A pulsed beam time of flight technique for studying ion-atom scattering

Nuclear Instruments and Methods 194 (1982) 681-684 North-Holland Publishing Company

681

A PULSED BEAM TIME OF FLIGHT TECHNIQUE FOR STUDYING ION-ATOM SCATTERING R i c h a r d M. S C H E C T M A N a n d T i m o t h y H A L L Department of Physics and Astronomy, The University of Toledo, Toledo, OH 43606, U.S.A.

A 10 ns wide pulsed He + beam is scattered at grazing incidence from a solid AI target and light from the 3889 ,~ 2s 35-3p 3p transition in HeI emitted in a narrow angular sector centered upon the scattering region is detected. By measuring time resolved decay of this intensity as a function of the scattering angle 8, information concerning both the angular dependence of the velocities of the scattered ions as well as concerning the overall angular distribution of the scattered particles can be obtained - all for the selected case of ions which have captured an electron into the 3p 3p state.

1. Introduction As part of a continuing program of studying asymmettles of the ion surface interaction at medium energies - 50 keV to 1.0 MeV - we have carried out a number of measurements of the polarization of light emitted following the transmission of fast ions through thin foils [i]. Toward this same end, we have also investigated the polarization of light emitted following the scattering of ions from solid surfaces [2]. Measurements of the scattered intensity, velocity distribution and atomic orientation and alignment as a function of surface inclination and scattering angle all serve as probes of this interaction [3] and the present paper describes a convenient novel experimental technique for carrying out certain of these experiments. At the energies described there, several ion angular distributions have been reported [4], but energy distributions and the contributions of the various possible charge states to these results are unknown; moreover, very little is known about the angular distributions of neutrals and essentially nothing is known concerning the possible dependence of the angular distribution upon the quantum state of the exiting ion or atom. At lower energies, combined energy and angular distributions have been performed [5] and neutral atoms exiting in the backward direction have been studied by either use of a stripping foil preceeding an electrostatic analyzer [6] or use of time of flight techniques [7]. At medium energies, backscattered neutrals have been studied using a cooled surface barrier detector [8]. Varelas and co-workers have studied the angular distribution of the fight emitted from a specific decay transition of He + ions scattered under channeling conditions [9]. Measurements of alignment and orientation of scattered beams have been reported [ 10] and attempts at measuring the angular distribution of these quantities have been made. The experimental technique described 0029-554X/82/0000-0000/$02.75 © 1982 North-Holland

here is designed to facilitate such measurements by removing many of the difficulties previously encountered.

2. Experimental method The geometry employed is illustrated schematically in fig. l a. A beam of ions is incident upon a solid /~ scattered ions

lal

incident beam

- "tilted V target, inclination

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polaroid#

PM lens.L

•0•

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r°tatable aperture A; selects viewing eEIon

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Fig. 1. Schematic diagram of the experimental arrangement. (a) Side view of the scattering region. (b) End-on view of the optical system. XI. ION SCATTERING / SURFACE STUDIES

R.M. Schectman, T. Hall / Pulsed beam TOF technique

682

surface tilted through an angle a. Scattering through a variety of angles occurs, but light arising preferentially from ions scattered in the vertical plane through an angle 0 with the original beam is collected by the optical system composed of the lens L which images the vertical plane upon the aperture disk A and the aperturephototube (PM) combination (see fig. lb). The angle 0 may be conveniently varied by simply rotating the aperture-photomultiplier combination about the axis a - a ' in a mount constructed for that purpose. Use of the narrow-band interference filter (F) allows selection of light from selected excited states in selected charge states of the excited outgoing ions, yielding information not easily obtainable otherwise. For example, in the feasibility tests described herein, He ÷ ions were incident upon an aluminum target and a filter centered at 3889A selected light from neutralized scattered particles excited to the 3p 3p levels while a filter centered at 4686 A selected those remaining singly ionized He + and excited to the n = 4 hydrogenic levels. Insertion of a rotatable sheet polarizer with and without a ~ / 4 retarding plate at the point indicated (P) allows convenient measurement of atomic alignment and orientation. The connection between the light intensity I(0) measured in this way and the scattered particle intensity N(O) is, for a sector-shaped viewing region centered on a line-shaped beam, as shown in fig. 1, given by

