- Email: [email protected]
Dependence of the electron emission statistics on the charge state of He ions impinging on metals
Nuclear Instruments and Methods
in
Physics Research B 125 (1997) 41-44
Beam Interactions with Materials 8 Atoms
EISEVIER
Dependence of the electron emission statistics on the charge state of He ions impinging on metals 0. Benka Ins~iturfir
Experimenralphysik,
* ,
T. Fink, A. Schinner, E. Steinbauer
Johanner KepIer Uniuersitiir Linz, AltenbergerstraJ?e 69. A-4040 Linz, Austria
Abstract The distribution of the number of emitted electrons is measured for the impact of He+ and He*+ ions on Al, Cu and Au targets. The ion energy is between 0.6 MeV (He’) and 4.8 MeV (He*‘>. The obtained results are very well represented by Polya distributions and the parameters of the fitted distributions are given. The measured distributions for He+ impact are explained by a model which takes into account stripping of the bound electron. It is found that the mean depth, where He+ ions penetrate the solid without losing their bound electron, is about 10 A and it decreases with increasing projectile energy. These results, obtained from measurements of the emission statistics, are also compared with results obtained from yield measurements.
1. Introduction The interaction of swift light ions with solid surfaces leads to kinetic electron emission for ion energies larger than 100 keV. Surveys of investigations of this ion induced electron emission (IIEE) process are given in reviews by Hasselkamp [I] and Rothard et al. [2]. An important quantity of IIEE is the mean number of emitted electrons per incident ion, the electron emission yield y. According to the most common theoretical models [3,4], y should be proportional to the stopping power S of a target surface layer with a thickness, which is equal to the mean escape length of the emitted electrons, with respect to the impinging ions, y = n/s,
(1)
with A depending only on the target material. In a recent work [5], the electron yield of Al, Cu and Au was studied for impact of H+, He+ and He*+ ions to test relation (1) for light ion impact. The energy of the ions was between 0.5 MeV (H+) and 4.8 MeV (He”). It was found that under the following conditions Eq. (1) is a good approximation to describe the yield for H+ and He*+ impact: For He*+ the stopping power of a bare He*+ ion has to be used (not the equilibrium stopping power of He ions). A correction factor f, which describes the effect of collective electric fields within the metal due to the positive charge channel along the ion path, has to be included.
* Corresponding author: Fax: [email protected]. 0168-583X/97/$17.00
The yield for He+ impact is explained by considering two contributions: the yield of a He+ ion (with one bound electron) and the yield of a He2+ ion plus a stripped electron with the same velocity. It was found that He+ ions contribute to the total yield with a probability of 35% almost independent of the ion velocity. This means that He+ ions pass a length of about 10 A in the solid without losing their bound electron. Another important property of IIEE is the statistical distribution of the number of emitted electrons. In a recent work [6], the emission statistics was studied for impact of light bare ions with energies in the MeV range on metals. It was shown that this distribution is very well represented by a Polya distribution:
+ 43-732-2468-9677; e-mail:
P,(&
b)=$(l+bp)yfJ[*
+(i-
l)b],
(2) where p is the mean value (emission yield) and b describes the deviation from a Poisson distribution. For projectiles with charge Ze and energy E it is found that b is roughly proportional to E/Z*. This result is explained by a model which proposes that the deviation from the Poisson distribution leading to the Polya distribution is caused by the electron cascade. The aim of the present work is to present measurements of the electron emission statistics for He+ and He*+ impact on metals and to compare the results of these measurements with results of previous yield measurements.
0 1997 Elsevier Science B.V. All rights reserved
PII SO 168-583X(96)00899-3
II. SECONDARY EMISSION - EXPERIMENT
42
0. Bmku ri al. /Nuci.
2. Experiment 0 20
The statistical distribution of the number of emitted electrons was measured for impact of He+ and He’+ ions on Al, Cu and Au targets. The ion energies were between 0.6 MeV (He+) and 4.8 MeV (He*‘). The experimental setup is described in detail in Refs. [6,7]. The projectiles were obtained from the I .6 MV tandem accelerator of Linz University. Measurements were performed in an UHV measuring chamber. The working pressure was 4 X IO- ‘O mbar with all valves open to the beamline. The targets were produced by vacuum evaporation on polished stainless-steel backings and subsequently moved to a manipulator in the UHV chamber without breaking the vacuum. The angle of incidence of the ion beam with respect to the surface normal of the target was 21”. The targets were on a potential of U = - 20 kV so that the emitted electrons were accelerated and focused to a solid-state detector (PIPS type). If n electrons are emitted per impinging ion, a single pulse will be produced at the detector corresponding to an energy neU, if the total energy of the electrons is deposited in the detector. Therefore, the number of emitted electrons can be obtained from energy spectra measured with the solid-state detector.
