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Suppression of Auger-neutralization in the scattering of noble gas ions from the surface of an insulator
-_ __ k!B
2 September 1996
&J
PHYSICS
EJ_SEVIER
LETTERS
A
Physics Letters A 220 ( 1996) 102- IO6
Suppression of Auger-neutralization in the scattering of noble gas ions from the surface of an insulator T. Hecht, C. Auth, A.G. Borisov, H. Winter Institurfir Physik der Humboldt-UniuersiCit :u Berlin, Inuulidenstrusse. 110, D-10115 Berlin, Germuny
Received 5 June 1996; accepted for publication 13 June 1996 Communicated by B. Fricke
Abstract We have scattered noble gas atoms and ions from a clean and flat LiF( lOO)-surface under a grazing angle of incidence and observed for He+- and Ne+-projectiles a substantial survival in the collision. We interpret our findings in terms of a suppression of the Auger-neutralizationfor these ions in the interaction with the surface of the insulator. Due to the high binding energies of valence band electrons and the large energy gap for LiF Auger transition rates seem to be clearly reduced in comparison to those rates for metal targets.
Charge exchange phenomena of atoms and ions interacting with solid surfaces are of considerable importance from fundamental as well as practical aspects. Of particular interest in this respect are studies with projectiles that approach the surface plane with low velocities and energies, i.e. hyperthermal beams with energies up to some eV. Then the electron transfer between solid and projectile will proceed via electron tunneling processes of relatively long range. These well established processes are (i> one-electron “resonant tunneling” between electronic states of solid and atom and (ii> two-electron “Auger-transfer” [ 1,2]. At metal surfaces, noble gas atoms are formed from singly charged projectiles via “Auger-neutralization” (AN) (for Ar also “resonant neutralization” (RN) might be possible) as sketched on the left side of Fig. 1 for a typical metal with workfunction W = 4-5 eV and Fermi-energy E, = 10 eV. Note 0375-%01/96/$12.00
in the AN-process one electron finally occupies an atomic groundstate, whereas the second electron is excited to an empty conduction band state or to a continuum state of the vacuum. Despite the fact that it is still a challenge to calculate Auger-transition rates from first principles [3,4], experimental studies and their analysis show that these rates are sufficiently large so that noble gas ions with eV-energies are completely neutralized well in front of the surface plane at distances of several a.u. (atomic unit of length = 0.053 nm) [ 1,5-81. Here we report on experiments of scattering noble gas ions from the surface of an insulator, LiF(lOO), where we find the interesting result that substantial fractions of ions survive the scattering event with the solid. The measurements are performed with keVions in the grazing incidence scattering geometry at typical incidence angles ain = l”, where the projectiles are “channeled” and reflected from the topmost that
Copyright 0 1996 Elsevier Science B.V. All rights reserved.
PII SO375-9601(96)00486-O
T. Hecht et al./Physics
voience bond
Fig. I. Sketch for the Auger-neutralization (AN) of some noble gas ions (in particular He+ ) in front of a metal surface (left side) and in front of the surface of an insulator (tight side).
layer of surface atoms. fn this type of ion-surface collision the approach to the surface proceeds with energies EZ c: E, sin2 Gin (EP = projectile energy), i.e. with eV-energies for keV-projectiles. The specific advantage in making use of this geometry here are well defined trajectories in the collision with the surface which allows us to study the complete interaction sequence with the solid. As an example of our studies we show in Fig. 2 the angular distributions for 4 keV He’- and He+projectiles, respectively, scattered from a LiF( lOO)surface kept at a tem~ra~re of about 3OOT in order
He+-He’
He, 4 heV
0
..p
0’. 01..
.
o .
0
0
GO
05
10
scoitering
!’ !
-
l
o0 0
l
l
%&
20
1 5 angle
.
(“)
Fig. 2. Angular distributions for neutral He-atoms (open circles) and He+-ions (full circles) as projectiles scattered from an Lilly}-su~ace and emerging as neutral atoms. The projectile energy is 4 keV. the neutral beams are produced via charge exchange in a gas target in the beamline operated with air.
