Energy spectra of secondary electrons emitted from metal surfaces under bombardment with high energy noble gas ions

NUCLEAR INSTRUMENTS

AND METHODS

132 ( I 9 7 6 )

483-488;

© NORTH-HOLLAND

PUBLISHING

CO.

E N E R G Y SPECTRA OF S E C O N D A R Y E L E C T R O N S E M I T T E D F R O M M E T A L S U R F A C E S U N D E R B O M B A R D M E N T W I T H H I G H E N E R G Y N O B L E GAS IONS* W. KRUGER, A. S C H A R M A N N and N. STILLER 1. Physikalisches lnstitut der Justus Liebig-Unioersit6t, Gieflen, Germany The investigation of the energy distribution of electrons released from metal targets by high energy ion bombardment showed an unexpected fine structure of monoenergetic electrons, which could be attributed to the decay of highly excited auto-ionizing states of the bombarding gas atoms. A comparison with spectra reported in literature revealed a rather good agreement with respect to the position of the lines on the energy scale. The measurements proved the practicability of the experimental method for such an investigation and will be applied by us in the future with greater refinements of electronics and data processing.

1. Introduction

Investigations of the secondary electrons which are released from metal targets under ion bombardment, * Dedicated to Prof. Dr. Ing. E.h.W. Hanle on the occasion of his 75th birthday.

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were a fruitful field of experimental research in the past1'2). Most of these experiments were done in the range of slowly moving ions 3) or at medium energies 4'5) whereas little information is available until now in the energy range between 100 keV and 1 MeV. The basic reasons for our investigations in this energy range were twofold: Firstly, we planned to align single crystalline targets, which were fixed on a goniometer stage by taking advantage of the minimum yield of secondary electrons if the ion beam is directed into a crystal channel, and secondly we expected to get a hint for the interpretation of previous measurements6), which probably is in relation to the topic of inelastic collisions in the target. Since the energy distribution spectra of secondary electrons provide considerable information on inelastic processes, we decided on their detailed investigation. The results showed a spectrum of monoenergetical electrons which was superimposed on a background similar to known measurements4). Since our electronic equipment was not designed for extreme resolution, the line shape of the peaks and close lying lines could not be resolved clearly. Thus our spectra resemble more to histograms. Model calculations, which simulated exactly the measuring proce-

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484

w . K R U G E R et al.

dure, showed that similar results can be obtained if a spectrometer with limited resolution like ours is applied to a dense spectrum with small linewidths. The main aim of this paper is therefore to demonstrate the correspondence of our preliminary experiments to already published spectra of autoionizing states of the incident ion.

and computed the ion current lio n and the secondary electron current/~-. A delay assured that the counting started only at the moment when the bias has reached the predetermined level. Since the stability of the ion beam current was very poor with respect to the required accuracy, the single measurements had to be repeated frequently in order to average out the deviations. For this reason the measuring cycle for each point lasted

2. Apparatus Fig. 1 shows the experimental setup of the target chamber and the principle of the measuring method. The polycristalline targets were mounted in a differentially pumped UHV-chamber and electrically well isolated by a BeO disk, which provided a good thermal conductivity. The residual gas pressure in the rear part was 1 x 10-9 torr while in the front part it was one order of magnitude less due to the ordinary high-vacuum pressure in the beam line system of the accelerator. During operation the pressure increased insignificantly in the target chamber. The ion beam was collimated by an 1 mm JZ~ aperture in the front chamber, so that no stray electrons from the edges of the aperture could influence the measurements. The current measured over the target is composed of the ion beam current lio n and the current of the secondary electrons I ewhich are released by the bombarding particles. If a high positive voltage is applied on the target, all the secondary electrons are suppressed apart from a negligible fraction with a kinetic energy high enough to overcome the potential barrier. The ratio of the currents with the bias voltage switched on and off yields the number of emitted electrons per ion:

