Neutralization of He+ and He2+ on Pb surfaces

Surface Science 257 (1991) 289-296 North-Holland

289

Neutralization of He+ and He2+ on Pb surfaces S. Schippers a, S. Oelschig b, W. Heiland b, L. Folkerts I.F. Urazgil’din d and A. Niehaus d ’ University of Osnabriick, D-4500 Osnabriick, Germany

‘, R. Morgenstern

‘, P. Eeken d,

and HMI Berlin, D-1000 Berlin, Germany b University of Osnabriick, D-4500 Osnabriick, Germany ’ Kernfysisch Versneller Instituut, University of Groningen, NL-9747 AA Groningen, Netherlands d Buys Ballot Luboratorium, University of Vtrecht, NL-3584 CC Utrecht, NetherlanA Received 28 November

1990; accepted for publication

6 May 1991

The neutralization of slow He+ and He’+ ions on Pb surfaces allows the study of Auger capture, resonant capture into autoionizing states and of a Landau-Zener-type two-state charge exchange process. In tlte primary energy range of 300 eV to 6 keV the processes are experimentally accessible by measuring the reflected ion energy spectra and the secondary electron energy spectra. The results lead to estimates of the transition rates (1.7-3.0 x 10” s-’ for Auger capture and 4.2 X lOI s-’ for resonant capture into autoionizing states) of the charge exchange processes and of the distances (3.55-4.0 A for the Landau-Zener process) where these processes in front of the metal surface occur.

1. Introduction The charge exchange between fast ions and solids contributes to the energy loss process, to secondary emission phenomena and radiation damage. At low energies, where surface phenomena are most important for the charge exchange the pioneering work of Hagstrum [1,2] has laid the base for the understanding of the physical phenomena. It was established that at low energies the “potential emission” of secondary electrons is caused mainly by an Auger-type charge exchange between the solid and the incoming ion. If the ionization energy of the ion is more than twice the work function Ei 2 2$, two electrons in the solid are perturbed such that one electron is captured by the ion and the second electron is emitted. The kinetic energy of the emitted electrons carries information about the band structure of the solid. The technique, i.e., the measurement and evaluation of these energy spectra has been named ion neutralization spectroscopy (INS). 0039-6028/91/$03.50

0 1991 - Elsevier science

In case of He*+ as an incoming ion also secondary electrons characteristic of the atomic system have been observed [3], i.e., electrons with a kinetic energy of 34 and 36 eV which is the signature of the doubly excited states of He, which decay in an autoionization process (AU) He * * --i He+ + e-. More recently, angular resolved electron spectroscopy of this process, i.e. the formation of the autoionizing states by two-electron capture, He*+ + 2e- + He * * for energies around 1 keV at Ni and Cu surfaces [4,5] allowed estimates of the time needed to capture the two electrons. Both electrons, as is concluded from Doppler-shift measurements of the electron kinetic energies, are captured on the incoming path of the ions within about lo-l5 s. The autoionization takes a time of the same order of magnitude, such that at grazing incidence the He particle is singly charged before the turning point of the trajectory. The capture process of the two electrons is probably a resonant transition (RC) (or resonant neutralization, RN, in Hagstrum’s nomenclature [2]).

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S. Schippers et al. / Neutralization of He + and He2 + on Pb surfaces

The two-electron capture process is however in competition with an Auger neutralization process He’+ + M + He++ M2++ e(AC). Here the letter M denotes the metal surface. A recent theoretical treatment of the resonant and the Auger capture process [6] resulted in estimates of the lifetimes corresponding to these processes for He+ interacting with electrons on a Ni metal surface. At low energies (2 keV) and grazing incidence, where the interaction with selectrons dominates, the Auger lifetime is 1.7 X lo-l5 s, corresponding to a mean free path of 5.2 A; the resonant lifetimes are estimated to be of the order of lo-l3 to lo-i4 s. The resonant capture plays a minor role as can be estimated from the cross sections of the two processes [7]. These lifetimes and mean-free-path data define a frame in time and space which affords the possibility of more quantitative studies of charge exchange experiments. It should be noted that the lifetimes quoted here are based on a dynamic theory, whereas in the data analysis used below, an adiabatic approximation is applied, i.e., the RC process is essentially modelled as a tunneling process [l-5]. Besides the Auger-neutralization or Auger capture (AC) and the resonant capture (RC) which occurs also with singly charged ions into excited states or into the affinity level of neutral atoms there is a quasiresonant (QRC) capture process possible which involves core states of surface atoms. It was first observed for He+-+ Pb, its signature is an oscillatory behaviour of the reflected ion yield [8] as a function of the velocity. It is a pure atom-atom interaction as shown by an experiment using a Pb atom beam as target [lo]. The effect is observed for a group of 9 elements in the periodic table [lo] (Ga, Ge, As, In, Sn, Sb, Tl, Pb, Bi). These elements have d-electron core states of 25 5 10 eV below the vacuum level, i.e., not too far from the Hels level at 24.5 eV. The theoretical treatment on the basis of a Landau-Zener-type interaction of the orbitals involved gave convincing agreement with most experimental findings [ll]. This type of “‘oscillatory charge exchange” is well known in atomic physics [12], the special case discussed here is labelled “Stueckelberg-oscillation” [13].