Letters A 220 (1996)
102-106
I03
to avoid a charging up of the target. The experiments were performed with beams well collimated by sets of horizontal and vertical slits (the narrow peak on the left side is due to a residual fraction of the projectile beam) at a base pressure of about lo-” mbar. A striking observation in these measurements are small fractions of He*-ions in the scattered beams for incoming ions, whereas for neutral projectiles of this energy the fraction of ions is negligible. With help of electric field plates between target and channeltron detector ions are deflected from the scattered beams, so that the data shown in Fig. 2 are recorded for the (dominant) neutral portion of the scattered projectiles. The distributions for incoming ions show a pronounced angular shift from the distribution for atoms which is at~ibuted to the effect of the image charge interaction on the incoming path of the trajectory for charged projectiles [8,9]. This interaction - not present for neutral atoms - increases the energy for the normal projectile motion and leads to effective larger angles of incidence than given by the macroscopic settings of the target. From the angular shift we deduce an energy gain of A Ei, = 0.9 eV for ions on the incident trajectory. Since the neutralization of the ions stops this interaction, one can deduce from A Ei, a mean distance of neutralization zE. This distance is estimated from an expression for the dynamical image potential given by Garcia de Abajo and Echenique [IO] for a charge moving parallel to the surface plane. With the dielectric response deduced from optical constants [I 1I we obtain A E,, = 0.38/42 and find from the measured A E,, z, = 2.9 a.u.; due to the locahsed nature of the F 2p-electrons of the target this distance is assumed to be referred to the topmost layer of surface atoms. It is interesting to note that this distance is clearly smaller than observed for metal surfaces and amounts to about the distance of closest approach of an eV-projectile to the surface lmin = 2.8 au. as calculated with help of “universal” interatomic potentials [12] for the LiF-surface. From this estimate we already conclude that the AN-rates have to be much smaller than for metal targets so that most of the incoming ions are neutralized around the turning point of the trajectory. Since the projectiles spend a relatively long time At = 1000 au. = 3 x IO-l1 s in the region Z,in < Z < tIZ,i,, + 1 a.u.) and substantial
104
T. Hecht et a/./Physics
fractions of ions still survive the scattering, AN-rates have to be indeed small. Detailed information on the survival of incident ions can be obtained from angular distributions separated via a “difference method” [13] for outgoing neutral atoms and He+-ions. The dis~bution for neutral incoming and outgoing atoms in Fig. 3 has been presented already in Fig. 2 and serves here for a comparison with the distribution of incoming and surviving ions. The fraction of surviving ions amounts here to about 15%, and the two data sets in Fig. 3 are normalized to the same peak height. In the same manner as neutral atoms also surviving ions are specularly reflected from the surface, i.e. the effects of the image charge interactions on the incident and outgoing trajectories are in the mean compensated. In addition to the observation that practically no ions are observed for neutral projectiles, this finding shows clearly that the detected ions must stem from a fraction of surviving incident ions. The broader angular distribution for He’-He+ - in comparison to He’-He0 - scattering is attributed to the increased normal energies due to image charge attraction. This leads to reduced distances of closest approach where the scattering potential shows an increased corrugation. In Fig. 4 we show results of our studies on the fractions of surviving ions as a function of the projectile velocity. The normal velocity components
fr
25 scattering
angle
(“)
Fig. 3. Angular distribution observed for the scattering of 3 keV Hei-ions from an LiF(100ksurface for particles emerging from the surface as neutral atoms (open circles) and as ions (full circles). The two distributions are normalized to the same height of the maximum. The fraction of surviving ions is about 15%.
Letters A 220 (1996) 102-106
80 s
,
z .-0 2
.
60-
0 . .
50 -
He+ - LiF
.
Ne* - LiF
.
Ar” - LIF
-
l
z
40-
75
30-
E .-0
20 -
:: z
1
I
l**
m-
l
0 0
l
Oo
lo00
0
. Ob b. @ I, 02
L C.l velocity
l
_, 0.3
(a.~.)