A

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[(Iio. + Ie-)-Iion]/Iio.. If further a lower voltage level is introduced by a second adjustable dc-voltage source, the fraction of electrons falling into a certain energy interval can be determined, that means, we obtain an energy distribution of the electrons in integral formSN(E)dEif the low level voltage is raised stepwise. The energy distribution N(E) is then calculated by differentiation. Fig. 2 shows a block diagram of the electronic equipment. The dc-source and the amplifier were adjusted and stabilized in the low or high voltage level by a square wave reference voltage of low switching frequency ( ~ 1 Hz) and by a feedback control system. The current of the ions and electrons were converted precisely into a voltage signal and then digitized in a voltage proportional pulse train. These signals at low and high bias voltage were fed into an SSR Digital Synchronous Computer, which counted both pulse rates

Fig. 3. Model calculations indicating the deformation of the dummy-spectrum A by a spectrometer with poor resolution (slope B-E) and drift (slope F-O).

ENERGY

SPECTRA

OF S E C O N D A R Y

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ELECTRONS

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Fig. 4. Spectrum of monoenergetic electrons released during the b o m b a r d m e n t of Cu with Ar + ions. The inserts are adopted from Ogurtsov et al. 8) and R u d d et alP) and compare the occurrence and the energetic positions of the peaks.

about 15 min so that only a small part of the spectrum could be scanned per day (cf. fig. 6.) 3. Results

and discussion

The maximum frequency of 10 kHz of the voltageto-frequency converter limited the resolution of the spectra to steps of 0.1 V. If smaller steps were used, the pulse rate became too low and the error of the measurements increased considerably. For this reason the measured spectra are rather coarse and no details of the lines are to be seen. However an approximate energetical localisation of the released electrons in the range of +0.I V could be stated. All spectra were controlled repeatedly concerning their reproducibility and gave, with reservations, identical results.

For testing the quality and reliability of the measurements a model calculation was carried out. An example is given in fig. 3. The function

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with A i, nl, x °, B and m as adjustable parameters serves as a dummy-measurement of the integral energy distribution. The first derivative of this function then represents the requested slope of an continuous background with superimposed peaks. The differentiation was made by simply computing the expression [F(x)-F(x+h)]/h. An extensive numerical algorithm for the differentiation could not be applied since the values of the function F(x) and respectively the values of the measurements, varied too much. The first slope VIII,

ELECTRON

AND X-RAY

EMISSION

w. KROGER et al.

486

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Fig. 5. Same spectrum as fig. 4 compared with results of Gerber et al.7). denoted with A shows the original function F'(x) evaluated with an increment h much smaller than the half width of the peaks. Curves B to E then indicate the deformations which F'(x) suffers with increasing increment. Regarding the real measurements and linewidths of autoionizing states, which are reported in literature, slope D will probably match best the present results. Similarly, a drift of the lower voltage level between two separate measurements covering the same energy interval can be simulated by a distinct shift of the increment. The slopes F - O show the change in the feature of the model spectrum, e.g. the smearing out of line groups and the disappearing of structures. On account of this behaviour the reproducibility of our measurements seemed us to be satisfactorily.

Figs. 4-7 show the results of our measurements in comparison with spectra of autoionizing states determined in gas collision experimentsT'8). Since the contact potential in our apparatus is unknown and the stray magnetic field of the ion-sputter-pumps introduces a certain shift in the energy scale, we fitted the results to an optimal agreement with the already published measurements. The identification of the lines should be adopted from the references. Only Ar + was used as a projectile, the target materials were Cu and Ag. Essentially we could observe lines emitted from states of the gas atoms, whereas no monoenergetic electrons could be attributed to decay processes which are specific for the target species. Comparing both spectra of the Cu and Ag measurements some of the peaks are shifted for a small amount

