- Email: [email protected]
Electron emission from temporary negative N−∗(1D) ions formed in slow collisions of N+ with insulator surfaces (LiF; CsI)
Nuclear Instruments
2&J __ _-
and Methods in Physics Research B 125 (1997) 63-66
Beam Interactions with Materials 8 Atoms
!!i!!! EISFVIER
Electron emission from temporary negative N- * ( *D) ions formed in slow collisions of N+ with insulator surfaces (LiF; CsI) P. Stracke, F. Wiegershaus,
St. Krischok, H. Miiller, V. Kempter
Physikalisches lnsfitut der Technischen Uniuersitiit Clausrhol, Leibnizsrr.4, 38678 Cluusthal-Zellerfeld. Germany
Abstract The emission of electrons in slow collisions (50 to 500 eV> of N+ ions with thin insulating films (LiF and CsI on tungsten) is reported. The electron spectra display a sharp feature which is due to the autodetachment of N- * (2~~; ‘D) to the N0(2p 3; 4S> ground state (on top of a smooth continuous background). In order to understand the dynamics of the formation of the temporary negative ion, the local character of the nitrogen interaction with the surface must be taken into account. The first step in the projectile-surface interaction is the resonant neutralization of the N+ projectile into states of the No 2p3 configuration, in particular into N * (*D). The second step is the attachment of an additional surface electron to the nitrogen atom; this capture is mediated by the strong interaction between the temporary negative N- * (‘D) ion and the hole in the surface anion (generated as the consequence of the electron transfer to the projectile).
1. Introduction We have recently presented evidence for the formation of N- *(ID) in collisions of N+ and several nitrogen-based molecules, NT, NO+ and N20+, with tungsten surfaces partially covered by alkali atoms [ 1,2]. The ion-impact electron spectra could be interpreted consistently by assuming that electron emission is observed from the autodetachment of N- * (2~~; ‘D) to the final state N0(2p3; 4S). It was proposed that the metastable negative ion species is formed by the resonant transfer of a surface electron to the states of the N 2p3 configuration, in particular to N * (2~~; *D). Generally, for collisions with metals the formation of negative ions is viewed as being due to the image potential shift of the affinity level; this brings it into resonance with occupied states at the surface and thus triggers the formation of a negative projectile ion by the resonant transfer of an electron from the surface [3]. Recently, the observation of large fractions of negative ions was reported for collisions of hydrogen [4], oxygen [5] and halogen [6] projectiles with insulator (LiF, LiCl, KI) surfaces. It was proposed [5,7] that in such cases the level shift follows essentially the Coulomb potential; it describes the interaction of the negative ion created by the electron transfer with the positively charged hole state created in that particular surface species which was involved in the very same electron transfer process. Promotion of the outermost level of the projectile may further facilitate the formation of negative ions [4]. In this paper we report an experiment that focuses on the detection of electrons emitted from the autodetachment of temporary negative ions, namely N- (ID), formed in
collisions of nitrogen projectiles with alkali halide surfaces rather than on the detection of the negative ions themselves. While we find no detectable formation of N- * in collisions with a clean metal [ 1,2], its formation sets in as soon as the surface becomes covered by an insulating alkali halide film [4-61.
l
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO1 68-583X(97)00926-3
2. Experimental
The apparatus was documented previously [8,9]. Briefly, ions (N+ and Ar+) are produced in a duo-plasmatron source. The mass analyzed ion beam impinges upon a W( 110) which was exposed to LiF molecules. The LiF films were prepared as follows (for details see Ref. [lo], and references cited therein): LF molecules were supplied to the clean substrate by thermal evaporation (1100 K) of LiF single crystals chips. During the exposure of the substrate to LiF the electronic properties of the surface changed from metallic to insulating as discussed in detail in [IO]. As judged on the basis of XI’S, MIES and UPS (He(I)) measurements, the electronic structure of bulk LiF was fully developed as soon as the thickness of the produced adlayer became of the order of lnm. In particular, (see also [lo]) no occupied surface states (such as colour centers) were detected with MIES in the bandgap of the insulating films. The surface was held at room temperature during the measurements. The incidence angle of the ion beam is 5” with respect to the surface; the emission angle of the ejected electrons
