- Email: [email protected]
Electron emission in slow collisions of protons with a LiF-surface
Nuclear Instruments
& -H
and Methods in Physics Research B 125 (1997) 67-70 NONil
B
Beam Interactions with Materials&Atoms
ELSEVIER
Electron emission in slow collisions of protons with a LiF-surface P. Stracke aT* , F. Wiegershaus a, S. Krischok a, V. Kempter a, P.A. Zeijlmans van Emmichoven b, A. Niehaus b, F.J. Garcia de Abajo ’ a Technical Uniuersiry Cluusrhal. Department ofPhysics, Leibnizsrr. 4. D-38678 Clausrhal-Zellerfeld.
Germuny
b Utrecht Uniuersiry, Debye Institute, Princetonplein 5,3584 CC Urrecht, The Nether1and.s ’ Universidad de1 Pais Vusco. Apartudo 649, 20080 Sun Sebastian, Spain
Abstract Electron spectra have been measured for collisions of protons with surfaces of LiF grown on a tungsten substrate. The measurements were carried out for 5” grazing incidence collisions at energies from 50 eV up to 1 keV. The experimental spectra are compared with theoretical spectra obtained from model calculations. The model is based on the assumption that electron emission is dominated by the mechanism of electron promotion via the 3du molecular orbital of the Hlquasi-molecular system. Using theoretically obtained functions for the scattering potential and for the 3da orbital energy, and calculating the spectra in semiclassical approximation for a distribution of impact parameters obtained from Monte Carlo simulations for the relevant scattering geometry, good agreement between experimental and theoretical spectra is obtained. This agreement is taken as strong evidence for the assumed ionization mechanism.
2. Experimental
1. Introduction Kinetic electron emission in slow collisions of ions with surfaces is not yet well understood, and appears to be very dependent on the ion/surface system. For collisions of light ions with metal surfaces, it has recently been shown that the dominant mechanism may be described as a perturbation of the loosely bound localized valence electrons by the incompletely screened charge of the passing projectile [I]. The electron spectra resulting from this “direct” ionization mechanism have the highest intensity at low electron energy and decay exponentially towards higher energies. Further, the absolute yield decreases strongly with collision energy and has an apparent threshold around a projectile velocity of 10’ m/s. In contrast to this behavior, it has recently been found [2] that in collisions of protons with a poly-crystalline LiF surface the electron yield does not decrease strongly in the energy region between 1 keV and 100 eV. In addition, we found that theoretical calculations based on the above described “direct” mechanism cannot explain the observed high yields at low collision energies, indicating that apparently a different mechanism for kinetic emission is responsible. In order to study the ill-understood main ionization mechanism in proton LiF collisions, we carried out measurements of the electron spectra resulting from such collisions in the low energy range.
l
Corresponding author.
016%583X/97/$17.00
Details of the apparatus can be found elsewhere [3,4] (see also Ref. [5] in this volume). Briefly, a mass analyzed H+ beam impinges upon a W(l10) surface covered by a LiF film (10 nm thick); it was produced by thermal evaporation (1100 K) of LiF single crystal chips. XPS, MIES and UPS (He(I)) measurements show that the film possesses already the electronic structure of bulk LiF surfaces. No occupied surface states could be found in the bandgap of the insulator film. The incidence angle of the ion beam is 5” with respect to the surface; the ejected electrons are energy analyzed under 90” with respect to the beam axis. The spectrometer records the electron spectra at constant pass energy with a resolution of AE = 0.2 eV (FWHM). The difference in the work functions of the surface and the analyzer was biased in such a way that electrons leaving the clean W(110) surface with zero energy arrive at the analyzer with 5.25 eV. Thus, the low energy cutoff of the spectra for the LiF spectra gives the work function of the LiF film (3.5 eV). A correction was applied to the spectra in order to compensate the energy dependence of the electrostatic analyzers transmission. However, the energy dependence of the collection effi-
ciency introduced by the biasing procedure is not fully compensated in this way. The weak peak-like structure at the low energy cutoff visible in all spectra is probably an artefact caused by this procedure. Fig. 1 displays the energy spectra of the electrons
0 1997 Elsexier Science B.V. All rights reserved
PII SOl68-583X(96)00800-2
II. SECONDARY EMISSION - EXPERIMENT
P. Stracke et al./Nucl.
0
5
11
15
20
Insrr. and Meth. in Phys. Res. B I25 (1997) 67-70
25
electron energy/ eV Fig. 1. Experimental electron spectra for 5” grazing incidence H+ -LiF collisions at various laboratory collision energies.