Fig. 4. Fractions of surviving ions as function of the projectile velocity for He+-ions (open circles), Ne+-ions (full circles), and Are-ions @ulI triangles). The energy for the motion normal to the surface plane is about 3 eV.
in these measurements are kept constant by a corresponding adjustment of the angle of incidence fin. In addition to He”-projectiles we also studied the neutralization of Ne*- and Arf-ions. As can be seen from the data in Fig. 4, we observe for Nef even higher fractions of surviving ions (up to 75%), whereas Ar+-ions are completely neutralized. The ion-fractions show a pronounced decrease with increasing projectile velocity and vanish practically at velocities of about 0.25-0.3 a.u. Here it is import~t to note that in this range of velocities the ion-fractions for neutral projectiles are negligible (less than 1%). In interpreting our results we first discuss the different behaviour of A.r+- in contrast to He+- and Nef-ions. This is simply explained by the energies of the F 2p-electrons forming the bulk valence band from about 12 to 16 eV [ 141. Then the (slightly shifted) binding energy of the Ar I 3s’ 3p6 ’ So-ground term E, = 15.8 eV is in resonance with this fully occupied band, and AI+-ions are efficiently neutralized via a resonant one-electron ~Meling process (“resonant neutralization”). The binding energies of the He I 1s2 ‘So- and Ne I 2s22p6 ’ S,-ground terms amount to E, = 24.6 eV and E, = 21.6 eV. Since furthermore no excited atomic states are in resonance with the valence band, neutralization has to proceed via a two-electron Auger-transfer (see also Fig. 1) 2151.
T. Hecht ef al./Physics
The new aspect of our work is related to specific features of the Auger-neutralization process in front of the surface of an insulator. For metals (see Fig. 1, left side) this process is very efficient, since the second electron can be excited to unoccupied electronic levels in the conduction band and in vacuum. However, for an insulator with a large band gap ( Eg = 14 eV for LiF [14]) available final states for the excited electrons can be a substantial problem. This is indicated for the He I-groundstate in front of a LiF-surface on the right side of Fig. 1. In addition, we expect from the large binding energies of LiF-valence band electrons a reduced spill-out to vacuum and closer distances for an efficient Auger neutralization than for metals. Based on the local character of the F 2p-electrons forming the valence band we propose that the neutralization of the He+- and Ne+-ions during the scattering in front of LiF proceeds via an interatomic Auger transition, where two electrons from different F--sites are involved. A similar process is discussed for the decay of inner shell holes at the alkali-sites of alkali-halides f 16-201. The ~ansition matrix element has the form Tk.a;.A
.B =
%(r,)~k(r2)
//
X-
I
~~-(r,)~~-(~*)
dr,
dr,,
(1)
I r12 I where ‘p,, (Pk are wave functions of the atomic groundstate and the excited electron, CD;-, @& are wavefunctions for electrons centered at F--sites A, B. Note that for the matrix element the overlap of cp, with (Pi?-(r,l is important and leads to the local nature of the Auger-process described here. In the adiabatic limit, the rate for the AN-process accompanied by the excitation of an electron to a continuum state in vacuum can be estimated from
r=
27r(
VB
P(E,) d+~“*PM
d&*
?;,a:A.R12S(t3,+E2-Ek--E,*),
(2)
Leners A 220 11996) 102-106
105