3. Results The measured energy spectra of the accelerated electrons were evaluated by a method similar to that used by Lakits et al. [8]; it is described in Ref. [7]. The probabilities pn for emission of n electrons are obtained from a fit of the measured spectra. All distributions of pn are very well approximated by a Polya distribution. Therefore the mean p and parameter b of the distribution are determined by a fit of the energy spectra assuming a Polya distribution of the probabilities p,. Figs. I and 2 show the obtained values b for impact of He+ and He2+ ions depending on the impact energy E of I 045 0.40
-
030
-
025
I.
4
u
’
I
I
’
1
l .
-
0.0
*
‘g
b
1’8’
n
0 00
0.0
0.5
1.0
I
,
1.5
Energy (MeV)
Fig. 2. Polya parameter h versus projectile energy for impact of He+ and He’+ ions on Cu and Au.
ions. It can be seen that b is almost proportional to E He2+ impact, but that there are significant deviations He+ impact, which increase with decreasing energy. high energies, the values b for He+ impact approach values for He”. Under the assumption of a Polya distribution, the emission yield y is given by the mean p and it can be compared to yield results from current integration measurements [5]. For yield values y > 4, where the probability p. for emitting no electron is negligibly small, reasonable agreement of both values is found. Generally, the yield from emission statistics measurements is higher than the yield from current integration measurements by 2-3%. the for for For the
4. Discussion To understand the behaviour of the values b it is helpful to take a closer look at the yield data. In Fig. 3 the results from Ref. [5] are shown. It can be seen that the yield for He + impact is also close to the yield for He2+
I
. . .
l
.
A
CJ
.
cl .
0 0
mi
0
.
’ 0.5
C"u
0
0
. .*
0.05
005
0
. -
0.10
cl
.
-
0.10
’
0 15
0
. -
0.00
I
-
0.35
D
.
. * I
hsrr. ud Meth. in Phys. Res. B 125 11997141-44
’ 1 .o
l
;
0
0
Al
.
’ 1.5
’
I 2.0
‘
Ewgy
’ 2.5
’ 3.0
.
’ 3.5
’ 4.0
,
’ 4.5
,
’ 5.0
(MeI’)
Fig. I. Polya parameter h versus projectile energy for impact of He+ and He” ions on Al.
01
0
’ 1
.
0
1
’
’
’
11
2
3
4
5
Energy (MeV)
Fig. 3. Emission yield versus projectile energy for impact of He+ (full symbols) and He’+ (open symbols) on Au, Cu, and Al.
43
0. Benka et nl./Nucl. Instr. and Meth. in Phys. Res. B 125 (1997) 41-44
impact and that the difference between both yields decreases with increasing energy at low energies. For some medium energies both yields are identical. This cross over is not found for the values b. In order to give a quantitative interpretation of the b values a model was developed, which takes into account the stripping of the bound electron of He+, similar to the model used for the interpretation of the yield data [5]: He+ ions impinge on the solid and travel for some distance with their bound electron, giving rise to a distribution Dl of the number of emitted electrons. After the stripping process, the He’+ ions and the stripped electrons will produce another distribution D2. Both distributions will convolute to a finally observed distribution D. The probability to find the He ion with a bound electron within me escape depth of the emitted electrons is defined as the value c. Consequently, the probability that the He ion is in the state He’+ plus a stripped electron equals 1 - c. All experimentally observed distributions were close to Polya distributions. Therefore, we assume the distributions Dl, D2 and D to be Polya distributions Pl(p,, b,), P2( p2, b,) and P( CL,b). It can be shown that two Polya distributions P 1 and P2 convolute to a Polya distribution, if, and only if (3) The resulting Polya parameters
are then given by:
cL=11,+/% and b=
db,
+ db,
(4) (k
+ CL*)*
and b2=?.