Still another effect in the He+-Pb (or in that case Sn) interaction field is reionization by violent collisions [14]. At small impact parameters electron promotion effects can cause previously neutralized ions to be reionized [15]. The convincing proof for the ionization process is obtained from experiments using neutral He as primary particles. Cross sections for this process are low at energies below 1 keV, i.e., the ion yield is of the order of lo-’ for scattering angles of 90” for He0 -+ Pb

1141. In the present paper we will discuss ion scattering yield measurements and secondary electron energy spectra using He+ and He2+ colliding with single-crystal Pb surfaces. The oscillations of the He+ ion yield are used to estimate the distance in front of the surface where the first neutralization step of He2+ occurs.

2. Experiments The experiments described here were performed in three different experimental setups at Osnabrtick, Groningen and Utrecht, respectively. The Osnabrtick system is a UHV experiment equipped with an ion source operating from 200 eV to 20 keV [16]. The magnetically mass analyzed beam is focussed on the target. The target is mounted on a goniometer with two axes of rotation, to control the impact angle 9 and the azimuthal angle @. An electrostatic energy analyzer can rotate around the target, such that angular resolved (scattering angle 0) energy spectra of the backscattered ions are measured. The energy spectra are also a means to measure the surface cleanliness. The surface structure is controlled by a rear view LEED (low-energy electron diffraction) system. The Osnabrtick system was used to study the azimuthal dependence of the He+-Pb(ll1) and Pb(ll0) Landau-Zener (Stueckelberg) ion yield oscillations [17]. At Groningen the UHV system designed to study the interaction of multiply charged ions with surfaces [18] was used to measure the backscattered ion yield of He+ and He’+ from Pb. The system allows the measurement of angular resolved energy spectra by means of an electro-

S. Shippers

et al. / Neutralization of He + and He2 + on Pb surfaces

static energy analyser. Its acceptance at the center of the target is 11.2 X lo-‘E(sr eV), E is the energy of the detected particles. Target manipulation etc. are equivalent to the Osnabrtick system. The Groningen facility allows however the use of He2+ primary beams with beam currents of 50 nA in the 1 keV region. The Utrecht system is an UHV apparatus with essentially similar features as the previous ones, its strength is however an electrostatic energy analyzer designed and built to measure angular resolved secondary electron spectra [19] especially in the low energy range. Sufficiently strong He2+ beams can be produced to study the capture of two electrons into autoionizing states [4,5] besides the low-energy secondary electrons due to potential emission. Different Pb single crystals were used in the experiments without finding any significant difference in the results.

3. Results The Pb single-crystal surfaces were prepared following the results of Frenken et al. [20]. After etching with a mixture (3 : 1) of 98% CH,COOH and 30% H,O, the crystals were mounted on the manipulator. Sputter and annealing cycles in UHV lead to satisfactory LEED patterns and “clean” ion scattering spectra, i.e., surface impurities like 0 and S are below l/100 of a monolayer [21]. Significant dependencies on the crystallographic orientation are found (fig. 1). In comparison with previous experiments on polycrystalline samples

291

I

I

I

1

0.5

1 .o

1.5

primory

I

ion kinetic

I

I

I

2.0 energy

[keV]

Fig. 2. He + ion yield from He+ + Pb(ll1) at random direction. The yield is not normalized on spectrometer constants.

