Ground-state Neutral Atoms Research Articles

Doppler shift laser fluorescence spectroscopy of sputtered and evaporated atoms under Ar+ bombardment

The temperature dependence of the sputtering yield and velocity distribution of sputtered neutral Ca and Cr atoms has been studied by Doppler shift laser fluorescence spectroscopy (DSLFS) for 15-keV-Ar+ ion bombardment.These investigations have been performed in the search for thermal spike effects and radiation-enhanced sublimation in sputtering at elevated temperatures. Ca targets have been studied up to about 270 °C and Cr targets up to 750 °C. The shapes of the velocity distributions do not change in the respective temperature ranges for both target materials. Furthermore, the absolute yield of sputtered neutral ground state atoms does not increase with temperature.This indicates that no radiation-enhanced thermal contribution is of relevant importance. Moreover, we can explain the velocity distributions in both cases by the linear cascade theory of sputtering. The thermal contribution due to thermal evaporation increases drastically above 230 °C for Ca and 550 °C for Cr. In both cases the corresponding velocity distribution can be fitted by a Boltzmann distribution.

Differential cross-section measurement for He 0-He 0 collisions

An apparatus has been designed and constructed in which a beam of helium atoms having energies of 0.5–1.0 keV is prepared by resonant charge exchange of He + on He, forming ground-state neutral atoms having the kinetic energy of the He + ions. The collimated neutral beam is allowed to scatter from a target gas of helium atoms. A resistance-strip magnetic electron multiplier with a tungsten cathode was used as the detector. The scattered particle signal has been determined as a function of the laboratory scattering angie 1° ⩽ θ ⩽ 16° with an angular resolution of 0.5°. Measured summed differential cross-sections for scattering He on the He are reported. They are strongly peaked in the forward direction.

Dynamic symmetry of atomic potential

The symmetry inherent in the periodic system of elements is discussed on the basis of the model one-electron potential in atoms. This potential approximates well that of the Thomas-Fermi theory and reproduces the well known n+l building-up rule describing the configurations of the ground-state neutral atoms. The modifications of the model potential accounting for the Coulomb tail allows us to analyse the (n+l) grouping in the Rydberg spectra of atoms. This grouping is shown to be closely connected with the n+l building-up rule for the ground states. The dynamic group of the periodic system (or of the atomic potential) is proposed. The specific doubling of the periods (as compared with the hydrogen type spectrum) is described by means of a group formally analogous to the isospin group. The operator of the 'isospin' projection on the chosen axis in the 'isospace' is found explicitly as an inversion operation in some sphere. The states classification according to the related quantum number allows us to give the group-theoretical interpretation of the 'secondary periodicity' effect in the periodic system.

Electrostatic potential–electronic density relationships in atoms. II

An exact equation has been derived that relates the derivative of the electrostatic potential at any radial distance from the nucleus of a neutral atom to the amount of electronic charge that is located beyond that radial distance. This equation assumes a particularly simple form at distances corresponding to minima in the radial density function, and can be used to show that at such a distance, rm, the electrostatic potential due to the electronic charge beyond rm is exactly half (but with opposite sign) of that produced by the nucleus and the electronic charge within the radial distance rm. When applied at the outermost radial density minimum, which has been proposed as the boundary between the core and valence regions of a ground-state neutral atom, the new equation can be used to significantly improve a previously derived atomic energy formula.

Electrostatic potential–electronic density relationships in atoms

Some electrostatic potential (V) –electronic density (ρ) relationships have been investigated for a number of neutral ground-state atoms, the largest being chlorine. The results provide further evidence of the physical significance of the outermost minimum in the radial density function, 4πr2ρ, which has previously been used to define the boundary between the core and valence regions in an atom. It was found, for all of the atoms, that the V–ρ behavior in the core regions agrees remarkably well with the Thomas–Fermi equation, V=4.785ρ2/3. In the valence regions, on the other hand, the Thomas-Fermi expression is no longer obeyed and the atoms show much less uniformity in their V–ρ behavior; a reasonable rough approximation, however, is V=αρ, where α depends upon the atom. By using this more realistic V–ρ relationship in the valence region, three correction terms can be derived which markedly improve the atomic energies obtained with the approximate formula E= (3/7) ZV0, in which V0 is the electrostatic potential at the nucleus. For the atoms K–Kr, the new formula reproduces the Hartree–Fock energies to within an average 0.25%. Further, the density is now predicted to decrease exponentially at large distances from the nucleus, correcting a weakness of the Thomas–Fermi theory.

Importance of the perpendicular transition leading to ground state atoms in the photodissociation of NaI

Measurements of the angular distribution of the alkali photofragment from photodissociation of a molecular beam of sodium iodide using linearly polarized light of wavelength 347.1 nm shows that a perpendicular transition from the 1Σ+ ionic ground state to the Ω = 1 covalent excited state accounts for 80 ± 9% of the absorption leading to ground state neutral atoms at that energy.

Yields and trapping of neutral deuterium beams containing 3∏u-molecules

Experimental yields of neutral ground-state atoms and molecules and long-lived 3∏u excited molecules from interactions of 30- to 120-keV ions in magnesium vapour are reported. Results of calculations that approximate trapping in magnetically confined plasmas of beams produced by either D+ or ions neutralized in Mg-vapour are given.

Born-Mayer interatomic potential for atoms with Z = 2 to Z = 36

Repulsive interatomic potentials for pairs of like neutral ground-state atoms are calculated on the basis of the statistical theory, the electron densities of atoms being taken from free-atom Hartree-Fock (HF) calculations. The results of numerical calculations are expressed in the Born-Mayer form in one or two intervals of internuclear separations and appropriate parameters of this fitting are tabulated.