in grazing (5”) collisions of H+ with the LiF film for various collision energies. At all studied collision energies the intensity is highest at zero kinetic electron energy and decays roughly exponentially towards higher energies. However some weak structure (which survives the smoothing procedure applied to the raw data) is superimposed and has to be considered meaningful between 4 and 11 (18) eV at 200 (1000) eV collision energy. emitted
3. Analysis From studies of ionization in low energy binary ionatom and atom-atom collisions it is well known that processes of “indirect” caused by ionization, “promotion” of electrons during the collision, can become dominant. One may distinguish two types of such indirect ionization processes: (i) due to electron promotion the potential energy curve describing the system enters the one electron continuum only during the collision, and (ii), by coupling of the promoted potential curve to another excited state that asymptotically correlates with the excitation of one of the collision partners, the system may remain in the continuum also after the collision. In processes of the first type, the ionization can only occur in the transient molecule, leading to a quasi-continuous electron spectrum, while in processes of the second type, depending on the autoionization lifetime of the excited state, the electron spectrum can vary from quasi-continuous to “peak-like”. For many binary collision systems in the gas phase such electron spectra have been observed [6]. In what follows, we analyze the H+-LiF spectra under the assumption that they are due to electron promotion. Since no peak-like structure is observed, we assume further that molecular autoionization of the type (i) described above is responsible. Electron promotion occurs at projectile-target atom distances that are considerably smaller than the distance
between Lif and F- in the surface. We therefore may distinguish between collisions involving these different target atoms. Electron promotion in collisions with the Li+ ion can be excluded because of the strong binding energy of the Li Is-electrons. In order to get a qualitative idea of the possibility of electron promotion in collisions with F-, we look at the diabatic one electron correlation diagram for the H-Fsystem, constructed according to the rules given in Ref. [7], and depicted in Fig. 2. In principle, we have to consider collisions with both, H+ ions and neutralized H projectiles. However, the probability for neutralization in one of several “distant” collisions is probably very high, due to the similar value of the electron binding energies of the H (Is) orbital and the F(2~) orbital (ca. 12 eV), respectively. This view is also supported by the work of Souda et al. [8], who calculate adiabatic population probabilities for the H (1s) orbital in (Cl-Lis-H)‘+ clusters as a function of the H-Cl distance. According to the correlation diagram, two of the F2p-electrons will be promoted via the 3da orbital, while a “hole” will be demoted via the 2pu orbital (see Fig. 2). The potential curve corresponding to this situation can in principle cross into the continuum and lead to autoionization of the transient molecule. The final state of this autoionization would be the ground state of the H-F molecule, with the 2pa,rr orbitals filled by the 8 available valence electrons, and with one electron in the continuum. This process may be depicted by the following scheme: H +F-+
H - F-( ..2pa’2pn43da2) +H-F(..2p~~2pn-~)
+e(E).
(I)
The above qualitative discussion shows that, in principle, molecular autoionization in binary collisions of the neutralized H atoms with F- ions could be responsible for the observed electron emission in collisions of protons with a LiF-crystal. In the following we show that mode1 calculations based on this process do indeed yield electron spectra and their collision energy dependence in close agreement with experiment.
Is-
~~~
Is0
Ne’ Fig. 2. One electron diabatic molecular for H-F-.
-
1Is(F)