period of 275 ns were obtained. However, because most of the accelerated beam was discarded, peak currents of only 5-10 /~A were obtained and this severely limited the accuracy of the results which were obtained. Data were collected using a low noise photomultiplier tube in the single photon mode with typical background rates being a few counts/second and were normalized to total charge collected at the insulated A1 target. For delayed coincidence time-of-flight spectra, a time-to-amplitude converter was started by pulses form the detector and stopped by pulses derived from the current pulse which signalled the arrival of the He + at the target. Output pulses were accumulated in a 1024 channel analyzer and were fed to a PDP-11 computer for analysis. A liquid nitrogen-cooled cold finger was positioned near the target to improve the local vacuum over the nominal ambient pressure of 1 × 10-6 Torr. Fig. 2a shows a typical total light intensity angular distribution of the sort described by eq. (1), for a target tilt angle of a - - 10 °. Fig. 2b shows a typical time-offlight spectrum obtained with the viewing region selecting sector set at 0 -- 20 ° -+ 2.5 ° - i.e., near the peak of the light intensity distribution. It is clear that due to the relatively weak incident beam, the statistical accuracy is limited and approximations must be introduced. The basic simplification introduced at this point is to as-

I ( 0 ) : X fN~(O) e x p [ - L , / v ( O ) ' r ] t)

X (l--exp[--(L 2-Li)/v(0)'r]).

(I)

Here Nv(0 ) represents the scattered intensity of particles with velocityv; L t and L 2 denote the limiting radii of the viewing sector; ~-denotes the meanlife of the decaying state; and a sum or integral over the velocity distribution is indicated. Since v may depend significantly upon scattering angle, additional information is dearly required to use this technique to obtain information about Nv(0 ). The needed information is acquired here by measuring - for each 0 - a pulsed beam time-of-flight spectrum. As will be discussed below, when these data are combined with measurements of I(0), significant knowledge of No(0) may be conveniently obtained.

3. Application of the technique

To demonstrate the feasibility of the system described here, it was tested using The University of Toledo 400 keV Van de Graaff accelerator. Since a high intensity pulsed ion source was not available, a beam of pulsed 100 keV He + ions was obtained by sweeping the beam horizontally across a vertical slit preceding the target. Beam pulses 7-10 ns wide with a repetition

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Fig. 2. Sample data obtained with this system. (a) Angular variation of the total light intensity. (b) Pulsed beam time-offlight spectrum, fixed O. The solid curve is a least-squares fit to eq. (2).

R.M. Schectman, T. Haft / Pulsed beam TOF technique sume that the velocity distribution function at each angle can be approximated by a two-component model, where the scattered beam at any angle consists of a fast elastic component, whose velocity v r is obtained from two-body kinematics and a much slower component, whose mean velocity vs results from multiple scatterings in the target [l i]. For the case of a finite number of discrete velocity components, if it is further assumed that the beam pulse is Gaussian in time with width o, the time spectrum for a sector-shaped slit extending from a distance L~ from the target to L 2 is

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Here ( is the detection efficiency and X the transition probability per unit time for emission of the detected photon and r is the meanlife of the decaying level. For the two velocity-component model described above, a non-linear least-squares fit of the data to eq. (2) was carried out at each angle with five free parameters: an overall amplitude, the beam width o, the zero of time