E N E R G Y S P E C T R A OF S E C O N D A R Y

in their energetic position, however this shift is in the limits of the expected error of _ 0.1 V. Special attention is payed to two characteristic lines denoted by M and R in the drawings. Gerber et al. 7) have identified M as a "molecular ionization process" of the form Ar + A r ~ A r * + A r * ~ A r + + e - +Ar. The simultaneous excitation of both colliding partners can be explained in terms of the promotion model of Fano and Lichten'°). R is classified as the decay of an excited negative Ar*--ion formed by charge exchange during the collision. Consequently this peak should not be observed in H + ~ A r collision experiments as can be verified from the insert of fig. 4. The scale with the marks A-J, which is drawn in just above the abszissa, denote the position of lines that had been identified by Rudd et al. 9) in Ar+ ~ A r collisions at 100 keV. The reasonable agreement in the energetic position of the lines shows that at most a Doppler broadening and no Doppler shift occurs. This fact suggests that the emission of the electrons is due to rather slowly moving Ar-atoms in an energy range of about 1500 eV. Fig. 5 shows that our results are also in accordance with measurements of Gerber et al. 7) regarding the line positions. We can accomplish the same comparison as the authors did, when we shift the spectrum for an amount of 13.45 eV. The high intensity of the low energy lines prove that the decay of the autoionizing levels into the first excited state Ar+(3s3p 6) is apparently favoured. In fig. 6 the spectrum of A r + ~ A g is divided into three parts according to three measuring cycles, which succeeded from one day to the other. The line intensities are not drawn to scale, however, it can be seen 300 keV

487

ELECTRONS

that the line M in the second part of the spectrum is much lower in its intensity, compared to the neighbouring lines. This suggests a certain dose dependence of the line, since the target was not yet saturated completely with Ar atoms in the beginning of the second run, whereas in the first measuring cycle, after a bombardment of more than 10 h, the line is quite prominent. Similar relations-were described recently by Soszka et al. 1~) and support the explanation that the electrons are emitted from outgoing Ar atoms, which move highly excited in the bulk material and decay just in front of the target or in a depth from where the electrons can be released without being scattered. Fig. 7 compares two spectra obtained at different ion energy. The intensities of several lines vary in their magnitude with respect to each other and suggest a dependence of the excitation cross section on the projectile energy. Especially the "molecular ionization peak" has decreased considerably, since in both cases the saturation of the target with argon atoms should be comparable. However considerations of the intensity of lines should only be made under precautions concerning the model calculations and the poor resolution of the instrument. 4. Conclusions

Though the experimental setup showed that the electronic equipment was improper for a detailed study of the fine structure, these preliminary results indicate the applicability of the reported experimental method. As an advantage of our experiments we can note that

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eV

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ELECTRON ENERGY

Fig. 7. Comparison of two spectra at different projectile energies. VIII. ELECTRON AND X-RAY EMISSION

488

w . K R O G E R et al.

1) All electrons released are recorded without regard to the emission angle. This yields maximum information concerning the multiplicity of involved atomic states. 2) If the idea of the formation of the excited states is correct the high density per unit volume of the gas atoms sticking in the metal and being adsorbed on a surface layer should provide lines with considerable intensity, and therefore a higher sensitivity, than gas collision experiments.

We thank Prof. Dr K. H. Schartner and Dr R. Hippler for valuable discussions and Mr G. Trylat for his assistance during the measurements.

References 1) K. H. Krebs, Fortschr. Pbysik 16 (1968) 419. 2) M. Kaminsky, Atomic and ionic impact phenomena on metal surfaces (Springer-Verlag, Berlin, Heidelberg, New York, 1965). a) U . A . Arifov, Interaction of atomic particles with a solid surface (Consultants Bureau, New York, 1969). 4) G. Wehner, Z. Physik 193 (1966) 439. 5) N. Colombie, C. Benazeth, J. Mischler and L. Viel, Rad. Effects 18 (1973) 251. 6) W. KrUger and A. Scharmann, Z. Physik 260 (1973) 417. 7) G. Gerber, R. Morgenstern and A. Niehaus, J. Phys. B 5 (1972) 1396; J. Phys. B 6 (1973) 493. s) G . N . Ogurtsov, I. P. Flaks and S. V. Avakyan, Sov. Phys. JETP 30 (1970) 16. 9) M.E. Rudd, T. Jorgensen and D.J. Volz, Phys. Rev. 151 (1966) 28. 10) U. Fano and W. Lichten, Phys. Rev. Lett. 14 (1967) 627. 11) W. Soszka and J. Lipiec, Surface Sci. 45 (1974) 371.