H+F’ orbital correlation
diagram
P. Stracke et al./Nucl.
69
fnstr. and Meth. in Phys. lies. B 1.25 13997) 67-75
In the model calculations we make to the largest possible extent the use of independent and realistic theoretical information on the collision system. The main steps in the calculations are the following: (i) It is assumed that the protons are neutralized at a distance of 4 atomic units in front of the first surface layer; (ii) the interaction of the projectile with the surface is described by the sum of two terms, a short range binary part, and a long range part representing the polarization interaction and the “Madelung-like” interaction with the ionic crystal; (iii) the relevant binary scattering potentials are calculated using a Hartree-Fock code, and the polarization interaction is described in terms of the individual polarizabilities; (iv) using the resulting potential, Monte Carlo calculations of projectile trajectories are carried out for the relevant experimental conditions (E,,,, O,,,) by replacing the real surface by a crystal slab containing 1000 Li’ and Fions; (v) from the Monte Carlo calculations a distribution function w( b, ECo,,, f&> of the probabilities for the occurrence of H-F- collisions with a certain impact parameter (b) is retrieved; (vi) from the one electron energies calculated in the Hartree-Fock code, the initial- and final state potentials V,(R) and V,(R), respectively, of the autoionization scheme (1) above are constructed; (vii) it is assumed that a finite distance dependent ‘.lifetime” and a corresponding energy “width” T(R) may be defined for the initial state. Such functions are not
10
easy
to calculate. We therefore estimate its functional form
as r(R)
= A exp( -R/R,,.)
(2)
with R,,= 2 at.u. a characteristic distance for autoionization transitions of the type indicated by scheme (1). The exponential form (2) has been found to be a good approximation for spontaneous electron emission in slow collisions [6]. The constant (A) does not influence the shape of the calculated spectra as long as the “collision time” is short compared to the lifetime = l/r. Typical collision times are in the present case of the order of 10 at.& and are thus considerably shorter than typical autoionization life times, which are of the order of 100 atu. For our model calculations we take A = 0.05 at.u. With the functions V&R), V,(R), T(R), and w(b, E,,,,, @I,,,,,>,it is possible to calculate the electron spectrum corresponding to a certain experimental condition. Various approximate methods have been used in the past. For the case of gas phase binary collisions this has been discussed in Ref. [6], and for the case of atomic collisions in surfaces, in Ref. [9]. By defining impact parameters (b) for the collision, we have already chosen a semiclassical description in which the trajectory is assumed to be well defined. In order to capture all quantum interference effects, we use in the present calculations the so called “eikonal approximation” [6], in which the transition amplitudes to a state defined by a certain energy (E) of the ejected electron are coherently summed along the trajectory S(R, Ecoll, b), the phase factor of the amplitudes being given in terms of the momenta of relative
15
20
25
electron energy [eVj
Fig. 3. Theoretical electron spectra for 5” grazing incidence HI-LiF collisions at three different laboratory are calculated for the mechanism of molecular autoionization indicated by scheme (1) (see text).
II. SECONDARY
collision energies.
EMISSION
The spectra
- EXPERIMENT
P. Strucke et al./ Nucl. Instr. und Meth. in Phys. Res. B 125 (1997)
70
motion in initial and final state, K,(S) and K&S, E), respectively. The numerical calculations in this approximation consist of the evaluation of the following expression
161:
Xdb
Xexp[iL:(Ki(S)
-Kr(S.
8))
ds’]]i’
(3)
with m being the reduced mass. In this expression the initial state momentum K,(S) is complex via the complex initial state potential in which the finite width T(R) is introduced as an imaginary part. Electron spectra calculated for three collision energies and for an incidence angle of 5” are shown in Fig. 3. The electron spectra have been shifted towards higher energy by 3.5 eV in order to facilitate comparison with the experimental spectra in Fig. 1. We notice that both, the observed slopes as well as their dependence on collision energy are reasonably well reproduced by the calculations. We would like to emphasize that these features do hardly at all depend on the value of the only two quantities that were somewhat arbitrarily chosen, namely (A) and (R,,) of the width function T(R). The weak oscillatory structure visible in the calculated spectra at the high energy side is due to quantum interference effects that partially survived the integration over impact parameters. This structure depends sensitively on details of the functions used in Eq. (3). and on the distribution of impact parameters. It is therefore not expected to be directly comparable to the experiment. However their appearance may be taken as an indication of the physical origin of the weak observed structure. The spectra calculated by (3) are given in units of partial probability per projectile in atomic units. If they are integrated over energy, one obtains the absolute yield, i.e. the quantity recently measured for the H+/LiF system. To carry out a quantitative comparison with these measured yields would require to calculate the impact paramefor the experimental situation ter distribution w( EC,,,,, Oco,,> of these experiments (poly-crystalline LiF), and in addition, assumptions would have to be made to take into account the loss of electrons when they are due to collisions in deeper layers. What we can say presently is that,
67-70
theoretical yields resulting from our model are consistent with the experimental yields of about one electron per proton at I keV collision energy. This can be seen as follows: we obtain a probability of 10% at 1 keV for collisions with impact parameter smaller than two atomic units. If we assume that effectively ten such collisions occur for a poly-crystalline surface at normal incidence, we predict one electron per projectile in agreement with experiment. Summarizing our analysis of kinetic electron emission in slow proton LiF collisions, we can state the following: the extent of agreement between results of calculations based on the “electron promotion-molecular autoionization” model indicated by scheme (I), and experimental results, is strong evidence for the validity of the model.