b
Insetting Eqs. (5) and (6) into Eq. (4) allows us to express b as a function of c. If the Polya parameters in Eqs. (5) and (6) are known, this relation can be used to obtain the probability c from experimental values b. We used the following approximations to obtain the required Polya parameters in Eqs. (5) and (6): The yield pn,_, was taken from the evaluation of the yield data for He + impact as given in Ref. [S]. The value b Hei was set equal to the b value for the impact of H+ ions with the same velocity. This is based on the assumption that b is proportional to (u/Qj2, where v and Q denote velocity and charge of the impinging ion, respectively [6]. Similarly the yield pHeZe was set equal to the sum of the yields for He’+ impact and the yield for the impact of an electron of equal velocity. Since we assume that He*+ does not change its charge state at our energies, the yield for He*+ was taken from the experimental results of Ref. [S]. Experimental yields for electrons were also obtained from Ref. [5]. The value for the parameter bHeZe was calculated by convolution of the distributions for He*+ and for the electron using Eq. (4): CL, and b, were set equal to the yield and the value b for He*+ impact. For p2 the measured yield for electron impact from Ref. [5] was used, for b, the b value for H* impact with the same velocity as the He*+ ion was taken because it is again assumed that the statistical distribution of the number of emitted electrons does not depend on the sign of the charge at first order (Ref. [6], see above).
It should be emphasised that even in the case that Eq. (3) is fulfilled only approximately, the convolution is reasonably close to a Polya distribution with p and b from Eq. (4). Let us assume that P,,,( pne,, b,,,) is the Polya distribution produced by He + ions which do not change their charge state in the surface layer. If the probability I - c for a He + ion to lose its electron is nonzero, the resulting Polya distribution P 1 has the parameters
0.3 0
PI = ‘PHel
_
b,=!k. C
0.1 -
(5)
Similarly, let us assume that P Hc_e(pneZe, bHeZe) is the distribution produced by a He” ion plus its stripped electron, which do not change the charge state in the surface layer. Then the resulting Polya distribution P 2 has the parameters pL2= ( 1 - C)pHeZe
0 l
0.2 -
and
(6)
l-c
. 0.0 1.0
n
ALy cu,y
0
A
0
0 0
A Au.y 0 Al,s 0
CU,S h. Au,s 1.5
*
0
A 0
0
0
He+ I , I 2.5 2.0 Energy (MeV)
I 3.0
3.5
Fig. 4. Probability c that the He+ ion has a bound electron for impact of He+ on Al, Cu and Au versus ion energy. Open symbols: results obtained from emission statistics measurements, full symbols: results obtained from yield measurements by current integration.
II. SECONDARY EMISSION - EXPERIMENT
0. Bet&a ef al. /Nucl.
44
Insrr. and Merh. in Phys. Res. 3 I25 (1997141-44
The values pHe,, b,,, pHcZe, bHcZc and measured values b for impact of He+ ions are now used to calculate c from Eq. (4). The so-obtained results are shown in Fig. 4 and they are compared to corresponding results from yield measurements [5]. It can be seen that for low energies both methods agree reasonably and yield the lowest c value for Cu targets. The results from the statistics measurement decrease with increasing energy, whereas the values obtained from the yield measurement are almost independent of the projectile energy. A decrease of the probability c with increasing energy might be expected, because the equilibrium charge of He ions in solids obtained from stopping power measurements 191 is increasing with increasing ion velocity. The penetration depth of HP+ ions can be defined as the distance the He’ ion moves from the surface until it strips off the electron. The penetration depths will have an exponential distribution with a mean value 1. Under the assumption of an exponentially distributed escape probability of the excited electrons (mean escape depth d), the mean penetration depth I can be calculated by convolution: ,=&C I-c’ A mean electron escape depth of d = 15 A [2] gives a mean penetration depth I of about 10 A, which decreases with increasing projectile energy. This result roughly agrees with values obtained from the evaluation of yield measurements [S]. The uncertainty of the calculated values c can be hardly estimated because of the large number of assumptions made. For the convolutions envolved in the evaluation procedure the required condition, Eq. (3). is not exactly fulfilled. However, at first approximation, yields where Q is the charge and u are proportional to (Q/u)‘, the velocity of a projectile, whereas the parameters b are proportional to (u/Qj2. Therefore, the product is almost independent of u and Q, thus fulfilling Eq. (3). The experimental fact that the measured distributions for He+ impact are very close to the Polya distribution also confirms the assumptions made. Another critical approximation is made for b,,, and of the value b for the electron, which are both set equal to the value b for H+ impact at the same velocity.