[g-10] and results in a “random” direction, i.e., @ = 30” with respect to the [121] direction, obtained in the Groningen setup (fig. 2), the positions of the maxima and minima on the energy scale are about the same. The “oriented” measurements differ in detail, i.e., the shape of the curves varies due to “shoulders” and small additional minima and maxima, and in magnitude. Changing the primary beam to He2+ results in an ion yield curve (fig. 3) which shows no significant oscillations. However, when changing the impact and scattering angle from \k = 45 o and 0 =

= He++ He+

0.5

l

G <

0.4

9 .E

0.3

.-S +

0.2

He++

lj,=lOo

1/1=450 $1=450

8=200 e=900 8=900

?

I

OJ

LOO

600

800

1000

Energy (eV

1200

0.1

woo

I

(left scale) and Fig. 1. He + ion yields from He+ -+ Pb(ll0) He+ -+ Pb(ll1) (right scale). The beam was directed along the [OOl] and [121] directions, respectively.

0.4

0.6 primary

0.8 ion kinetic

1.0 energy

1.2

1.4

[keV]

Fig. 3. He+ ion yields from He2+ --) Pb(ll1). For comparison also the corresponding part of fig. 2 is drawn (broken line).

S. Schippers et al. / Neutralization

292

90 o to 4 = 10 o and 8 = 20 O, respectively, weak but significant oscillations are found. The orientation was kept at a “random” direction as for the He+ results of fig. 2. The minima and maxima for He2+ occur again at the same energy values as for He+, indicating that the He*+ oscillations are in fact due to He+ which already has been formed by capturing one electron before entering into the Landau-Zener interaction. The finding of the independence of the oscillations on the impact angle is in agreement with the theory ,[ll] and previous experimental results 1221.The result of fig. 3 is the first indication, that at the larger impact angle the time available for forming the He+ state may be too short. The third set of experimental results are the secondary electron energy spectra measured at Utrecht. Fig. 4 shows a series of such spectra with the impact angle as a parameter. The primary energy is 1 keV. At grazing incidence the autoionization (AU) electrons from the decay of the He * *,

10

20 electron

energy

30 [eV]

Fig. 4. Electron spectra from He2+ + Pb(ll1) (full lines) and He+ -L Pb(ll1) (broken lines) for different angles of incidence. The ordinates of the He+ curves are not to xaie. ‘Ike projectile’s kinetic energy was 1 keV in each case.

ofHe +

and He’ + on Pb surfaces (1111

IllO) Top view w

Surface Atom looll

Top view

Surface Atom

(CIxx!Bm me9_

+z

Fig. 5. Orientation of es and t2s Sd-orbitals on the Pb(ll0) Pb(lll) surfaces.

and

states are found. With increasing impact angle, to the i.e., increasing velocity pe~endicul~ surface, the AU electron peaks vanish and only a typical INS electron distribution remains.

4. Discussion While the Pb 6s and 6p states forming the conduction band are delocalized and therefore do not retain their atomic character, the deeper lying 5d levels are localized, atomic in character, and the ~~espon~g orbitals are oriented in space. Since 5d orbitals of n~~bou~ng surface atoms tend to interfere with each other as little as possible the orientation of the orbitals depends on the surface structure as shown by fig. 5 for the (110) and for the (111) surface. The bold arrows denote t,, and the light arrows es orbitals. The experimental finding that the ion yield is about five times higher when scattering at a Pb(ll1) surface as compared to a Pb(ll0) surface can partly be accounted for by the higher density of surface layer atoms: for a Pb(ll1) surface it is 1.63 times higher than for a Pb(ll0) surface. The secondary electron emission experiments were performed in a “random” direction, as in fig. 2, in order to avoid the additional effects which depend on the orbital orientation. In order to decide which particular transitions take place during the neutralization of the incident He*+ ion we performed model calculations of electron spectra applying the model of Zeijlmans van Emmichoven et al. [4] to the He-Pb(ll1) system. As the density of Pb valence states we use the DOS given by Ley et al. [23]. The development of the populations n, of the different relevant He

S. Shippers

et al. / Neutralization of He + and He’ + on Pb surfaces

states are calculated along a trajectory solving the rate equation dn, = zi (&ink dz/dt

dz

z(t) by

gikni)

.