Acknowledgements This paper presents joint work carried out within the Human Capita1 and Mobolity Network “Charge Transfer Processes at Surfaces” (ERB CHRXCT 94 0571) of the European Union. Financial support of the Deutsche Forschungsgemeinschaft (grants: Ke 155/22 and /23) is also acknowledged. This work was also performed as part of the program of the “Stichting voor Fundamenteel Onderzoek der Materie (FOM)“, with financial support from the “Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)“.
References
[II G.
Spierings, 1. Urazgil’din, P.A. Zeijlmans van Emmichoven and A. Niehaus, Phys. Rev. Lett. 74 (19%) 453. 121M. Vana, F. Aumayr, P. Varga and H.P. Winter, Europhys. Lett. 29 (1995) 61. [31 H. Schall, W. Huber, H. Hoermann, W. Maus-Friedrichs and V. Kempter, Surf. Sci. 210 (1989) 163. [41 H. Brenten, H. Mueller, K.H. Knorr, D. Kruse, H. Schall and V. Kempter, Surf. Sci. 243 (1991) 309. [51 P. Stracke, F. Wiegershaus, S. Krischok, H. Mueller and V. Kempter, these Proceedings (IISC-I I), Nucl. Instr. and Meth. B 125 (1997) 63. [61 A. Niehaus, Phys. Rep. 186 ( 1990) 149. [71 M. Barat and W. Lichten, Phys. Rev. A 6 (1972) 211. k31R. Souda, K. Yamamoto, W. Hayami, B. Tilley, T. Ayzawa and Y. Ishizawa, Surf. Sci. 324 (1995) L349. [91 Z. Sroubeck and 3. Fine, Phys. Rev. B 51 (1995) 5635.
& -H
and Methods in Physics Research B 125 (1997) 67-70 NONil
B
Beam Interactions with Materials&Atoms
ELSEVIER
Electron emission in slow collisions of protons with a LiF-surface P. Stracke aT* , F. Wiegershaus a, S. Krischok a, V. Kempter a, P.A. Zeijlmans van Emmichoven b, A. Niehaus b, F.J. Garcia de Abajo ’ a Technical Uniuersiry Cluusrhal. Department ofPhysics, Leibnizsrr. 4. D-38678 Clausrhal-Zellerfeld.
Germuny
b Utrecht Uniuersiry, Debye Institute, Princetonplein 5,3584 CC Urrecht, The Nether1and.s ’ Universidad de1 Pais Vusco. Apartudo 649, 20080 Sun Sebastian, Spain
Abstract Electron spectra have been measured for collisions of protons with surfaces of LiF grown on a tungsten substrate. The measurements were carried out for 5” grazing incidence collisions at energies from 50 eV up to 1 keV. The experimental spectra are compared with theoretical spectra obtained from model calculations. The model is based on the assumption that electron emission is dominated by the mechanism of electron promotion via the 3du molecular orbital of the Hlquasi-molecular system. Using theoretically obtained functions for the scattering potential and for the 3da orbital energy, and calculating the spectra in semiclassical approximation for a distribution of impact parameters obtained from Monte Carlo simulations for the relevant scattering geometry, good agreement between experimental and theoretical spectra is obtained. This agreement is taken as strong evidence for the assumed ionization mechanism.