5. Conclusion Results of measurements of the statistical distribution of the number of emitted electrons for impact of He+ and He2+ ions are presented. The measured distributions are very well approximated by Polya distributions. The Polya parameters for He+ impact are explained by a model which takes into account the stripping of the bound electron. The measured number distribution is assumed to be a convolution of distributions generated by unstripped and by stripped ions. These individual contributions are calculated using a previously proposed cascade model for emission statistics [6]. Using this model and the experimental data the stripping probability for He+ ions is calculated. Reasonable agreement is found with comparable results obtained from yield measurements by current integration. This agreement also confirms the cascade model for electron emission induced by light ion impact on metals with ion energies in the MeV region.
Acknowledgement This work was supported by the Austrian “Fonds zur Fijrderung der wissenschaftlichen Forschung”, Projects No. PO752 1-PHY and P09504-PHY.
References
[ 11 D. Hasselkamp, in: Particle Induced Electron Emission II, Springer Tracts in Modem Physics, Vol. 123 (Springer, Berlin, 1992). 121 H. Rothard, K.O. Groeneveld and J. Kemmler, in: Particle Induced Electron Emission II, Springer Tracts in Modem Physics, Vol. 123 (Springer, Berlin, 1992). [3] E.J. Stemglass, Phys. Rev. 108 (1957) 1. [4] J. Schou, Scanning Microsc. 2 (1988) 607. 1510. Benka, M. Pfaffenlehner and A. Schinner, Nucl. In&. and Meth. B 117 (1996) 350. [6] 0. Benka, A. Schinner and T. Fink, Phys. Rev. A 51 (1995) 2281. 1710. Benka, E. Steinbauer, 0. Bolik and T. Fink, Nucl. Instr. and Meth. B 93 (1994) 156. [8] G. Lakits. F. Aumayr and H.P. Winter. Rev. Sci. Instr. 60 (1989) 3151. 191Ch. Eppacher and D. Semrad, Nucl. Instr. and Meth. B 67 (1992) 138.
in
Physics Research B 125 (1997) 41-44
Beam Interactions with Materials 8 Atoms
EISEVIER
Dependence of the electron emission statistics on the charge state of He ions impinging on metals 0. Benka Ins~iturfir
Experimenralphysik,
* ,
T. Fink, A. Schinner, E. Steinbauer
Johanner KepIer Uniuersitiir Linz, AltenbergerstraJ?e 69. A-4040 Linz, Austria
Abstract The distribution of the number of emitted electrons is measured for the impact of He+ and He*+ ions on Al, Cu and Au targets. The ion energy is between 0.6 MeV (He’) and 4.8 MeV (He*‘>. The obtained results are very well represented by Polya distributions and the parameters of the fitted distributions are given. The measured distributions for He+ impact are explained by a model which takes into account stripping of the bound electron. It is found that the mean depth, where He+ ions penetrate the solid without losing their bound electron, is about 10 A and it decreases with increasing projectile energy. These results, obtained from measurements of the emission statistics, are also compared with results obtained from yield measurements.
1. Introduction The interaction of swift light ions with solid surfaces leads to kinetic electron emission for ion energies larger than 100 keV. Surveys of investigations of this ion induced electron emission (IIEE) process are given in reviews by Hasselkamp [I] and Rothard et al. [2]. An important quantity of IIEE is the mean number of emitted electrons per incident ion, the electron emission yield y. According to the most common theoretical models [3,4], y should be proportional to the stopping power S of a target surface layer with a thickness, which is equal to the mean escape length of the emitted electrons, with respect to the impinging ions, y = n/s,
(1)
with A depending only on the target material. In a recent work [5], the electron yield of Al, Cu and Au was studied for impact of H+, He+ and He*+ ions to test relation (1) for light ion impact. The energy of the ions was between 0.5 MeV (H+) and 4.8 MeV (He”). It was found that under the following conditions Eq. (1) is a good approximation to describe the yield for H+ and He*+ impact: For He*+ the stopping power of a bare He*+ ion has to be used (not the equilibrium stopping power of He ions). A correction factor f, which describes the effect of collective electric fields within the metal due to the positive charge channel along the ion path, has to be included.
