Similarities in photon and ion emissions induced by sputtering

It is now well known that the photon and ion emissions which result from the sputtering of metals by ion bombardment depend strongly on the surface composition. Recently R. Kelly and C.B. Kerkdijk, and G. Blaise and M. Bernheim, reported respectively on the intensification of both emissions produced by the adsorption of oxygen on a metal surface (this intensification corresponds to the so-called chemical effect). Their experiments were carried out approximately under the same sputtering conditions. Photon and ion yields were studied as a function of oxygen pressure and primary current density. A comparison of the results reveals some similarities in the behaviour of the two emissions. Our purpose is to underline these similarities and their consequences concerning the processes which give rise to photon and ion emissions.     

Oscillatory Singularities in Bianchi Models with Magnetic Fields

An idea which has been around in general relativity for more than 40  years is that in the approach to a big bang singularity solutions of the Einstein equations can be approximated by the Kasner map, which describes a succession of Kasner epochs. This is already a highly non-trivial statement in the spatially homogeneous case. There the Einstein equations reduce to ordinary differential equations and it becomes a statement that the solutions of the Einstein equations can be approximated by heteroclinic chains of the corresponding dynamical system. For a long time, progress on proving a statement of this kind rigorously was very slow but recently there has been new progress in this area, particularly in the case of the vacuum Einstein equations. In this paper we generalize some of these results to cases where the Einstein equations are coupled to matter fields, focussing on the example of a dynamical system arising from the Einstein–Maxwell equations with symmetry of Bianchi type VI₀. It turns out that this requires new techniques since certain eigenvalues are in a less favourable configuration than in the vacuum case. The difficulties which arise in that case are overcome by using the fact that the dynamical system of interest is of geometrical origin and thus has useful invariant manifolds.     

Charge-induced fragmentation of sodium clusters

The fission of highly charged sodium clusters with fissilities X>1 is studied by ab initio molecular dynamics. Na4+24 is found to undergo predominantly sequential Na+3 emission on a time scale of 1 ps, while Na(Q+)(24) ( 5< or =Q< or =8) undergoes multifragmentation on a time scale > or =0.1 ps, with Na+ increasingly the dominant fragment as Q increases. All singly charged fragments Na(+)(n) up to size n = 6 are observed. The observed fragment spectrum is, within statistical error, independent of the temperature T of the parent cluster for T< or =1500 K. These findings are consistent with and explain recent trends observed experimentally.     

Adsorption of gases studied by secondary ion emission mass spectrometry

The adsorption of an active gas, like oxygen, on the surface of a metal or an alloy leads to an intensification of the positive ion emission produced by sputtering. In the present paper this phenomenon (called chemical ion emission) is observed by blowing the gas on the surface of the sample while it is sputtered. The general features of the chemical effect on M⁺ ion production are discussed in terms of coverage resulting from equilibrium between the rate of atoms sticking to the surface and the sputtering rate of adsorbed atoms, and in terms of ionization probability depending upon the coverage. Examples are given for pure metals (Al, W, Ni) and alloys (NiCr, CuBe, CuAl, …). Attention is drawn to the effects observed on nickel single crystals (100) and (110). At a critical coverage an incorporation process takes place, producing both changes in the work function and in the ionization probability. These results, largely consistent with those obtained by classical methods, show that secondary ion emission can be used for adsorption studies. Finally, investigations of metal-oxygen interaction by the dynamic method (present work) and the static method of secondary ion mass analysis will be discussed and compared.     

NpAs2: Magnetic form factor and tentative crystal field model

A polarized neutron diffraction experiment at 4.2 K was carried out on two NpAs₂ single crystals with different orientations of the crystallographic axes with respect to the magnetic field. Low temperature atomic positions are given together with the Fermi length of the Np atom: 1.015X10^-12cm. The valence state of the Np ion is 4+, the ordered moment value is 1.46μ_B, a strong magnetocrystalline anisotropy and a field variation of the ordering temperature have been evidenced for the ferromagnetic state. 58 magnetic structure factors have been measured. A tentative crystal field model is presented.     