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and - the principle aims of this experiment - the velocity of the slow-component vs and the ratio of the scattered intensity of the slow component to the fast, N s / N f. The solid curve in fig. 2b is the computed fit. The asymmetric shape of the data of fig. 2b indicates that at least some of the emergent ions move slowly enough for their travel time past the slit, (L 2 - L i ) / v , to be comparable with the 105 ns meanlife of the decaying 3p 3p level. It should be noted that a one component model with velocity fixed at the value for elastic scattering from a binary collison was unable to fit the data, while a one component model with the velocity free to vary could fit the data only by requiring aphysical values for the other fit parameters. For the two component model, both o and t o did not vary significantly from run to run, as would be expected. The variation of vs with 0 and the slow/fast ratio measured in this way are shown in fig. 3. As might have been anticipated, the slow component decreases significantly in velocity as the angle increases and the fast component is more forward peaked than the slow. Once vs(0 ) and N J N t ( O ) are known, eq. (1) together with the total intensity measurements of fig. la allow Nt to be determined. In practice, a further small correction to account for effects of finite beam size needs to be carried out and a computer program to effect this correction by means of a Monte Carlo integration has been developed. The resultant values of Nf(0) are displayed in fig. 4.

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Fig. 3. Sample results based upon the two velocity component model. (a) Angular variation of the slow velocity component. (b) Angular variation of the relative amplitudes of the two velocity components.

4. Conclusions

The potential of this technique for studying the scattering of ions by solids through the light emitted by outgoing excited particles has been demonstrated. It is clear that realization of the full potential of this technique requires use of a high intensity pulsed ion-beam. XI. ION SCATTERING / SURFACE STUDIES

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R.M. Schectman, T. H a l l / Pulsed beam TOF technique

The authors wish to acknowledge the contributions m a d e b y Dr. R. H i g h t in the design and construction of the a p p a r a t u s described here. They also wish to acknowledge the invaluable assistance of J. Wells and N. Schaffel with the accelerator operation. This work was s u p p o r t e d in part b y the U.S. N a t i o n a l Science F o u n d a tion.

[5] [6] [7] [8]

References [1] See, e.g., R.M. Schectman, R.D. Hight, S.T. Chen, L.J. Curtis, H.G. Berry, T.J. Gay and R. DeSerio, Phys. Rev. A22 (1980) 1591.; ~J. Gay, H.G. Berry, R. DeSerio, H.P. Garnir, R.M. Schectman, N. Schaffel, R.D. Hight and D.J. Burns, Phys. Rev. A23 (1981) 1745. [2] H.G. Berry, G. Gabrielse, A.E. Livingston, R.M. Schectman and J. Desesquelles, Phys. Rev. Lett. 38 (1977) 1475. [3] See, e.g., Inelastic ion surface collisions, eds., N.H. Tolk, J.C. Tully, W. Heiland and C.W. White, (Academic Press, New York, 1977). [4] H.G. Andre, R. FrOhling, H.J. Pli3hn and J.D. Silver, Phys.

[9]

[10]

[11]

Rev. Lett. 37 (1976) 18; H.G. Berry, G. Gabrielse and A.E. Livingston, Phys. Rev. A16 (1977) 1915. P. Bertrand, F. Delaunay, C. Bulens and J.M. Streydis, Surf. Sci. 68 (1977) 108. W. Eckstein, V.A. Molchanov and H. Verbeek, Nucl. Instr. and Meth. 149 (1978) 599. T.M. Buck, Y.S. Chen, G.H. Weatley and W.F. Van der Weg, Surf. Sci. 47 (1975) 244. W.F.S. Poehlman, R.S. Bhattacharya, D.A. Thompson and H.D. Barber, Surf. Sci. 100 (1980) 14. C. Varelas and Sizmann, Surf. Sci. 71 (1978) 51: C. Varelas, K. Goltz and R. Sizmann, Surf. Sci. 80 (1979) 524; N. Graser and C. Varelas, this Conference. H.G. Berry, G. Gabrielse, A.E. Livingston, R.M. Schectman and J. Desesquelles, Phys. Rev. Lett. 38 (1977) 1473: N.H. Tolk, J.C. Tully, J.S. Kraus, W. Heiland and S.H. Neff, Phys. Rev. Lett. 41 (1978) 643 and 42 (1979) 1475: H.J. Andfft, H. Winter, R. Fr6hling, N. Kerchner, H,J. PliShn, W. Wittmann, W. Graser and C. Varelas, Nucl. Instr. and Meth. 170 (1980) 527. This represents an extension to higher energies of the two-component model of W. Heiland and E. Taglauer, Nucl. Instr. and Meth. 132 (1976) 539.