II. SECONDARY EMISSION - EXPERIMENT
P. Strucke et al./Nucl.
64
Instr. und Meth. in Phys. Res. B 125 (1997) 63-66
is 90” with respect to the beam axis. The spectra of the slow electrons were taken in the following way: the difference in the work functions of the crystal and the analyzer was overcompensated by biasing the electrostatic analyzer (energy resolution 0.2 eV FWHM) in such a way that electrons leaving the clean W(ll0) crystal with zero energy arrive at the electrostatic analyzer with 5.25 eV. In this way the low energy cutoff of the spectra gives directly the variation of the surface workfunction with exposure of the alkali halide molecules. This biasing procedure affects the collection efficiency below about 10 eV electron energy, and the spectra do not represent truly angle-resolved electron spectra. It was checked that this procedure did not produce artefacts in the low energy part of the spectra.
3. Results
1
5
/
/Zi?=iq Figs. 1 and 2 display the results for N+ (50 eV> collisions with W( 1 IO) as a function of the exposure to LF and CsI molecules, respectively. For the clean W(1 10) substrate the electron emission is exclusively due to the Auger capture process. Fig. 3 displays the corresponding results for Arf (50 eV) collisions. Again, for the clean W(1 IO) substrate the electron emission is exclusively due to the Auger capture process. Common to both cases (argon and nitrogen collisions) is the emergence of a structureless “background” which is peaked at the low-energy cutoff of the spectra, i.e. at zero kinetic energies of the electrons emitted from the surface. It will be demonstrated in Ref. [I I] that this emission is a particular type of kinetic emission. This part
10
energy
15
/
eV
Fig. 2. Energy spectra of the electrons emitted in 50 eV collisions of N+ ions with a W(1 IO) surface exposed to CsI molecules.
of the spectra will not be considered in detail here. Only for Nt collisions in addition a sharp feature, labeled AU(N- *(‘D)) (FWHM 0.2 eV, corresponding to the apparatus resolution), about 1.4 eV above the low-energy onset in the spectra, is observed which will be discussed in Section 4. For nitrogen collisions with metallic substrates (WC1 10) and Pd(l1 I) partially covered by alkali atoms)
2.4 Lo
o 4 *
1.4.. 1.2..
2.0
0.4
1
5
10 energy
Fig. 1. Energy spectra of the electrons emitted in 50 eV collisions of N+ ions with a W( I IO) surface exposed to LIF molecules.
15 /
eV
Fig. 3. Energy spectra of the electrons emitted in 50 eV collisions of Ar+ ions with a W(l10) surface exposed to LiF molecules.
P. Stmcke et al./ Nucl. Instr. and Meth. in Phys. Rex B 125 (1997) 63-66
this feature was identified as due to the intra-atomic Auger decay of N- ‘(ID) to the nitrogen Nc4S> final state; the excitation energy of ‘D (I .4 eV) is given by the energy distance from the onset of the spectra [1,2]. We like to stress the following point of importance for the considerations in Section 4: N- (‘D) is not formed in collisions with the clean W-surface as documented by the absence of the sharp feature in the bottom spectra of Figs. 1 and 2. l
4. Discussion At 50 eV collision energy and grazing incidence promotion effects such as discussed in Ref. [4] will be neglected. Fig. 4 shows the one-electron energies of the LF surface (characterized by its wide bandgap of 14.1 eV [ 121) and of the nitrogen projectile. When approaching the surface closer than about 10 a.u., the N+ projectile ion is efficiently neutralized possibly in all three multiplet states 4S, 2D, and *P of the N0(2s22p3) ground state configuration. Fig. 4 shows also the energy of the affinity level, i.e. of the binding energy of an additional 2p electron at the projectile for the case that N- *(ID) is formed by electron attachment to N ‘(‘D). Due to the interaction of the negative ion with the hole created in the surface anion during the process of the negative ion formation, the level energy varies approximately as l/R; here R is the distance between the projectile and the particular anion that was involved in the electron transfer process (see Ref. [7] for more details). A sizeable probability for negative ion formation exists as soon as the affinity level drops into near-resonance with states of the valence band [5,7] (below