2. Experimental
1. Introduction Kinetic electron emission in slow collisions of ions with surfaces is not yet well understood, and appears to be very dependent on the ion/surface system. For collisions of light ions with metal surfaces, it has recently been shown that the dominant mechanism may be described as a perturbation of the loosely bound localized valence electrons by the incompletely screened charge of the passing projectile [I]. The electron spectra resulting from this “direct” ionization mechanism have the highest intensity at low electron energy and decay exponentially towards higher energies. Further, the absolute yield decreases strongly with collision energy and has an apparent threshold around a projectile velocity of 10’ m/s. In contrast to this behavior, it has recently been found [2] that in collisions of protons with a poly-crystalline LiF surface the electron yield does not decrease strongly in the energy region between 1 keV and 100 eV. In addition, we found that theoretical calculations based on the above described “direct” mechanism cannot explain the observed high yields at low collision energies, indicating that apparently a different mechanism for kinetic emission is responsible. In order to study the ill-understood main ionization mechanism in proton LiF collisions, we carried out measurements of the electron spectra resulting from such collisions in the low energy range.
l
Corresponding author.
016%583X/97/$17.00
Details of the apparatus can be found elsewhere [3,4] (see also Ref. [5] in this volume). Briefly, a mass analyzed H+ beam impinges upon a W(l10) surface covered by a LiF film (10 nm thick); it was produced by thermal evaporation (1100 K) of LiF single crystal chips. XPS, MIES and UPS (He(I)) measurements show that the film possesses already the electronic structure of bulk LiF surfaces. No occupied surface states could be found in the bandgap of the insulator film. The incidence angle of the ion beam is 5” with respect to the surface; the ejected electrons are energy analyzed under 90” with respect to the beam axis. The spectrometer records the electron spectra at constant pass energy with a resolution of AE = 0.2 eV (FWHM). The difference in the work functions of the surface and the analyzer was biased in such a way that electrons leaving the clean W(110) surface with zero energy arrive at the analyzer with 5.25 eV. Thus, the low energy cutoff of the spectra for the LiF spectra gives the work function of the LiF film (3.5 eV). A correction was applied to the spectra in order to compensate the energy dependence of the electrostatic analyzers transmission. However, the energy dependence of the collection effi-
ciency introduced by the biasing procedure is not fully compensated in this way. The weak peak-like structure at the low energy cutoff visible in all spectra is probably an artefact caused by this procedure. Fig. 1 displays the energy spectra of the electrons
0 1997 Elsexier Science B.V. All rights reserved
PII SOl68-583X(96)00800-2
II. SECONDARY EMISSION - EXPERIMENT
P. Stracke et al./Nucl.
0
5
11
15
20
Insrr. and Meth. in Phys. Res. B I25 (1997) 67-70
25
electron energy/ eV Fig. 1. Experimental electron spectra for 5” grazing incidence H+ -LiF collisions at various laboratory collision energies.
in grazing (5”) collisions of H+ with the LiF film for various collision energies. At all studied collision energies the intensity is highest at zero kinetic electron energy and decays roughly exponentially towards higher energies. However some weak structure (which survives the smoothing procedure applied to the raw data) is superimposed and has to be considered meaningful between 4 and 11 (18) eV at 200 (1000) eV collision energy. emitted
3. Analysis From studies of ionization in low energy binary ionatom and atom-atom collisions it is well known that processes of “indirect” caused by ionization, “promotion” of electrons during the collision, can become dominant. One may distinguish two types of such indirect ionization processes: (i) due to electron promotion the potential energy curve describing the system enters the one electron continuum only during the collision, and (ii), by coupling of the promoted potential curve to another excited state that asymptotically correlates with the excitation of one of the collision partners, the system may remain in the continuum also after the collision. In processes of the first type, the ionization can only occur in the transient molecule, leading to a quasi-continuous electron spectrum, while in processes of the second type, depending on the autoionization lifetime of the excited state, the electron spectrum can vary from quasi-continuous to “peak-like”. For many binary collision systems in the gas phase such electron spectra have been observed [6]. In what follows, we analyze the H+-LiF spectra under the assumption that they are due to electron promotion. Since no peak-like structure is observed, we assume further that molecular autoionization of the type (i) described above is responsible. Electron promotion occurs at projectile-target atom distances that are considerably smaller than the distance
between Lif and F- in the surface. We therefore may distinguish between collisions involving these different target atoms. Electron promotion in collisions with the Li+ ion can be excluded because of the strong binding energy of the Li Is-electrons. In order to get a qualitative idea of the possibility of electron promotion in collisions with F-, we look at the diabatic one electron correlation diagram for the H-Fsystem, constructed according to the rules given in Ref. [7], and depicted in Fig. 2. In principle, we have to consider collisions with both, H+ ions and neutralized H projectiles. However, the probability for neutralization in one of several “distant” collisions is probably very high, due to the similar value of the electron binding energies of the H (Is) orbital and the F(2~) orbital (ca. 12 eV), respectively. This view is also supported by the work of Souda et al. [8], who calculate adiabatic population probabilities for the H (1s) orbital in (Cl-Lis-H)‘+ clusters as a function of the H-Cl distance. According to the correlation diagram, two of the F2p-electrons will be promoted via the 3da orbital, while a “hole” will be demoted via the 2pu orbital (see Fig. 2). The potential curve corresponding to this situation can in principle cross into the continuum and lead to autoionization of the transient molecule. The final state of this autoionization would be the ground state of the H-F molecule, with the 2pa,rr orbitals filled by the 8 available valence electrons, and with one electron in the continuum. This process may be depicted by the following scheme: H +F-+
H - F-( ..2pa’2pn43da2) +H-F(..2p~~2pn-~)
+e(E).
