eV, E a2 = 2.39 ± 0.13 eV for Na. The second-order part of the desorption is probably due to interactions or collisions of the atoms as they move along the surface. Above 1100 K the work function decreases by 0.03 ± 0.01 eV and the desorption of K and Na from this surface is predominantly first order with E a1 = 2.35±0.07 eV for K and E a1 = 2.45± 0.12 eV for Na. The (100) surface in the temperature range of 800 to 1550 K has φ + = 4.67 ± 0.08 eV and a first order desorption energy E a1 = 2.54 ± 0.14 eV for K. The work functions from the thermionic emission of electrons are φ c − = 4.55 ± 0.1and φ e − =4.53 ± 0.1eVfor the (111) and (100) surfaces, respectively, for T ⩾ 1000 K.

Surface characterization of sodium tungsten bronzes

Single crystals of the sodium tungsten bronzes Na 0.6WO 3 and Na 0.8WO 3 were investigated using a variety of surface-sensitive techniques. The predominant work functions of the (100) orientation of these materials were measured to be 4.65±0.05 eV and 4.50±0.05 eV for Na 0.6WO 3 and Na 0.8WO 3 respectively. However, a second work function appeared irreproducibly between 3.2 and 3.7 eV for both crystals. At present, we conclude that the low work function state is a result of a local change in surface structure or composition brought about by annealing. Potassium impurity preferentially segregates at the surface upon annealing at 500°C but could not unequivocally be related to the formation of the low work function state. Neither crystal readily adsorbs CO and O 2, even while at elevated temperatures. The activation energies of sodium desorption were measured to be 12.5 and 30.1 kcal/mole for Na 0.6WO 3 and Na 0.8WO 3 respectively.

Auger widths of core levels in metallic sodium and lithium

The Auger decay rates of the $1s$ level in lithium and $2p$ level in sodium are calculated using a diagrammatic many-body technique. The lowest-order nonvanishing contribution to the self-energy of these core states gives Auger widths of the order of ${10}^{\ensuremath{-}3}$ eV for Li and ${10}^{\ensuremath{-}4}$ eV for Na. However, higher-order terms are found to diverge showing that a renormalized theory must be used for such calculations. We find that for systems with low-lying excitations, there is a many-body mechanism which enhances the Auger width. Using a self-consistent renormalized theory, we find widths of the order of 20 meV for both Li and Na. The sodium result is consistent with predictions based on observed soft-x-ray spectra, but for lithium it is still one order of magnitude too small to be able to attribute the observed threshold behavior as purely due to an Auger core-level width.

Excitation in atom-atom collisions. K+Hg, K+K, and Na+K

Fast alkali beams produced by sputtering and charge exchange have been utilized to study collisional excitation of potassium atoms by mercury, potassium and sodium. K(4 2P) was detected by its radiative decay. The threshold energies for excitation of K(4 2P) are 4.0 eV for K+Hg. 1.6 eV for K+K, and 2.0 eV for Na+K. Cross sections are of the order of 3A 2 at 20 eV for all three systems.

Thermal Desorption of Na and K from α Iron

The activation energies for the thermal desorption of Na and K from iron filaments were measured by allowing a flux of Na and K neutral atoms to strike the Fe filament for predetermined periods of time and then applying a linear temperature sweep of 0.895°K/sec. The desorption energies for an unoxygenated iron surface were found to be 2.8 and 2.6 eV for Na and K from iron, respectively. The effect of oxygen application on the desorption energies is also investigated.

Collision-Induced Vibrational Excitation and Dissociation of Hydrogen with Alkali Atoms and Ions from 2 to 50 eV

Inelastic energy losses for Na0, Na+, K0, and K+ backscattered from H2 and D2 have been measured. At center-of-mass energies below15 ∼ eV for Na and ∼ 20 eV for K, the collisions yield vibrational excitations which are compared with the one-dimensional exact classical calculations of Secrest. The vibrational excitation in the Na+–H2 and Na0–H2 system is in excellent agreement with three-dimensional classical calculations of Cheng and Wolfsberg. Above these energies the molecules are dissociated but the energy loss distribution of the alkali projectile remains sharp and in the case of Na–H approaches the maximum loss possible at 50 eV.

