Orientation of Atoms by Collisions with Inclined Surfaces

Abstract

We have measured large orientations for Ar n states excited by 300-keV Ar + impact on solid Cu surfaces at grazing incidence. We report circulary polarized light fractions of up to 76% and interpret the excitation mechanism as an orbital-angular-momentum effect. As an application of this new excitation scheme, a zero-field level-crossing experiment is presented.We report new experiments which confirm and extend the recent observation that the excitation of fast ion beams by impact with inclined solid amorphous surfaces can lead to appreciable excited-state orientations. 1 We present measurements of angular distributions of the scattered ion current which support the assumption that near grazing incidence, there is a well-defined direction for the outgoing oriented atoms, as in "tilted-foil" experiments. 2 We also give the first results for the application of this "tilted-surface" effect to atomic level-crossing studies.The experimental setup is shown in Fig. 1. With the axes indicated, the direction of observation was along Ox. The incident beam direction is along Oz, the target surface normal makes an angle a with the -y axis in the yz plane, and we anticipate the results given below by ascribing an angle 9 to the average forward-scattered particle direction. The target was amorphous Cu, of dimensions 30 mmxl5 mmxl mm, and its surface to spectrometer FIG. 1. Experimental setup. The beam cross section is 0.5x 5 mm 2 " was mechanically polished flat to 2 jum. It was mounted on a shaft which was along Ox, and the angle a could be set to a precision of ± 0.17°. A 1-mm-diam pickup wire was mounted parallel to and off center from the x axis in such a way as to describe a circle of radius 7 cm about that axis. The scattered beam current could be measured with this wire as a function of y, in 1.4° steps. The pickup wire and the target could be held independently at variable potentials to suppress secondary-electron emission.The optical system consisted of an achromatic A/4 plate and a linear polarizer, followed by a single imaging lens, a 0.3-m grating spectrometer, and a photo multiplier tube in photon-counting mode. The viewing region was approximately 7 mm in diameter in the yz plane, and was centered on the x axis. Spectra were built up by repetitive sweeping of the wavelength proportional to collected charge of the unbiased target (our results are independent of secondary-electron emission) and recording the photon counts in a synchronized multichannel scaler. Similarly level-crossing curves were obtained by sweeping the magnetic flux B z , proportional to time at less than ± 5% beam fluctuations and recording photons of a particular line and polarization. B z , was applied by Helmholtz coils aligned along the scattered beam direction Oz'.In order to determine excited-state orientations of free scattered ions we measured the normalized Stokes parameter S/I= {la" -Io + )/(Ia" + Ia + ) by rotation of the A/4 plate in steps of 90°, where Jcr" and Ia + are, respectively, right and left circularly polarized light inte...

Production of high circular polarization by ion surface scattering of ferromagnets

“…The apparent asymmetry in the treatment of the orbital and the spin part of p(to) stresses that we are mainly interested in the influence of the target-spin polarization on S/I, whereas-in the present investigation -the p~kl)(LL) are treated as adjustable parameters (irrespective of the question of the way in which PL may be constructed from pt and PLy). This point will become clear from our attempt -given below-to take account of the values that were previously determined from experiment [2] 9…”

“…If, on the other hand, the ion-target collision would merely generate an ionic excited state without any electron exchange, no substantial influence of the target magnetization is expected to occur because of the very short interaction time (< 10-~3 s). Assuming LS-coupling for the partners participating in an ion-target collision as done in [2], the effect of electron-target spin polarization in a loss-capture model may be accounted for in the following manner: Let us assume that-as a first step-an impinging ion loses another electron to the target. The intermediate ion may be described by a definite angular momentum L1 and spin $1 if in addition to LS-coupling we assume that in a parentage approximation of the final state only one parent contributes significantly [9].…”

“…Attempts to theoretically explain or predict the degree of atomic alignment or orientation are mainly based on either of the following physical mechanisms: (1) The net polarization is produced by electron capture of the projectile into anisotropically populated, but well-defined angular-momentum (L) states assuming an isotropic spin distribution both in the target as well as in the ensemble of outgoing or scattered atoms or ions [1][2][3][4][5]. (2) The anisotropy in the excited atomic states results from the transfer of target-electron-spin polarization to the final state of the atom or ion formed without any influence of the orbital motion of the electrons [6,7].…”

“…It is the purpose of the present note to investigate, theoretically, the combined effect of orbital and spin polarization mechanisms on the degree of excited state polarization. The problem is of practical interest as can easily be understood by referring to two experimental investigations performed by Rau and Sizmann [6] and Andr~i et al [2], resp., with different techniques and physical aims. Rau and Sizmann were mainly interested in studying the electron-spin polarization at ferromagnetic Ni surfaces via ground-state electron * This work is part of the research program of the "Sonderforschungsbereich 161" supported by the Deutsche Forschungsgemeinschaft capture of surface-scattered deuterons.…”

Observation of the HeI 33 P polarization after medium energy He+ ? Ne collisions with impact parameter selection

“…collision processes for the He + energies from 10 to 355 keV.As one possible approach to the understanding of the microscopic mechanisms yielding the recently discovered large orientation of scattered ions (atoms) after ion-beam surface interaction at grazing incidence (IB-SIGI) [1][2][3][4][5] we investigate the orientation of scattered ions (atoms) resulting from ion-atom collisions with impact parameter selection. The experimental problem in such investigations is usually solved by time-consuming standard coincidence measurements [6].…”