(I)

The transition rates gki from state k to state i are estimated using Slater’s rules, as explained in ref. [4]. In favour of a better physical description of the AC process the calculation of the corresponding rates has been slightly modified. The mean binding energy of both electrons has been substituted by the initial binding energy of the downgoing electron. The AC rates are multiplied by a factor exp[(E, - E2)/a]’ which accounts for the dependence of the Auger matrix element on the energy separation of the two perturbed metal electrons with binding energies El and E,. The constant u has been taken to be 100 a.u. The spectral contribution from each transition is obtained from the corresponding transition energies ekj

and the whole spectrum as the sum over all contributions ‘(‘)

= c

kfi

‘ki(‘)*

(3)

293

An additional parameter interpolating between an adiabatic and a diabatic version of the transition energies is kept at the value established for the He-Cu and He-Ni systems [4]. Fig. 6 represents a useful scheme comprising all relevant He states taking part in the neutralization process [4]. The states are ordered from bottom to top by the number of 1s holes and from left to right by the number of excited electrons. Several types of transitions are principally possible: Auger capture (AC), autoionization (AU), de-excitation ionization (DI), Auger de-excitation (AD), resonant capture (RC), resonant ionization (RI) and quasi-resonant capture (QRC). Since the electrons emitted via PI and AD are energetically indistinguishable these two processes are comprised in one, named Penning ionization (PI). Within the calculation QRC can be neglected because it is relatively unimportant as can be estimated from fig. 2: the ratio of detected and incident particles is of the order of 10p6, assuming an isotropic distribution of the scattered helium ions and taking the analyzer acceptance into account. For the hydrogen-like He+ *(2s) state we take into account the level splitting caused by the linear Stark effect due to the electric field of the image charge. The He+ *(3s) state however, which at infinite projectile-target separation lies well below the Pb

Fig. 6. Scheme comprising He’+/metal processes with Auger capture (AC), autoionization (AU), Penning ionization (PI, i.e., Auger de-excitation and de-excitation ionization), resonant capture (RC), resonant ionization (RI) and quasi-resonant capture (QRC). The numbers in the brackets are the “coordinates” within the scheme of the specified transition’s initial state, the first one being the number of Is holes, the second is the number of excited electrons.

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S. Shippers

et al. / Neutralization

of He + and He2 + on Pb surfaces

Table 1 Maximum values of the transition rates of the neutralization processes of the He’+ ---tPb(ll1) interaction according to our model calculations Transition

Rate (s-r)

Life-time (s)

AW, 0) W2,O) RC(2,L) RC(2,0> PK2,2) PI(2,l) AU(2, 2) =)

3.ox10’5 1.7 x 10’5 9.6 x 1Or4 4.2 x lOI 4.0 x 1or4 2.8 x 1014 1.7 x lOI

3.3x10-16 5.9x10-‘6 1.0x10-‘5 2.4 x lo- I5 2.5x10-” 3.6X10_‘5 5.9x1o-‘5

a) The value for AU(2,2) is taken from gas-phase experiments [251.

10

20 30 electron energyleVI

Fig. 7. Calculated electron energy spectra for He2+ -+ Pb(lll), Eki, = 1 keV. work function, is reionized already before the linear Stark splitting sets in. The two He* * states for which autoionization is known to occur in the gas phase [24] are taken into account, i.e., He* *(2s2)rS and He**(2p2)‘D. Their autoionizations rates are 1.7 X 1014 and 8.3 X 1013 s-r, respectively [25]. These values serve as a point of references for the remaining rates, which depend on the distance between surface and projectile. Agreement between the measured (fig. 4) and the calculated spectra (fig. 7) has been obtained with the maximum values of the transition rates listed in table 1. The result of the calculation is that the main contribution to the spectra originates from AC(I, 0), AC(2, 0) and AU(2, 2). These transitions give rise to the broad structures at O-15, 15-30 eV and to the double peak structure around 35 eV at * = 2”, respectively. There are also some minor contributions from PI(2, 1) and PI(2, 2) the latter being responsible for the electrons at the high-energy side of the AU structure. The increase of the impact angle YJ implies a higher vertical velocity of the projectile and correspondingly a shorter interaction time. Eventually the chain of the neutralisation events is dominated by the fastest processes, i.e., AC(1, 0) and AC(2, 0). Contributions from slower processes, e.g. AU,