* Corresponding author: Fax: [email protected]. 0168-583X/97/$17.00
The yield for He+ impact is explained by considering two contributions: the yield of a He+ ion (with one bound electron) and the yield of a He2+ ion plus a stripped electron with the same velocity. It was found that He+ ions contribute to the total yield with a probability of 35% almost independent of the ion velocity. This means that He+ ions pass a length of about 10 A in the solid without losing their bound electron. Another important property of IIEE is the statistical distribution of the number of emitted electrons. In a recent work [6], the emission statistics was studied for impact of light bare ions with energies in the MeV range on metals. It was shown that this distribution is very well represented by a Polya distribution:
+ 43-732-2468-9677; e-mail:
P,(&
b)=$(l+bp)yfJ[*
+(i-
l)b],
(2) where p is the mean value (emission yield) and b describes the deviation from a Poisson distribution. For projectiles with charge Ze and energy E it is found that b is roughly proportional to E/Z*. This result is explained by a model which proposes that the deviation from the Poisson distribution leading to the Polya distribution is caused by the electron cascade. The aim of the present work is to present measurements of the electron emission statistics for He+ and He*+ impact on metals and to compare the results of these measurements with results of previous yield measurements.
0 1997 Elsevier Science B.V. All rights reserved
PII SO 168-583X(96)00899-3
II. SECONDARY EMISSION - EXPERIMENT
42
0. Bmku ri al. /Nuci.
2. Experiment 0 20
The statistical distribution of the number of emitted electrons was measured for impact of He+ and He’+ ions on Al, Cu and Au targets. The ion energies were between 0.6 MeV (He+) and 4.8 MeV (He*‘). The experimental setup is described in detail in Refs. [6,7]. The projectiles were obtained from the I .6 MV tandem accelerator of Linz University. Measurements were performed in an UHV measuring chamber. The working pressure was 4 X IO- ‘O mbar with all valves open to the beamline. The targets were produced by vacuum evaporation on polished stainless-steel backings and subsequently moved to a manipulator in the UHV chamber without breaking the vacuum. The angle of incidence of the ion beam with respect to the surface normal of the target was 21”. The targets were on a potential of U = - 20 kV so that the emitted electrons were accelerated and focused to a solid-state detector (PIPS type). If n electrons are emitted per impinging ion, a single pulse will be produced at the detector corresponding to an energy neU, if the total energy of the electrons is deposited in the detector. Therefore, the number of emitted electrons can be obtained from energy spectra measured with the solid-state detector.
3. Results The measured energy spectra of the accelerated electrons were evaluated by a method similar to that used by Lakits et al. [8]; it is described in Ref. [7]. The probabilities pn for emission of n electrons are obtained from a fit of the measured spectra. All distributions of pn are very well approximated by a Polya distribution. Therefore the mean p and parameter b of the distribution are determined by a fit of the energy spectra assuming a Polya distribution of the probabilities p,. Figs. I and 2 show the obtained values b for impact of He+ and He2+ ions depending on the impact energy E of I 045 0.40
-
030
-
025
I.
4
u
’
I
I
’
1
l .
-
0.0
*
‘g
b
1’8’
n
0 00
0.0
0.5
1.0
I
,
1.5
Energy (MeV)
Fig. 2. Polya parameter h versus projectile energy for impact of He+ and He’+ ions on Cu and Au.
ions. It can be seen that b is almost proportional to E He2+ impact, but that there are significant deviations He+ impact, which increase with decreasing energy. high energies, the values b for He+ impact approach values for He”. Under the assumption of a Polya distribution, the emission yield y is given by the mean p and it can be compared to yield results from current integration measurements [5]. For yield values y > 4, where the probability p. for emitting no electron is negligibly small, reasonable agreement of both values is found. Generally, the yield from emission statistics measurements is higher than the yield from current integration measurements by 2-3%. the for for For the
4. Discussion To understand the behaviour of the values b it is helpful to take a closer look at the yield data. In Fig. 3 the results from Ref. [5] are shown. It can be seen that the yield for He + impact is also close to the yield for He2+
I
. . .
l
.
A
CJ
.
cl .
0 0
mi
0
.
’ 0.5
C"u
0
0
. .*
0.05
005
0
. -
0.10
cl
.
-
0.10
’
0 15
0
. -
0.00
I
-
0.35
D
.
. * I
hsrr. ud Meth. in Phys. Res. B 125 11997141-44
’ 1 .o
l
;
0
0
Al
.