Born-Mayer-Type Interatomic Potential for Neutral Ground-State Atoms withZ=2toZ=105

Born-Mayer parameters are given which permit, with good accuracy (to within 6%), a greatly simplified computation of a previously derived interatomic potential, $U(R)$, based on the Thomas-Fermi-Dirac (TFD) approximation. The numerical values of $A$ and $b$ appearing in $U(R)=A \mathrm{exp}(\ensuremath{-}bR)$ are tabulated in two sets of commonly used units for 104 homonuclear pairs of neutral ground-state atoms having $Z=2$ to $Z=105$. Approximate lower and upper limits of applicability, ${R}_{l}$ and ${R}_{u}$, are also listed, as is the magnitude of the maximum percent error ($\ensuremath{\epsilon}$) for each fit. ${R}_{l}$ is generally $\ensuremath{\sim}1.5{a}_{0}({a}_{0}=0.52917 \AA{})$, while ${R}_{u}\ensuremath{\sim}3.5{a}_{0}$. The effective upper limit probably lies at $\ensuremath{\sim}6\ensuremath{-}8{a}_{0}$. Also, with the aid of the given table and the combining rule ${U}_{12}\ensuremath{\simeq}{({U}_{11}{U}_{22})}^{\frac{1}{2}}$, the interaction energies of a total of 5356 heteronuclear diatoms can readily be obtained.

Auto-Ionizing States in the Alkali Atoms with Microsecond Lifetimes

Discrete atomic energy levels lying between the first and second ionization potentials which are metastable against both auto-ionization and radiative decay have been observed in the alkali elements. These levels arise from the excitation of an electron from the outermost closed shell of the atom and have characteristic lifetimes in the microsecond region. A beam of metastable alkali atoms is produced by electron bombardment of neutral ground-state atoms and is detected by collecting the charged products of the decay. Stern-Gerlach magnetic-deflection experiments have been performed to show unambiguously that the signals arise from paramagnetic atoms and not from possible stray-photon effects in the apparatus. The excitation energies, electron-bombardment production cross sections, natural lifetimes, and the tentative spectroscopic assignments of these atoms are listed below: The energy and lifetime of the metastable lithium atom are in good agreement with the available theoretical estimates. The metastable states in the heavier alkali atoms are classified in relation to the known ${\mathrm{I}}^{b}$ spectral terms. Magnetic and electric fields have been found to be effective in reducing the lifetime of the metastable atoms. The Zeeman quenching is discussed in a separate paper. The Stark quenching rate has been found to be proportional to the square of the electric field and to increase rapidly with atomic number $Z$. It is suggested that the Stark quenching results from a second-order interaction which involves the product of the electric field and the spin-orbit or other magnetic operators.

Further Study of the Reaction of Hot Hydrogen with Methane

The reaction of hot hydrogen atoms with methane (hot hydrogen in the form of recoil tritium produced by nuclear reaction) has been studied in further detail. Earlier work had been incomplete because in the presence of moderator, small amounts of HT, the abstraction product, were produced by what were recognized as spurious processes. An exhaustive investigation of possible limitations of the recoil method indicated that if helium is used as a moderator, not all the recoil tritium reaches chemical energies as neutral ground-state atoms. This finding is in accordance with the adiabatic principle. The use of neon or argon as moderators eliminated this difficulty and permitted estimation of the true yield of HT and the total reactivity integral for the reaction. The consequent complete analysis of the system results in several new significant findings: (1) Values of the reactivity integral IHT measured using a helium moderator are probably too high and require downward revision (other products are apparently not subject to this effect); (2) The most stringent test of the kinetic theory of hot reactions to date discloses no deviation despite the approximate nature of the assumptions underlying this theory; (3) The relative energy loss on collision of hot T with Ne, Ar, and He seems to accord with the soft-sphere predictions of Estrup; (4) Contrary to earlier belief, the abstraction product HT is formed at higher average energies than the displacement product CH3T; (5) The pressure dependence of the double displacement product CH2T·, suggests a generalization regarding the mechanisms by which such products are formed.

Measurements of Electron Impact Excitation Cross Sections of Laser States of Argon(II)

Cross sections for the excitation of the upper laser levels 4p2D5/2, 2D3/2, 2P3/2, and 2S1/2 of Ar (II) by electron impact with ground-state neutral atoms have been found by measuring the incoherent light produced by a triode-type structure filled with argon to pressures near 0.1 torr. Peak values in the vicinity of 5×10−19 cm2 are found. By operating the same structure in an argon-ion laser cavity, an increase in coherent output is measured. The amount of the increase in output may be used in conjunction with the measured upper-state cross section to estimate the cross section for electron excitation of the lower laser levels.

Production of Metastable Hg Atoms by Charge Exchange

Neutral mercury atoms, formed by charge exchange processes between positive mercury ions and mercury vapor atoms, were found to liberate a copious yield of secondary electrons from a metal surface. Values of γ as high as 0.40 were observed. The source of positive ions was the plasma of a mercury arc discharge. Since such a high yield of secondary electrons is not indicative of ground state neutral atoms, the atoms were assumed to be metastable. To test this hypothesis the neutrals were allowed to decay in their flight to the target. A second method of quenching the supposed metastable atoms consisted of admitting minute quantities of H2 and Ar gas into their flight path. In both cases the secondary emission was reduced by several orders of magnitude thus verifying the metastability hypothesis. The 4.66-, 5.43-, and 9.0-eV metastable states were observed along with a long-lived state having a mean lifetime of 6.0 μsec. As an experimental side light of great importance, it was observed that Hg− ions were formed in the collimated beam of excited neutral atoms.

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.