B. Andresen

Atomic excitations in sputtering processes

Abstract The sputtering of Mg, Cu, Zn, Cia, Au, and TI by Xe+ impact has been studied by optical spectrometry in a 10−9 Torr vacuum. The excitation functions are compared with total sputtering yields. The relative level populations of the sputtered particles follow an (n*)p behavior, almost identical to that for gas single collisions and beam-foil excitations, whereas two intensity distributions based on local thermodynamic equilibrium are shown not to be satisfied.

Adiabatic and diabatic decoupling for selected H-H* scattering states

For thermal atomic collisions a new simple expression for the partial inelastic scattering cross section, not involving any derivatives or highly cancelling components, is derived in a two-state distorted wave description. It is shown that an R-dependent diabatic representation is not possible, R being the internuclear distance, and a near-adiabatic representation is suggested as an alternative to the usual adiabatic representation. To test the numerical stability of the new formula and to compare the representations, transition probabilities for thermal H-H* collisions are calculated by the method of distorted waves using the complete radial coupling operators.

A Study of Molecular Effects in Beam-Foil Spectroscopy

Relative populations of ns + nd levels in hydrogen as functions of the principal quantum number n have been measured with beams of H+, H2+, and H3+ impinging on thin carbon foils at 25 keV/amu and 100 keV/amu. Enhancements of 20% and 45% for dimer and trimer clusters are observed uniformly for all levels. A possible explanation in terms of screening of the Coulomb repulsion between the protons inside the foil, thus reducing the effective thickness of the foil, is given. All relative populations closely follow an np power law with p = - 4.0 and - 3.7 at 25 keV/amu and 100 keV/amu, respectively, in perfect analogy with atomic collision experiments. O+/O2+-foil excitations at 100 keV and 155 keV show a similar molecular effect, but in reverse with a larger mean charge produced by the dimer.

A comparison of gas-phase, plasma, sputtering and beam-foil excitations

Abstract A comparison of relative populations of ZnI and ZnII levels excited in equilibrium plasma, sputtering, gas-phase single collision, and beam foil sources is presented, and includes data on the first reported excitation studies under single collision conditions for a (transition metal atom)-(transition metal ion) interaction. A search for continuum emission from atomic chromium, like that seen during sputtering of chromium metal, produced a negative result.

A Study of the Beam-Foil Excitation Mechanism Using Carbon Projectiles

Beam-foil excitation functions have been measured for some optical transitions in C II-C IV in the projectile energy range 100-450 keV, and in C IV-C VI from 4 to 20 MeV. The full set of population functions, representing eight different classes of core-outer shell arrangements, are reproduced by three parameters accounting for 1s vacancies, 2s vacancies, and outer shell populations according to the independent-electron model. The excitation functions at 4-20 MeV were measured with foils tilted 0°, 30° and 45° and the photon yields for C III-C VI lines found to depend substantially upon the foil tilt angle. Relative level populations were measured for some C IV levels. The results show that the relative level population within the same term series decreases slightly faster than n-3. The relative population for fixed principal quantum number increases from s to p levels, decreases to d levels, and shows another increase from d to g levels. This relative level population picture is independent of foil tilt angle.

A study of relative level populations in beam-foil excited Li, Be, and Mg

We have studied the optical beam-foil spectra of Li, Be, and Mg in the wavelength region 200–550 nm by use of a quantum efficiency calibrated monochromator at projectile energies of 210 keV for Li, 156, 256, and 356 keV for Be, and 100 keV for Mg. The relative population for different Rydberg levels of the same term series decreases in most cases somewhat faster than (n*)−3, as is also observed in ion-atom encounters under single collision conditions. The relative level population for levels of the same principal quantum number increases generally with the orbital angular momentum quantum number, and superimposed on this increase is a peaking forp levels. The relative population of levels of the same charge state and with the same core configuration is unaffected by change of the projectile energy.

An evaluation of the methods of determining excited state population distributions from sputtering sources

Abstract The method of calculating relative initial level populations of excited states of sputtered atoms is developed in principle and compared with those in current use. The reason that the latter, although mathematically different, have generally led to similar population distributions is outlined.

Semiclassical calculation of charge transfer: tunneling at large angular momentum

Abstract The semiclassical approximation suggested earlier is compared with the exact quantum mechanical solution for a model charge exchange process. The main objective is to study collisions at large impact parameters corresponding to the transition from the classically allowed to the classically forbidden region of non-adiabatic coupling. Semiclassical equations for the extended exponential—linear model of non-adiabatic coupling are shown to give a reasonable description of tunneling.

Conditions for the indistinguishability of thermal equilibrium and single atomic collision relative level populations

Abstract Single atomic collision and thermodynamic equilibrium level populations are compared. The population distributions for relatively cascade free levels with similar lifetimes are shown to be easily described by a Boltzmann relation even if the excitation mechanism is non-thermal.

Observed equivalence of arc, sputtering, and gas phase single collision relative populations for some ZnI levels

Abstract Equivalent relative populations of Znl levels of similar lifetime and small cascade contribution in arc, sputtering, and single collision sources are demonstrated, and imply that the physical basis of the Andersen and Hinthorne theory is only one possible description of the excitation mechanism operative during sputtering.