where P(E) is the density of states of the LiF-valence band (VB), E,, E* are binding energies of valence band electrons, E, = k*/2 is the free electron energy, and E,* is the binding energy of the atomic state slightly modified close to the surface. In order to keep the discussion simple we neglect here the hole-hole interaction in the final state [16] which is expected to have energies l/R,, G== eV (F, = “high frequency” dielectric constant) for our system. As a result of energy conservation (S-function in Eq. (2)) for LiF with F 2p-valence band energies E, ,E* 2 12 eV the number of valence band states available for AN of He+-ions (E, = 24 eV1 is small. This will reduce the AN-rates. For Ne+-ions (E, = 21 eV) AN should be suppressed completely in this respect. The projectile energies Ep are clearly larger than the interaction energies E, so that the dynamics of the local AN-process has to be considered. The final interaction times lead to a “collisional broadening”, and the a-function has to be replaced by a distribution of finite width. Then the number of available states for AN and con~quently also transition rates for the AN-process are expected to be enhanced. The pronounced decrease of the ion-fractions with increasing velocity might be attributed to this effect. Aside from the kinematics also unoccupied surface states I211 and imperfections of the crystal can cause a general e~~cement of AN-rates. Recent work on electron spectra obtained with He+ions impinging on a very thin LiF-film [22] seems to support this assumption, even though this work may not directly be compared with our studies performed with a single crystal. For further info~ations on the neutralization process, we performed some simple calculations of the occupation of the ion P, during the scattering process. We assumed classical trajectories deduced by using screened interatomic potentials and parameterized the total AN-rate according to W( ~1 = W. expt- z/z,1 (z is referred to the surface plane). Also an energy gain via image charge interaction A Ei, is taken into account. For the neutralization of He+-ions in front of a metal surface the parameters W, = 1 a.u. and z, from about 1 au. to 1.5 au. are reported [1,3,8]. This rate is sufficiently large that P, (obtained from the master equation d P+/dt = - WP,) vanishes, before
106
T. Hechr et al./Physics
the projectiles reach the distance of closest approach. The effective distance of neutralization is about z, = 6 to 8 au., and no ions survive on the trajectory. From the data for He+- and Ne+-ions at the lowest velocities of our study (Fig. 4) and from the poor dependence of the ion-fractions on the normal energy Ez [23] we can reproduce the data with the parameters z, = 1.5 a.u. and W, = 0.006 a.u. (He), W, = 0.002 a.u. (Ne). These estimates on the ANrates are more than two orders of magnitude smaller than the rates observed for metal-surfaces. In conclusion, we report on first studies on charge fractions of noble gas ions in grazing collisions with a LiF(lOO)-surface. For He+- and Ne+-projectiles we observe that substantial fractions of ions survive the scattering process despite long interaction times with the solid. We interpret this finding by small Augerneutralization rates due to the specific electronic structure of the insulator; i.e. high binding energies of valence band electrons and a large band gap. In additional experiments we observed no effect on the fractions of surviving ions for projectiles of higher charges (He’+, Ne2+, etc.). This indicates that ions of larger binding energies seem to efficiently capture electrons to form He+- and Ne+-ions before the turning point of the trajectory is reached. The effect of suppression of the Auger-process is reduced with increasing projectile energy which we attribute to collisional broadening effects. We hope that this work and our attempts to interpret the data will stimulate further studies on this interesting problem.
Acknowledgements
This work is supported by the Deutsche Forschungsgemeinschaft (DFG) under contract Wi
Letters A 220 (1996) 102-106
1336/l-l. One of us (A.G.B.) gratefully acknowledges generous support from the Alexander von Humboldt foundation.
References
111H.D. Hagstrum, l21J. Los and J.J.C.