(I)
The above qualitative discussion shows that, in principle, molecular autoionization in binary collisions of the neutralized H atoms with F- ions could be responsible for the observed electron emission in collisions of protons with a LiF-crystal. In the following we show that mode1 calculations based on this process do indeed yield electron spectra and their collision energy dependence in close agreement with experiment.
Is-
~~~
Is0
Ne’ Fig. 2. One electron diabatic molecular for H-F-.
-
1Is(F)
H+F’ orbital correlation
diagram
P. Stracke et al./Nucl.
69
fnstr. and Meth. in Phys. lies. B 1.25 13997) 67-75
In the model calculations we make to the largest possible extent the use of independent and realistic theoretical information on the collision system. The main steps in the calculations are the following: (i) It is assumed that the protons are neutralized at a distance of 4 atomic units in front of the first surface layer; (ii) the interaction of the projectile with the surface is described by the sum of two terms, a short range binary part, and a long range part representing the polarization interaction and the “Madelung-like” interaction with the ionic crystal; (iii) the relevant binary scattering potentials are calculated using a Hartree-Fock code, and the polarization interaction is described in terms of the individual polarizabilities; (iv) using the resulting potential, Monte Carlo calculations of projectile trajectories are carried out for the relevant experimental conditions (E,,,, O,,,) by replacing the real surface by a crystal slab containing 1000 Li’ and Fions; (v) from the Monte Carlo calculations a distribution function w( b, ECo,,, f&> of the probabilities for the occurrence of H-F- collisions with a certain impact parameter (b) is retrieved; (vi) from the one electron energies calculated in the Hartree-Fock code, the initial- and final state potentials V,(R) and V,(R), respectively, of the autoionization scheme (1) above are constructed; (vii) it is assumed that a finite distance dependent ‘.lifetime” and a corresponding energy “width” T(R) may be defined for the initial state. Such functions are not
10
easy
to calculate. We therefore estimate its functional form
as r(R)
= A exp( -R/R,,.)
(2)
with R,,= 2 at.u. a characteristic distance for autoionization transitions of the type indicated by scheme (1). The exponential form (2) has been found to be a good approximation for spontaneous electron emission in slow collisions [6]. The constant (A) does not influence the shape of the calculated spectra as long as the “collision time” is short compared to the lifetime = l/r. Typical collision times are in the present case of the order of 10 at.& and are thus considerably shorter than typical autoionization life times, which are of the order of 100 atu. For our model calculations we take A = 0.05 at.u. With the functions V&R), V,(R), T(R), and w(b, E,,,,, @I,,,,,>,it is possible to calculate the electron spectrum corresponding to a certain experimental condition. Various approximate methods have been used in the past. For the case of gas phase binary collisions this has been discussed in Ref. [6], and for the case of atomic collisions in surfaces, in Ref. [9]. By defining impact parameters (b) for the collision, we have already chosen a semiclassical description in which the trajectory is assumed to be well defined. In order to capture all quantum interference effects, we use in the present calculations the so called “eikonal approximation” [6], in which the transition amplitudes to a state defined by a certain energy (E) of the ejected electron are coherently summed along the trajectory S(R, Ecoll, b), the phase factor of the amplitudes being given in terms of the momenta of relative
15
20
25
electron energy [eVj
Fig. 3. Theoretical electron spectra for 5” grazing incidence HI-LiF collisions at three different laboratory are calculated for the mechanism of molecular autoionization indicated by scheme (1) (see text).