N+(Zp’; 3P)
65
about 3 au.). During the separation of the negative ion from the surface the large band gap width prevents the attached electron from tunneling back into the surface. Instead, the N- * (‘D) ion undergoes autodetachment to the N”(4S) ground state (lifetime against autodetachment about IO-” s [15]) after having departed from the surface. This model predicts correctly several peculiarities of the results: (1) The signal AU(N- * ) becomes considerably larger when the LiF is replaced by a CsI film on tungsten (band gap width about 7 eV [13]). Consequently, the affinity level comes closer to resonance with the valence band states, and the probability for the transfer of the additional 2p electron to the projectile increases correspondingly [5]. (2) No formation of N- * is observed on the clean W(110) substrate. Here, the variation of the affinity level with decreasing projectile-surface distance L is given by the image potential 1/4z (in a.u.) [3]. The resulting level shift is too small to bring it into resonance with filled metal states at large work functions. (3) Very little N- * is formed in the initial phase of exposure before a closed insulator film develops, i.e., as long as the work function decreases with exposure [lo]. The strong l/R interaction of the negative ion with the surface requires that the hole created in the electron transfer to the projectile stays localized at one particular surface anion, i.e. its lifetime is longer than the collision time [7]. During the initial phase of the film formation the lifetime of the hole in one of the surface-adsorbed alkali halide molecules can be expected to be comparatively short because of the strong vertical molecule-surface interaction [14]. Only after a first layer of the insulating film has been completed, the vertical interaction weakens considerably at the expense of the lateral interaction between the halide molecules. The lifetime of the hole becomes then sufficiently long to give rise to the strong l/R-interaction between the negative projectile ion and the surface.
N0(2p3; 2 D) Acknowledgement Financial support of the Deutsche Forschungsgemeinschaft and the European Union through the HCM network ERBCHRXCT 940571 is gratefully acknowledged. V.K. is indebted to Drs. A.G. Borisov, J.P. Gauyacq, H. Winter and V. Puchin for illuminating discussions.
References
-14.a
LiF
-12.1
2D
-14.5
5
N(2s22p3)
Fig. 4. Energy level diagram (schematically) tile in front of an alkali halide surface.
for a nitrogen projec-
[l] H. Miller, R. Hausmann, H. Brenten and V. Kempter, Surf. Sci. 303 (1994) 56. [2] H. Miller, D. Gador, F. Wiegershaus and V. Kempter, J. Phys. B 29 (19%) 715. [3] J. Los and J.J.C. Geerlings, Phys. Rep. 190 (1990) 133. 141 R. Souda, K. Yamamoto. W. Hayami, B. Tilley, T. Aizawa and Y. Ishizawa, Surf. Sci. 324 (1995) L349.
II. SECONDARY
EMISSION
- EXPERIMENT
66
P. Stracke et al./Nucl.
Instr. and Meth. in Phys. Res. B 125 (1997) 63-66
[5] C. Auth, A.G. Borisov and H. Winter, Phys. Rev. Lett. 75 (1995) 2292. [6] H. Winter, A. Mertens, C. Auth and A.C. Borisov, to be published. [7] A.G. Borisov, V. Sidis and H. Winter, to be published [8] H. Schall, W. Huber, H. HBrmann, W. Maus-Friedrichs and V. Kempter, Surf. Sci. 210 (1989) 163. [9] H. Brenten, H. Miiller, K.H. Knorr, D. Kruse. H. Schall and V. Kempter, Surf. Sci. 243 (1991) 309. [IO] F. Wiegershaus, S. Krischok, D. Ochs, W. Maus-Friedrichs and V. Kempter, Surf. Sci. 345 (1996) 91. [ 1l] P. Stracke, F. Wiegershaus, S. Krischok. V. Kempter, P.A.
[12] [13]
1141 (151
Zeijlmans van Emmichoven, A. Niehaus, F.J. Garcia de Abajo, these Proceedings (JJSC- 111,Nucl. Instr. and Meth. B 125 (1997) 67. D.A. Lapiano-Smith, E.A. Eklund, F.J. Himpsel and L.J. Terminello, Appl. Phys. Lett. 59 (1991) 2174. A. Hit&e, S. Ptilm, H. Miiller, R. Hausmamr, J. Giinster, S. Dieckhoff, W. Maus-Friedrichs and V. Kempter, Surf. Sci. 291 (1993) 67. R. Souda, Int. J. Mod. Phys. B 8 (1994) 679. R.D. Cowan, Charlotte Froese Fischer, J.E. Hansen and V. Kempter, J. Phys. B (1996), in press.