Phys. Rev. 96 (1954) 336. Geerlings, Phys. Rep. 52 (1989) 655. R. Monreal and H. Alducin, Phys. Rev. A 49
131 N. Lorente, (1994) 4716. [41 T. Folden and A. Zwartkmis, Surf. Sci. 269/270 (1992) 601. [51 H.D. Hagstrum, Phys. Rev. B 8 (1973) 107. [61P.A. Zeijlmans van Emmichoven, P.A.A.F. Wouters and A. Niehaus, Surf. Sci. 195 (1988) 115; Comm. At. Mol. Phys. 24 (1990) 65. [71 H. Miller, R. Hausmarm, H. Brenten and V. Kempter, Surf. Sci. B 78 (1993) 239. [al H. Winter, J. Phys. Condens. Matter 5 (1993) 1. [91 H. Winter, Europhys. Lett. 18 (1992) 207. [lOI F.J. Garcia de Abajo and P.M. Echenique, Phys. Rev. B 46 (1992) 2663. PII E.O. Pahlik and W.R. Hunter. in: Handbook of optical constants of solids (Academic Press, New York, 1985). [I21 J.P. Biersack and J.F. Ziegler, Nucl. Instr. Meth. 194 (1982) 93. [I31 H. Winter, Phys. Rev. A 46 (1992) R13. t141 D.A. Lapiano-Smith, E.A. Eklund and F.J. Himpsel. Appl. Phys. Lett. 59 (1991) 2174. I151 P. Varga and U. Diebold, in: Low energy ion-surface interactions, ed. J.W. Rabalais (Wiley, New York, 1994) p. 355. 1161P.H. Citrin, J.E. Rowe and S.B. Christman, Phys. Rev. B 14 ( 1976) 2642. [17] J.A.D. Matthew and Y. Komninos, Surf. Sci. 53 (1975) 716. [18] Y. Yafet and R.E. Watson, Phys. Rev. B 16 (1977) 895. [19] T.A. Green and D.R. Jenninson, Phys. Rev. B 36 (1987) 6112. [20] G.K. Wertheim, J.E. Rowe, D.N.E. Buchanan and P.H. Citrin, Phys. Rev. B 5 1 (1995) 13669. [21] P. Wurz, I. Sarntheim, W. Husinsky, G. Bets, P. Nordlander and Y. Wang, Phys. Rev. B 43 (1991) 6729. [22] F. Wiegershaus, S. Krischok, D. Ckhs, W. Maus-Friedrich and V. Kempter, Surf. Sci., submitted for publication. [23] T. Hecht et al., to be published.
2 September 1996
&J
PHYSICS
EJ_SEVIER
LETTERS
A
Physics Letters A 220 ( 1996) 102- IO6
Suppression of Auger-neutralization in the scattering of noble gas ions from the surface of an insulator T. Hecht, C. Auth, A.G. Borisov, H. Winter Institurfir Physik der Humboldt-UniuersiCit :u Berlin, Inuulidenstrusse. 110, D-10115 Berlin, Germuny
Received 5 June 1996; accepted for publication 13 June 1996 Communicated by B. Fricke
Abstract We have scattered noble gas atoms and ions from a clean and flat LiF( lOO)-surface under a grazing angle of incidence and observed for He+- and Ne+-projectiles a substantial survival in the collision. We interpret our findings in terms of a suppression of the Auger-neutralizationfor these ions in the interaction with the surface of the insulator. Due to the high binding energies of valence band electrons and the large energy gap for LiF Auger transition rates seem to be clearly reduced in comparison to those rates for metal targets.
Charge exchange phenomena of atoms and ions interacting with solid surfaces are of considerable importance from fundamental as well as practical aspects. Of particular interest in this respect are studies with projectiles that approach the surface plane with low velocities and energies, i.e. hyperthermal beams with energies up to some eV. Then the electron transfer between solid and projectile will proceed via electron tunneling processes of relatively long range. These well established processes are (i> one-electron “resonant tunneling” between electronic states of solid and atom and (ii> two-electron “Auger-transfer” [ 1,2]. At metal surfaces, noble gas atoms are formed from singly charged projectiles via “Auger-neutralization” (AN) (for Ar also “resonant neutralization” (RN) might be possible) as sketched on the left side of Fig. 1 for a typical metal with workfunction W = 4-5 eV and Fermi-energy E, = 10 eV. Note 0375-%01/96/$12.00
in the AN-process one electron finally occupies an atomic groundstate, whereas the second electron is excited to an empty conduction band state or to a continuum state of the vacuum. Despite the fact that it is still a challenge to calculate Auger-transition rates from first principles [3,4], experimental studies and their analysis show that these rates are sufficiently large so that noble gas ions with eV-energies are completely neutralized well in front of the surface plane at distances of several a.u. (atomic unit of length = 0.053 nm) [ 1,5-81. Here we report on experiments of scattering noble gas ions from the surface of an insulator, LiF(lOO), where we find the interesting result that substantial fractions of ions survive the scattering event with the solid. The measurements are performed with keVions in the grazing incidence scattering geometry at typical incidence angles ain = l”, where the projectiles are “channeled” and reflected from the topmost that
Copyright 0 1996 Elsevier Science B.V. All rights reserved.