vanish. Our values for the AC and RC life times (table 1) are about one order of magnitude lower than the values estimated from the dynamical theory [6]. However the ratio between the AC and RC rates is the same. The values listed in table 1 are to be considered as upper limits for the distance dependent rates. According to eq. (2) the spectral contributions are also governed by the populations of the initial states, so that transitions may occur at distances where the corresponding transition rates are lower than given in table 1. Furthermore, our findings are in agreement with theoretical results of Arifov et al. [26], who predict that electrons from AC transitions are most probably emitted if the separation of the two states ranges from 15 to 30 eV. The corresponding interval on the energy scale of the emitted electrons is shifted by twice the metal’s work function, i.e., for Pb to 7-22 eV. Disappearance of AU electrons for high incident vertical velocity - corresponding to high angles of incidence in our case - has already been observed by Hagstrum and Becker [3] for the system He’+ on Ni(ll0) and by de Zwart et al. [18,27] for Ar9+ on polycrystalline W. Already the former authors explained their results by a competition between AC and RT into autoionising states in terms of transition probabilities. More recently Folkerts and Morgenstern [28] were able to reproduce the latter result by a drastically simplified model calculation along the lines described above only taking into account one AC, one RC and the AU process.

S. &kippers et ol. / ~euttaliza~~~ of He t and He2 + on Pb surfaces

distance above surface

[A]

Fig. 8. AC(2,O) transition probability densities for different angles of incidence. The two dotted vertical lines mark the region where theory assumes the Landau-Zener curve crossing to be.

The disappearance of the oscillations in the He+ yield for increasing angle of incidence of the He2+ projectile we ascribe to the dependence of the AC transition probabilities on the vertical velocity as exemplified within fig. 8, where the transition probability density for AC(2, 0) has been calcualted using the formula of de Zwart [18]. The position s, of the transition probability density’s maximum approaches the surface for increasing angle of incidence, i.e., s, = 4.8 A for \k=2” and s,=3.55 A for *=45“. Since de Zwart’s calculation is based on the jellium model he measures distances from the cutoff of the electron density. In order to use the topmost surface layer as a reference plane the valyes for s, have been obtained by adding 1.43 A, i.e., half an interlayer spacing for a Pb(l~l) surface, to the values obtained by application of de Zwart’s formula. The values of s, are valid only if we assume that only one process takes place. A qualitative analysis [3] shows that in case of competing processes the distances given above might change by about 0.5 A. However there is now clear evidence that for higher impact angles incidence the AC processes under consideration occur closer to the surface. We consider especially AC(2,O) because the result of our calculation is that AC(2,O)

295

is the dominating process for the formation of He+. The Landau-Zener process requires He+ to be formed before the projectile reaches the crossing of the bonding and anti-bonding HePb+ quasimolecular potential curves. According to calculations by Zartner [29] the crossing point is located 3-5 A in front of the surface, this region being marked by the two dotted vertical bars within fig. 8. In view of these theoretical data our interpretation of our experimental data is as follows: While at low angles of incidence the formation of He+ by AC(2,0), PI(2, 1) and AU(2, 2) takes place before the projectile enters the Landau-Zener interaction region, at high angles of incidence He+ is formed by AC(2,O) only beyond the LandauZener crossing point. Therefore oscillations in the He+ ion yield vanish at increasing angles of incidence of the He2+ projectile. This interpretation implies that the Landau-Zener curve crossing is located between 3.55 and 4.0 A in front of the surface, i.e., between the values of s, for ?lj = 45’ and @= 10”.

5. Conclusion The quasi-resonant Landau-Zener type neutralization in He+ -+ Pb collisions is shown to depend on the surface topography only slightly. In He2++ Pb collisions this interaction is triggered by previous neutralization to He+. Interpretation of secondary electron spectra with the aid of model calculations shows that neutralization is mainly due to Auger capture and resonant capture into He* * states with subsequent autoio~zation. The processes are shown to occur closer to the surface the higher the projectile’s vertical velocity is. Our result concerning the relative importance of these processes agrees to the findings of other authors for He+-’ Ni systems.

References [l] H.D. Hagstrum, Phys. Rev. 96 (1954) 336. [2] H.D. Hagstrnm, in: Inelastic Ion-Surface Collisions, Eds.