Binding and charge transfer associated with alkali metal adsorption on single crystal nickel surfaces

Work function and thermal desorption energy measurements of alkali metal coated single crystal Ni surfaces are reported and correlated with previously reported surface crystallography measurements. It is shown that the minimums in the work function versus coverage curves do not generally correspond to significant changes in surface structure. Initial dipole moments calculated from the work function measurements are 7.4 ± 0.5, 7.2 ± 0.5 and 3.2 ± 0.3 Debye units for Na on Ni(111), (100) and (110), respectively, and 3.2 ± 0.3, 5.3 ± 0.3 and 7.0 ±0.5 Debye units for Na, K and Cs on Ni(110), respectively. A quantum mechanical theory developed by Gadzuk predicts dipole moments in good agreement with these experimental values for alkali metal coated Ni. The thermal desorption energies corresponding to the initial stage of adsorption measured for Na on Ni(111), (100), and (110) were 2.54, 2.52 and 2.18 ± 0.1 eV, respectively, and 2.18, 2.56 and 2.72 ± 0.1 eV for Na, K and Cs on Ni (110), respectively. The shapes of the desorption energy and frequency factor versus coverage curves were related to the surface crystallography. Finally, correlation of the desorption energy with the work function measurements emphasized the importance of ionic bonding at low coverages.

Mean Residence Times of Alkali Ions on Polycrystalline Wolfram Surfaces as Studied with a Pulsed‐Molecular‐Beam Mass Spectrometer

AbstractThe mean residence time τi of alkali ions on both atomically clean and strongly gas‐covered surfaces of polycrystalline wolfram has been studied with a recently developed technique of combining modulated atomic and/or molecular beams from vapors of alkali metals and alkali halides with phase‐sensitive mass‐spectrometic detection. The experimental arrangement allows the direct determination of the mean residence time τi of an ion on a metal surface under extremely clean surface conditions and at very small incident beam intensities (109− 1012 particles cm.−2 sec.−1) to avoid an influence of surface coverage by the beam material itself. In addition, it allows mass‐spectrometric determination of the effect of the composition of the incident beam on τi. The dependence of τi on the surface temperature T is given by Frenkel's equation τi = τ exp (Ei/kT), where Ei is the ion desorption energy. With alkali chloride sources, the values tabulated below were determined over ranges within the temperature region 1100 °K < T < 1700 °K. The atomic and molecular composition of the beam significantly affected τi, e.g., for an incident beam of neutral sodium atoms, τ = (0.85 ± 0.05) × 10−13 sec. Ei = 2.69 ± 0.03 eV for Na+ on clean W. magnified imageThe present experiments are being extended to determine the mean residence time τα of neutral alkali atoms on wolfram surfaces. Then with the aid of the more precisely determined values of Ei and Eα and with additional knowledge of the effective work function of the surface and of the ionization energy of the incident particle, it will be possible to decide such an important question as whether or not the surface states of adsorbed atoms differ from those for ions.

Atomic Negative Ions. Second Period

Results of Hartree-Fock calculations on the ions ${\mathrm{Na}}^{\ensuremath{-}}$, ${\mathrm{Al}}^{\ensuremath{-}}$, ${\mathrm{Si}}^{\ensuremath{-}}$, ${\mathrm{P}}^{\ensuremath{-}}$, and ${\mathrm{Cl}}^{\ensuremath{-}}$ in states of the lowest electronic configuration, along with correlation and relativistic corrections, are given. The data are used to compute electron affinities for the atoms of the second row of the periodic table, and the stability of the excited states of the ions relative to the neutral ground-state atoms. The computed electron affinities for atoms on which experimental determinations are not available are 0.78 eV for Na, 0.52 eV for Al, 1.39 eV for Si, and 0.78 eV for P. The computed electron affinities for S and Cl are 2.12 and 3.56 eV in fair agreement with the experimental values of 2.07\ifmmode\pm\else\textpm\fi{}0.07 and 3.613\ifmmode\pm\else\textpm\fi{}0.003 eV, respectively. For the second-row negative ions, several excited states are lower in energy than the corresponding ground-state neutral atoms; the computed stabilities for the species ${\mathrm{Al}}^{\ensuremath{-}}(^{1}D)$, ${\mathrm{Si}}^{\ensuremath{-}}(^{2}D)$ and ${\mathrm{Si}}^{\ensuremath{-}}(^{2}P)$ are 0.23, 0.58, and 0.08 eV, respectively. While the $^{1}D$ state of ${\mathrm{P}}^{\ensuremath{-}}$ is estimated to be very close to that of $\mathrm{P}(^{4}S)$, our estimates are not accurate enough to say definitely whether it is above or below.