PII SO375-9601(96)00486-O
T. Hecht et al./Physics
voience bond
Fig. I. Sketch for the Auger-neutralization (AN) of some noble gas ions (in particular He+ ) in front of a metal surface (left side) and in front of the surface of an insulator (tight side).
layer of surface atoms. fn this type of ion-surface collision the approach to the surface proceeds with energies EZ c: E, sin2 Gin (EP = projectile energy), i.e. with eV-energies for keV-projectiles. The specific advantage in making use of this geometry here are well defined trajectories in the collision with the surface which allows us to study the complete interaction sequence with the solid. As an example of our studies we show in Fig. 2 the angular distributions for 4 keV He’- and He+projectiles, respectively, scattered from a LiF( lOO)surface kept at a tem~ra~re of about 3OOT in order
He+-He’
He, 4 heV
0
..p
0’. 01..
.
o .
0
0
GO
05
10
scoitering
!’ !
-
l
o0 0
l
l
%&
20
1 5 angle
.
(“)
Fig. 2. Angular distributions for neutral He-atoms (open circles) and He+-ions (full circles) as projectiles scattered from an Lilly}-su~ace and emerging as neutral atoms. The projectile energy is 4 keV. the neutral beams are produced via charge exchange in a gas target in the beamline operated with air.
Letters A 220 (1996)
102-106
I03
to avoid a charging up of the target. The experiments were performed with beams well collimated by sets of horizontal and vertical slits (the narrow peak on the left side is due to a residual fraction of the projectile beam) at a base pressure of about lo-” mbar. A striking observation in these measurements are small fractions of He*-ions in the scattered beams for incoming ions, whereas for neutral projectiles of this energy the fraction of ions is negligible. With help of electric field plates between target and channeltron detector ions are deflected from the scattered beams, so that the data shown in Fig. 2 are recorded for the (dominant) neutral portion of the scattered projectiles. The distributions for incoming ions show a pronounced angular shift from the distribution for atoms which is at~ibuted to the effect of the image charge interaction on the incoming path of the trajectory for charged projectiles [8,9]. This interaction - not present for neutral atoms - increases the energy for the normal projectile motion and leads to effective larger angles of incidence than given by the macroscopic settings of the target. From the angular shift we deduce an energy gain of A Ei, = 0.9 eV for ions on the incident trajectory. Since the neutralization of the ions stops this interaction, one can deduce from A Ei, a mean distance of neutralization zE. This distance is estimated from an expression for the dynamical image potential given by Garcia de Abajo and Echenique [IO] for a charge moving parallel to the surface plane. With the dielectric response deduced from optical constants [I 1I we obtain A E,, = 0.38/42 and find from the measured A E,, z, = 2.9 a.u.; due to the locahsed nature of the F 2p-electrons of the target this distance is assumed to be referred to the topmost layer of surface atoms. It is interesting to note that this distance is clearly smaller than observed for metal surfaces and amounts to about the distance of closest approach of an eV-projectile to the surface lmin = 2.8 au. as calculated with help of “universal” interatomic potentials [12] for the LiF-surface. From this estimate we already conclude that the AN-rates have to be much smaller than for metal targets so that most of the incoming ions are neutralized around the turning point of the trajectory. Since the projectiles spend a relatively long time At = 1000 au. = 3 x IO-l1 s in the region Z,in < Z < tIZ,i,, + 1 a.u.) and substantial
104
T. Hecht et a/./Physics
fractions of ions still survive the scattering, AN-rates have to be indeed small. Detailed information on the survival of incident ions can be obtained from angular distributions separated via a “difference method” [13] for outgoing neutral atoms and He+-ions. The dis~bution for neutral incoming and outgoing atoms in Fig. 3 has been presented already in Fig. 2 and serves here for a comparison with the distribution of incoming and surviving ions. The fraction of surviving ions amounts here to about 15%, and the two data sets in Fig. 3 are normalized to the same peak height. In the same manner as neutral atoms also surviving ions are specularly reflected from the surface, i.e. the effects of the image charge interactions on the incident and outgoing trajectories are in the mean compensated. In addition to the observation that practically no ions are observed for neutral projectiles, this finding shows clearly that the detected ions must stem from a fraction of surviving incident ions. The broader angular distribution for He’-He+ - in comparison to He’-He0 - scattering is attributed to the increased normal energies due to image charge attraction. This leads to reduced distances of closest approach where the scattering potential shows an increased corrugation. In Fig. 4 we show results of our studies on the fractions of surviving ions as a function of the projectile velocity. The normal velocity components
fr
25 scattering
angle
(“)
Fig. 3. Angular distribution observed for the scattering of 3 keV Hei-ions from an LiF(100ksurface for particles emerging from the surface as neutral atoms (open circles) and as ions (full circles). The two distributions are normalized to the same height of the maximum. The fraction of surviving ions is about 15%.