Magnetic properties of the uranium sesquichalcogenides

Magnetic measurements on the orthorhombic uranium sesquichalcogenides and orthorhombic U₃Te₄ were performed in the temperature range 4.2–1000 K and in magnetic fields up to 140 kOe. The temperature dependence of the reciprocal susceptibility was found to be curvilinear for all the investigated compounds. At low temperatures U₂Te₃ and U₃Te appeared to be antiferromagnetic below 106 K. U₂Se₃ and U₂S₃ seem to have a more considerable ferromagnetic contribution below 60 and 80 K, respectively. The magnetic properties are discussed in terms of crystal structure and related properties of the investigated compounds.     

Electrical resistivity of neptunium mono and diarsenide

The electrical resistivity of NpAs and NpAs₂ has been measured over the temperature range 4.2–300 K. Both compounds are semi-metallic contrary to semi-conducting transition reported in NpAs. The present results agree qualitatively with previously observed complex magnetic behaviour.     

Isotopieverschiebungskonstanten von Th,U, Pu und Am

The isotope shifts of unperturbed electron configurations have been determined from isotope shift measurements in the spectra of Th, U, Pu, and Am. The screening of the 7s electron charge density at the nucleus by 6d, 7s, and 7p electrons is discussed. It turns out that the same screening factors as for the 6s electron in lighter elements can be used. The screening of the 7s electron charge density at the nucleus by one 5f electron amounts to about 25%: [δT(f ⁿ s)?δT(f ⁿ )]/[δT(f ⁿ }s)?δ(f ^n} )]=0.7₅. The charge density at the nucleus due to the filleds (andp _1/2) shells is considerabely screened by anf electron. The isotope shiftδT(f^n}-1 d ^m +2)?δT(f ⁿ d ^m ) produced by this effect is of the same order of magnitude as the isotope shiftδT(f ⁿ d ^m s)-δT(f ⁿ d ^m ) due to ans electron. The experimental isotope shift constants are found to be:Β C _exp(Th²³⁰–Th²³²)=880±120;Β C _exp(U²³³–U²³⁵)=1000±180;Β C _exp(U²³⁴–U²³⁶)=1070±200;Β C _exp(U²³⁶–U²³⁸)=1080±180;Β C _exp(Pu²³⁸–Pu²⁴⁰)=1200±120;Β C _exp(Pu²³⁹–Pu²⁴¹)=1060±100;Β C _exp(Pu²⁴⁰–Pu242)=900 ±90;Β C _exp(Am²⁴¹–Am²⁴³)=890±50 [10^?3cm^?1]. The ratiosΒ C _exp/C _th are discussed.     

Anharmonic effects on theoretical IR line shapes of H-bonds

In the spirit of Y. Maréchal and A. Witkowski's [J. Chem. Phys. 48 (1968) 2697] work, one revisits, for weak H-bonds, the dependence of the angular frequency ω and of the equilibrium position q_e of the υ_X–H high frequency mode q, on the position coordinate Q of the low frequency υ_X–HY mode. One considers: ω=ω^o+bQ+cQ² and q_e=gQ+fQ². That leads to the anharmonic potential U: U=k₁q²+∑k_rQ^r+∑∑k_nmqⁿQ^m+k₁₅qQ⁵. Here k_r and k_mn (r=2–5, n=1,2 and m=1–4) are interrelated through b, c, g and f. By aid of the Hamiltonian involving U, we find the direct damped auto correlation function of υ_X–H, which, by Fourier transform, gives the IR spectral density (SD). When only b≠0, the SD is nothing but that given in a previous paper [P. Blaise, O. Henri-Rousseau, Chem. Phys. 243 (1999) 229]. When the adiabatic approximation is performed, this SD becomes that of N. Rösch and M. Ratner [J. Chem. Phys. 61 (1974) 3444] which reduces in turn to that of Maréchal and Witkowski in the absence of damping. With respect to b≠0, c produces a narrowing of the SD if c>0 and a subtle broadening if c<0. Besides, g induces the same narrowing for g>0 and g<0, while f gives subtle changes very sensitive to the sign of f and to the values of b, c and g. The situation b<0 and f>0 which is physically the most probable, leads to SDs which are the most evoking experimental profiles.