2&J __ _-
and Methods in Physics Research B 125 (1997) 63-66
Beam Interactions with Materials 8 Atoms
!!i!!! EISFVIER
Electron emission from temporary negative N- * ( *D) ions formed in slow collisions of N+ with insulator surfaces (LiF; CsI) P. Stracke, F. Wiegershaus,
St. Krischok, H. Miiller, V. Kempter
Physikalisches lnsfitut der Technischen Uniuersitiit Clausrhol, Leibnizsrr.4, 38678 Cluusthal-Zellerfeld. Germany
Abstract The emission of electrons in slow collisions (50 to 500 eV> of N+ ions with thin insulating films (LiF and CsI on tungsten) is reported. The electron spectra display a sharp feature which is due to the autodetachment of N- * (2~~; ‘D) to the N0(2p 3; 4S> ground state (on top of a smooth continuous background). In order to understand the dynamics of the formation of the temporary negative ion, the local character of the nitrogen interaction with the surface must be taken into account. The first step in the projectile-surface interaction is the resonant neutralization of the N+ projectile into states of the No 2p3 configuration, in particular into N * (*D). The second step is the attachment of an additional surface electron to the nitrogen atom; this capture is mediated by the strong interaction between the temporary negative N- * (‘D) ion and the hole in the surface anion (generated as the consequence of the electron transfer to the projectile).
1. Introduction We have recently presented evidence for the formation of N- *(ID) in collisions of N+ and several nitrogen-based molecules, NT, NO+ and N20+, with tungsten surfaces partially covered by alkali atoms [ 1,2]. The ion-impact electron spectra could be interpreted consistently by assuming that electron emission is observed from the autodetachment of N- * (2~~; ‘D) to the final state N0(2p3; 4S). It was proposed that the metastable negative ion species is formed by the resonant transfer of a surface electron to the states of the N 2p3 configuration, in particular to N * (2~~; *D). Generally, for collisions with metals the formation of negative ions is viewed as being due to the image potential shift of the affinity level; this brings it into resonance with occupied states at the surface and thus triggers the formation of a negative projectile ion by the resonant transfer of an electron from the surface [3]. Recently, the observation of large fractions of negative ions was reported for collisions of hydrogen [4], oxygen [5] and halogen [6] projectiles with insulator (LiF, LiCl, KI) surfaces. It was proposed [5,7] that in such cases the level shift follows essentially the Coulomb potential; it describes the interaction of the negative ion created by the electron transfer with the positively charged hole state created in that particular surface species which was involved in the very same electron transfer process. Promotion of the outermost level of the projectile may further facilitate the formation of negative ions [4]. In this paper we report an experiment that focuses on the detection of electrons emitted from the autodetachment of temporary negative ions, namely N- (ID), formed in
collisions of nitrogen projectiles with alkali halide surfaces rather than on the detection of the negative ions themselves. While we find no detectable formation of N- * in collisions with a clean metal [ 1,2], its formation sets in as soon as the surface becomes covered by an insulating alkali halide film [4-61.
l
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO1 68-583X(97)00926-3
2. Experimental
The apparatus was documented previously [8,9]. Briefly, ions (N+ and Ar+) are produced in a duo-plasmatron source. The mass analyzed ion beam impinges upon a W( 110) which was exposed to LiF molecules. The LiF films were prepared as follows (for details see Ref. [lo], and references cited therein): LF molecules were supplied to the clean substrate by thermal evaporation (1100 K) of LiF single crystals chips. During the exposure of the substrate to LiF the electronic properties of the surface changed from metallic to insulating as discussed in detail in [IO]. As judged on the basis of XI’S, MIES and UPS (He(I)) measurements, the electronic structure of bulk LiF was fully developed as soon as the thickness of the produced adlayer became of the order of lnm. In particular, (see also [lo]) no occupied surface states (such as colour centers) were detected with MIES in the bandgap of the insulating films. The surface was held at room temperature during the measurements. The incidence angle of the ion beam is 5” with respect to the surface; the emission angle of the ejected electrons