Letters A 220 (1996) 102-106
80 s
,
z .-0 2
.
60-
0 . .
50 -
He+ - LiF
.
Ne* - LiF
.
Ar” - LIF
-
l
z
40-
75
30-
E .-0
20 -
:: z
1
I
l**
m-
l
0 0
l
Oo
lo00
0
. Ob b. @ I, 02
L C.l velocity
l
_, 0.3
(a.~.)
Fig. 4. Fractions of surviving ions as function of the projectile velocity for He+-ions (open circles), Ne+-ions (full circles), and Are-ions @ulI triangles). The energy for the motion normal to the surface plane is about 3 eV.
in these measurements are kept constant by a corresponding adjustment of the angle of incidence fin. In addition to He”-projectiles we also studied the neutralization of Ne*- and Arf-ions. As can be seen from the data in Fig. 4, we observe for Nef even higher fractions of surviving ions (up to 75%), whereas Ar+-ions are completely neutralized. The ion-fractions show a pronounced decrease with increasing projectile velocity and vanish practically at velocities of about 0.25-0.3 a.u. Here it is import~t to note that in this range of velocities the ion-fractions for neutral projectiles are negligible (less than 1%). In interpreting our results we first discuss the different behaviour of A.r+- in contrast to He+- and Nef-ions. This is simply explained by the energies of the F 2p-electrons forming the bulk valence band from about 12 to 16 eV [ 141. Then the (slightly shifted) binding energy of the Ar I 3s’ 3p6 ’ So-ground term E, = 15.8 eV is in resonance with this fully occupied band, and AI+-ions are efficiently neutralized via a resonant one-electron ~Meling process (“resonant neutralization”). The binding energies of the He I 1s2 ‘So- and Ne I 2s22p6 ’ S,-ground terms amount to E, = 24.6 eV and E, = 21.6 eV. Since furthermore no excited atomic states are in resonance with the valence band, neutralization has to proceed via a two-electron Auger-transfer (see also Fig. 1) 2151.