II. SECONDARY
collision energies.
EMISSION
The spectra
- EXPERIMENT
P. Strucke et al./ Nucl. Instr. und Meth. in Phys. Res. B 125 (1997)
70
motion in initial and final state, K,(S) and K&S, E), respectively. The numerical calculations in this approximation consist of the evaluation of the following expression
161:
Xdb
Xexp[iL:(Ki(S)
-Kr(S.
8))
ds’]]i’
(3)
with m being the reduced mass. In this expression the initial state momentum K,(S) is complex via the complex initial state potential in which the finite width T(R) is introduced as an imaginary part. Electron spectra calculated for three collision energies and for an incidence angle of 5” are shown in Fig. 3. The electron spectra have been shifted towards higher energy by 3.5 eV in order to facilitate comparison with the experimental spectra in Fig. 1. We notice that both, the observed slopes as well as their dependence on collision energy are reasonably well reproduced by the calculations. We would like to emphasize that these features do hardly at all depend on the value of the only two quantities that were somewhat arbitrarily chosen, namely (A) and (R,,) of the width function T(R). The weak oscillatory structure visible in the calculated spectra at the high energy side is due to quantum interference effects that partially survived the integration over impact parameters. This structure depends sensitively on details of the functions used in Eq. (3). and on the distribution of impact parameters. It is therefore not expected to be directly comparable to the experiment. However their appearance may be taken as an indication of the physical origin of the weak observed structure. The spectra calculated by (3) are given in units of partial probability per projectile in atomic units. If they are integrated over energy, one obtains the absolute yield, i.e. the quantity recently measured for the H+/LiF system. To carry out a quantitative comparison with these measured yields would require to calculate the impact paramefor the experimental situation ter distribution w( EC,,,,, Oco,,> of these experiments (poly-crystalline LiF), and in addition, assumptions would have to be made to take into account the loss of electrons when they are due to collisions in deeper layers. What we can say presently is that,
67-70
theoretical yields resulting from our model are consistent with the experimental yields of about one electron per proton at I keV collision energy. This can be seen as follows: we obtain a probability of 10% at 1 keV for collisions with impact parameter smaller than two atomic units. If we assume that effectively ten such collisions occur for a poly-crystalline surface at normal incidence, we predict one electron per projectile in agreement with experiment. Summarizing our analysis of kinetic electron emission in slow proton LiF collisions, we can state the following: the extent of agreement between results of calculations based on the “electron promotion-molecular autoionization” model indicated by scheme (I), and experimental results, is strong evidence for the validity of the model.
Acknowledgements This paper presents joint work carried out within the Human Capita1 and Mobolity Network “Charge Transfer Processes at Surfaces” (ERB CHRXCT 94 0571) of the European Union. Financial support of the Deutsche Forschungsgemeinschaft (grants: Ke 155/22 and /23) is also acknowledged. This work was also performed as part of the program of the “Stichting voor Fundamenteel Onderzoek der Materie (FOM)“, with financial support from the “Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)“.
References
[II G.
Spierings, 1. Urazgil’din, P.A. Zeijlmans van Emmichoven and A. Niehaus, Phys. Rev. Lett. 74 (19%) 453. 121M. Vana, F. Aumayr, P. Varga and H.P. Winter, Europhys. Lett. 29 (1995) 61. [31 H. Schall, W. Huber, H. Hoermann, W. Maus-Friedrichs and V. Kempter, Surf. Sci. 210 (1989) 163. [41 H. Brenten, H. Mueller, K.H. Knorr, D. Kruse, H. Schall and V. Kempter, Surf. Sci. 243 (1991) 309. [51 P. Stracke, F. Wiegershaus, S. Krischok, H. Mueller and V. Kempter, these Proceedings (IISC-I I), Nucl. Instr. and Meth. B 125 (1997) 63. [61 A. Niehaus, Phys. Rep. 186 ( 1990) 149. [71 M. Barat and W. Lichten, Phys. Rev. A 6 (1972) 211. k31R. Souda, K. Yamamoto, W. Hayami, B. Tilley, T. Ayzawa and Y. Ishizawa, Surf. Sci. 324 (1995) L349. [91 Z. Sroubeck and 3. Fine, Phys. Rev. B 51 (1995) 5635.