II. SECONDARY EMISSION - EXPERIMENT
P. Strucke et al./Nucl.
64
Instr. und Meth. in Phys. Res. B 125 (1997) 63-66
is 90” with respect to the beam axis. The spectra of the slow electrons were taken in the following way: the difference in the work functions of the crystal and the analyzer was overcompensated by biasing the electrostatic analyzer (energy resolution 0.2 eV FWHM) in such a way that electrons leaving the clean W(ll0) crystal with zero energy arrive at the electrostatic analyzer with 5.25 eV. In this way the low energy cutoff of the spectra gives directly the variation of the surface workfunction with exposure of the alkali halide molecules. This biasing procedure affects the collection efficiency below about 10 eV electron energy, and the spectra do not represent truly angle-resolved electron spectra. It was checked that this procedure did not produce artefacts in the low energy part of the spectra.
3. Results
1
5
/
/Zi?=iq Figs. 1 and 2 display the results for N+ (50 eV> collisions with W( 1 IO) as a function of the exposure to LF and CsI molecules, respectively. For the clean W(1 10) substrate the electron emission is exclusively due to the Auger capture process. Fig. 3 displays the corresponding results for Arf (50 eV) collisions. Again, for the clean W(1 IO) substrate the electron emission is exclusively due to the Auger capture process. Common to both cases (argon and nitrogen collisions) is the emergence of a structureless “background” which is peaked at the low-energy cutoff of the spectra, i.e. at zero kinetic energies of the electrons emitted from the surface. It will be demonstrated in Ref. [I I] that this emission is a particular type of kinetic emission. This part
10
energy
15
/
eV
Fig. 2. Energy spectra of the electrons emitted in 50 eV collisions of N+ ions with a W(1 IO) surface exposed to CsI molecules.
of the spectra will not be considered in detail here. Only for Nt collisions in addition a sharp feature, labeled AU(N- *(‘D)) (FWHM 0.2 eV, corresponding to the apparatus resolution), about 1.4 eV above the low-energy onset in the spectra, is observed which will be discussed in Section 4. For nitrogen collisions with metallic substrates (WC1 10) and Pd(l1 I) partially covered by alkali atoms)
2.4 Lo
o 4 *
1.4.. 1.2..
2.0
0.4
1
5
10 energy
Fig. 1. Energy spectra of the electrons emitted in 50 eV collisions of N+ ions with a W( I IO) surface exposed to LIF molecules.
15 /
eV
Fig. 3. Energy spectra of the electrons emitted in 50 eV collisions of Ar+ ions with a W(l10) surface exposed to LiF molecules.
P. Stmcke et al./ Nucl. Instr. and Meth. in Phys. Rex B 125 (1997) 63-66
this feature was identified as due to the intra-atomic Auger decay of N- ‘(ID) to the nitrogen Nc4S> final state; the excitation energy of ‘D (I .4 eV) is given by the energy distance from the onset of the spectra [1,2]. We like to stress the following point of importance for the considerations in Section 4: N- (‘D) is not formed in collisions with the clean W-surface as documented by the absence of the sharp feature in the bottom spectra of Figs. 1 and 2. l
4. Discussion At 50 eV collision energy and grazing incidence promotion effects such as discussed in Ref. [4] will be neglected. Fig. 4 shows the one-electron energies of the LF surface (characterized by its wide bandgap of 14.1 eV [ 121) and of the nitrogen projectile. When approaching the surface closer than about 10 a.u., the N+ projectile ion is efficiently neutralized possibly in all three multiplet states 4S, 2D, and *P of the N0(2s22p3) ground state configuration. Fig. 4 shows also the energy of the affinity level, i.e. of the binding energy of an additional 2p electron at the projectile for the case that N- *(ID) is formed by electron attachment to N ‘(‘D). Due to the interaction of the negative ion with the hole created in the surface anion during the process of the negative ion formation, the level energy varies approximately as l/R; here R is the distance between the projectile and the particular anion that was involved in the electron transfer process (see Ref. [7] for more details). A sizeable probability for negative ion formation exists as soon as the affinity level drops into near-resonance with states of the valence band [5,7] (below