T. Hecht ef al./Physics
The new aspect of our work is related to specific features of the Auger-neutralization process in front of the surface of an insulator. For metals (see Fig. 1, left side) this process is very efficient, since the second electron can be excited to unoccupied electronic levels in the conduction band and in vacuum. However, for an insulator with a large band gap ( Eg = 14 eV for LiF [14]) available final states for the excited electrons can be a substantial problem. This is indicated for the He I-groundstate in front of a LiF-surface on the right side of Fig. 1. In addition, we expect from the large binding energies of LiF-valence band electrons a reduced spill-out to vacuum and closer distances for an efficient Auger neutralization than for metals. Based on the local character of the F 2p-electrons forming the valence band we propose that the neutralization of the He+- and Ne+-ions during the scattering in front of LiF proceeds via an interatomic Auger transition, where two electrons from different F--sites are involved. A similar process is discussed for the decay of inner shell holes at the alkali-sites of alkali-halides f 16-201. The ~ansition matrix element has the form Tk.a;.A
.B =
%(r,)~k(r2)
//
X-
I
~~-(r,)~~-(~*)
dr,
dr,,
(1)
I r12 I where ‘p,, (Pk are wave functions of the atomic groundstate and the excited electron, CD;-, @& are wavefunctions for electrons centered at F--sites A, B. Note that for the matrix element the overlap of cp,
r=
27r(
VB
P(E,) d+~“*PM
d&*
?;,a:A.R12S(t3,+E2-Ek--E,*),
(2)
Leners A 220 11996) 102-106
105
where P(E) is the density of states of the LiF-valence band (VB), E,, E* are binding energies of valence band electrons, E, = k*/2 is the free electron energy, and E,* is the binding energy of the atomic state slightly modified close to the surface. In order to keep the discussion simple we neglect here the hole-hole interaction in the final state [16] which is expected to have energies l/R,, G== eV (F, = “high frequency” dielectric constant) for our system. As a result of energy conservation (S-function in Eq. (2)) for LiF with F 2p-valence band energies E, ,E* 2 12 eV the number of valence band states available for AN of He+-ions (E, = 24 eV1 is small. This will reduce the AN-rates. For Ne+-ions (E, = 21 eV) AN should be suppressed completely in this respect. The projectile energies Ep are clearly larger than the interaction energies E, so that the dynamics of the local AN-process has to be considered. The final interaction times lead to a “collisional broadening”, and the a-function has to be replaced by a distribution of finite width. Then the number of available states for AN and con~quently also transition rates for the AN-process are expected to be enhanced. The pronounced decrease of the ion-fractions with increasing velocity might be attributed to this effect. Aside from the kinematics also unoccupied surface states I211 and imperfections of the crystal can cause a general e~~cement of AN-rates. Recent work on electron spectra obtained with He+ions impinging on a very thin LiF-film [22] seems to support this assumption, even though this work may not directly be compared with our studies performed with a single crystal. For further info~ations on the neutralization process, we performed some simple calculations of the occupation of the ion P, during the scattering process. We assumed classical trajectories deduced by using screened interatomic potentials and parameterized the total AN-rate according to W( ~1 = W. expt- z/z,1 (z is referred to the surface plane). Also an energy gain via image charge interaction A Ei, is taken into account. For the neutralization of He+-ions in front of a metal surface the parameters W, = 1 a.u. and z, from about 1 au. to 1.5 au. are reported [1,3,8]. This rate is sufficiently large that P, (obtained from the master equation d P+/dt = - WP,) vanishes, before
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the projectiles reach the distance of closest approach. The effective distance of neutralization is about z, = 6 to 8 au., and no ions survive on the trajectory. From the data for He+- and Ne+-ions at the lowest velocities of our study (Fig. 4) and from the poor dependence of the ion-fractions on the normal energy Ez [23] we can reproduce the data with the parameters z, = 1.5 a.u. and W, = 0.006 a.u. (He), W, = 0.002 a.u. (Ne). These estimates on the ANrates are more than two orders of magnitude smaller than the rates observed for metal-surfaces. In conclusion, we report on first studies on charge fractions of noble gas ions in grazing collisions with a LiF(lOO)-surface. For He+- and Ne+-projectiles we observe that substantial fractions of ions survive the scattering process despite long interaction times with the solid. We interpret this finding by small Augerneutralization rates due to the specific electronic structure of the insulator; i.e. high binding energies of valence band electrons and a large band gap. In additional experiments we observed no effect on the fractions of surviving ions for projectiles of higher charges (He’+, Ne2+, etc.). This indicates that ions of larger binding energies seem to efficiently capture electrons to form He+- and Ne+-ions before the turning point of the trajectory is reached. The effect of suppression of the Auger-process is reduced with increasing projectile energy which we attribute to collisional broadening effects. We hope that this work and our attempts to interpret the data will stimulate further studies on this interesting problem.
Acknowledgements
This work is supported by the Deutsche Forschungsgemeinschaft (DFG) under contract Wi
Letters A 220 (1996) 102-106
1336/l-l. One of us (A.G.B.) gratefully acknowledges generous support from the Alexander von Humboldt foundation.
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