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5. S&tippers et al. / Ne~t?~~ation of He + and He2 ’ on Pb surfaces

N.R. Tolk, J.C. Tuliy, W. Heiland and C.W. White (Academic Press, New York, 1977) p. 1. [3] H.D. Hagstrum and GE. Becker, Phys. Rev. B 8 (1973) 107. [4j P.A. Zeijlmans van Emmicboven, P.A.A.F. Wouters and A. Niehaus, Surf. Sci. 195 (1988) 115. [S] P.A.A.F. Wouters, P.A. Zeijlmans van Emmichoven and A. Niehaus, Rad. Eff. Defects Solids 109 (1989) 111. [6] R. Monreal, E.C. Goldberg, F. Flores, A. N&rmann, H. De&s and W. Heiland, Surf. Sci. 211/212 (1989) 271. [7] F. Sols and F. Flares, Phys. Rev. B 30 (1984) 4878. [S] R.L. Erickson and D.P. Smith, Phys. Rev. Lett. 34 11975) 297. [9] A. Zartner, E. Tagfauer and W. Heiland, Phys. Rev. Len. 40 (1978) 1259. [lo] T.W. Rusch and R.L. Erickson, in: Inelastic Ion-Surface Collisions, Eds. N.H. Totk, J.C. Tully, W. Heiland and C.W. White (Academic Press, New York, 1977) p. 73. [ll] J.C Tully, Phys. Rev. B 16 (1977) 4324. [12] W. Lichten, Phys. Rev. 131 (1963) 2292; 139 (1965) A27. [13] N.F. Mott and H.W. Massey, The Theory of Atomic Collisions (1975) pp. 351, 657. [14] R. Souda, T. Aizawa, C. Oshima, M. Aono, S. Tsuneyuki and M. Tsukada, Surf. Sci. 187 (1987) L592. 1151 S. Tsuneyuki, N. Shima and M. Tsukada, Surf. Sci. 186 (1987) 26. [16] J. Moller, K.J. Snowdon, W. Heiiand and H. Niehus, Surf. Sci. 178 (1986) 475. [17] S. Gels&i& Diploma&it, Osnabriick, 1988, unpublished:

W. Heiland, Proc. Nate-ASI, A&ante, Spain, May 1990 (Plenum, New York, 1991). [18] S.T. de Zwart, Thesis, Groningen 1987; S.T. de Zwart, A.G. Drentje and A.L. Boers, Nucl. Instrum. Methods B 9 (1985) 608; S.T. de Zwart, A.G. Drentje, A.L. Boers and R. Morgenstern, Surf. Sci. 217 (1989) 298. 1191 P.A.A.F. Wouters, P.A. Zeijlmans van Emmichoven, J.M. Fluit and A. Niehaus, Meas. Sci. Technol. 1 (1990) 41. [20] J.W.M. Frenken, Dissertation, Amsterdam (1986); J.W.M. Frenken and J.F. van der Veen, Phys. Rev. Lett. 54 (1985) 134. [21] W. Heiland and E. Taglauer, in: Methods of Experimental Physics, Vol. 22, Eds. M.G. Lagally and R.L. Park (Academic Press New York, 1985) p. 299. [22] N.H. Tolk, J.C. Tully, J. Kraus, C.W. White and S.N. Neff, Phys. Rev. Lett. 36 (1976) 747. 1231 L. Ley, R. Pollak, S. Kowalczyk and D. Shirley, Phys. Lett. A 41 (1972) 429. [24] G. Gerber, R. Morgenstem and A. Niehaus, J. Phys. B (At. Mol. Phys.) 6 (1973) 493. [25] P.J. Hicks, S. Cvejanovic, J. Comer, F.H. Read and J.M. Sharp, Vacuum 24 (1974) 573. [26] V.A. Arifov, L.M. Kishinevskii, E.S. Mukhamadiev and E.S. Parihs, Sov. Phys-Tech. Phys. 18 (1973) 118. (271 S.T. de Zwart, Nucl. Instrum. Methods B 23 (1987) 239. [28] L. Folkerts and R. Morgenstem, J. Phys. (Paris) 50 (1989) Cl-541. 1291 A. Zartner, Thesis, TU Milnchen, 1979.