N+(Zp’; 3P)
65
about 3 au.). During the separation of the negative ion from the surface the large band gap width prevents the attached electron from tunneling back into the surface. Instead, the N- * (‘D) ion undergoes autodetachment to the N”(4S) ground state (lifetime against autodetachment about IO-” s [15]) after having departed from the surface. This model predicts correctly several peculiarities of the results: (1) The signal AU(N- * ) becomes considerably larger when the LiF is replaced by a CsI film on tungsten (band gap width about 7 eV [13]). Consequently, the affinity level comes closer to resonance with the valence band states, and the probability for the transfer of the additional 2p electron to the projectile increases correspondingly [5]. (2) No formation of N- * is observed on the clean W(110) substrate. Here, the variation of the affinity level with decreasing projectile-surface distance L is given by the image potential 1/4z (in a.u.) [3]. The resulting level shift is too small to bring it into resonance with filled metal states at large work functions. (3) Very little N- * is formed in the initial phase of exposure before a closed insulator film develops, i.e., as long as the work function decreases with exposure [lo]. The strong l/R interaction of the negative ion with the surface requires that the hole created in the electron transfer to the projectile stays localized at one particular surface anion, i.e. its lifetime is longer than the collision time [7]. During the initial phase of the film formation the lifetime of the hole in one of the surface-adsorbed alkali halide molecules can be expected to be comparatively short because of the strong vertical molecule-surface interaction [14]. Only after a first layer of the insulating film has been completed, the vertical interaction weakens considerably at the expense of the lateral interaction between the halide molecules. The lifetime of the hole becomes then sufficiently long to give rise to the strong l/R-interaction between the negative projectile ion and the surface.
N0(2p3; 2 D) Acknowledgement Financial support of the Deutsche Forschungsgemeinschaft and the European Union through the HCM network ERBCHRXCT 940571 is gratefully acknowledged. V.K. is indebted to Drs. A.G. Borisov, J.P. Gauyacq, H. Winter and V. Puchin for illuminating discussions.
References
-14.a
LiF
-12.1
2D
-14.5
5
N(2s22p3)
Fig. 4. Energy level diagram (schematically) tile in front of an alkali halide surface.
for a nitrogen projec-
[l] H. Miller, R. Hausmann, H. Brenten and V. Kempter, Surf. Sci. 303 (1994) 56. [2] H. Miller, D. Gador, F. Wiegershaus and V. Kempter, J. Phys. B 29 (19%) 715. [3] J. Los and J.J.C. Geerlings, Phys. Rep. 190 (1990) 133. 141 R. Souda, K. Yamamoto. W. Hayami, B. Tilley, T. Aizawa and Y. Ishizawa, Surf. Sci. 324 (1995) L349.
II. SECONDARY
EMISSION
- EXPERIMENT
66
P. Stracke et al./Nucl.
Instr. and Meth. in Phys. Res. B 125 (1997) 63-66
[5] C. Auth, A.G. Borisov and H. Winter, Phys. Rev. Lett. 75 (1995) 2292. [6] H. Winter, A. Mertens, C. Auth and A.C. Borisov, to be published. [7] A.G. Borisov, V. Sidis and H. Winter, to be published [8] H. Schall, W. Huber, H. HBrmann, W. Maus-Friedrichs and V. Kempter, Surf. Sci. 210 (1989) 163. [9] H. Brenten, H. Miiller, K.H. Knorr, D. Kruse. H. Schall and V. Kempter, Surf. Sci. 243 (1991) 309. [IO] F. Wiegershaus, S. Krischok, D. Ochs, W. Maus-Friedrichs and V. Kempter, Surf. Sci. 345 (1996) 91. [ 1l] P. Stracke, F. Wiegershaus, S. Krischok. V. Kempter, P.A.
[12] [13]
1141 (151
Zeijlmans van Emmichoven, A. Niehaus, F.J. Garcia de Abajo, these Proceedings (JJSC- 111,Nucl. Instr. and Meth. B 125 (1997) 67. D.A. Lapiano-Smith, E.A. Eklund, F.J. Himpsel and L.J. Terminello, Appl. Phys. Lett. 59 (1991) 2174. A. Hit&e, S. Ptilm, H. Miiller, R. Hausmamr, J. Giinster, S. Dieckhoff, W. Maus-Friedrichs and V. Kempter, Surf. Sci. 291 (1993) 67. R. Souda, Int. J. Mod. Phys. B 8 (1994) 679. R.D. Cowan, Charlotte Froese Fischer, J.E. Hansen and V. Kempter, J. Phys. B (1996), in press.