’ 1.5
’
I 2.0
‘
Ewgy
’ 2.5
’ 3.0
.
’ 3.5
’ 4.0
,
’ 4.5
,
’ 5.0
(MeI’)
Fig. I. Polya parameter h versus projectile energy for impact of He+ and He” ions on Al.
01
0
’ 1
.
0
1
’
’
’
11
2
3
4
5
Energy (MeV)
Fig. 3. Emission yield versus projectile energy for impact of He+ (full symbols) and He’+ (open symbols) on Au, Cu, and Al.
43
0. Benka et nl./Nucl. Instr. and Meth. in Phys. Res. B 125 (1997) 41-44
impact and that the difference between both yields decreases with increasing energy at low energies. For some medium energies both yields are identical. This cross over is not found for the values b. In order to give a quantitative interpretation of the b values a model was developed, which takes into account the stripping of the bound electron of He+, similar to the model used for the interpretation of the yield data [5]: He+ ions impinge on the solid and travel for some distance with their bound electron, giving rise to a distribution Dl of the number of emitted electrons. After the stripping process, the He’+ ions and the stripped electrons will produce another distribution D2. Both distributions will convolute to a finally observed distribution D. The probability to find the He ion with a bound electron within me escape depth of the emitted electrons is defined as the value c. Consequently, the probability that the He ion is in the state He’+ plus a stripped electron equals 1 - c. All experimentally observed distributions were close to Polya distributions. Therefore, we assume the distributions Dl, D2 and D to be Polya distributions Pl(p,, b,), P2( p2, b,) and P( CL,b). It can be shown that two Polya distributions P 1 and P2 convolute to a Polya distribution, if, and only if (3) The resulting Polya parameters
are then given by:
cL=11,+/% and b=
db,
+ db,
(4) (k
+ CL*)*
and b2=?.
b
Insetting Eqs. (5) and (6) into Eq. (4) allows us to express b as a function of c. If the Polya parameters in Eqs. (5) and (6) are known, this relation can be used to obtain the probability c from experimental values b. We used the following approximations to obtain the required Polya parameters in Eqs. (5) and (6): The yield pn,_, was taken from the evaluation of the yield data for He + impact as given in Ref. [S]. The value b Hei was set equal to the b value for the impact of H+ ions with the same velocity. This is based on the assumption that b is proportional to (u/Qj2, where v and Q denote velocity and charge of the impinging ion, respectively [6]. Similarly the yield pHeZe was set equal to the sum of the yields for He’+ impact and the yield for the impact of an electron of equal velocity. Since we assume that He*+ does not change its charge state at our energies, the yield for He*+ was taken from the experimental results of Ref. [S]. Experimental yields for electrons were also obtained from Ref. [5]. The value for the parameter bHeZe was calculated by convolution of the distributions for He*+ and for the electron using Eq. (4): CL, and b, were set equal to the yield and the value b for He*+ impact. For p2 the measured yield for electron impact from Ref. [5] was used, for b, the b value for H* impact with the same velocity as the He*+ ion was taken because it is again assumed that the statistical distribution of the number of emitted electrons does not depend on the sign of the charge at first order (Ref. [6], see above).
It should be emphasised that even in the case that Eq. (3) is fulfilled only approximately, the convolution is reasonably close to a Polya distribution with p and b from Eq. (4). Let us assume that P,,,( pne,, b,,,) is the Polya distribution produced by He + ions which do not change their charge state in the surface layer. If the probability I - c for a He + ion to lose its electron is nonzero, the resulting Polya distribution P 1 has the parameters
0.3 0
PI = ‘PHel
_
b,=!k. C
0.1 -
(5)
Similarly, let us assume that P Hc_e(pneZe, bHeZe) is the distribution produced by a He” ion plus its stripped electron, which do not change the charge state in the surface layer. Then the resulting Polya distribution P 2 has the parameters pL2= ( 1 - C)pHeZe
0 l
0.2 -
and
(6)
l-c
. 0.0 1.0
n
ALy cu,y
0
A
0
0 0
A Au.y 0 Al,s 0
CU,S h. Au,s 1.5
*
0
A 0
0
0
He+ I , I 2.5 2.0 Energy (MeV)
I 3.0
3.5
Fig. 4. Probability c that the He+ ion has a bound electron for impact of He+ on Al, Cu and Au versus ion energy. Open symbols: results obtained from emission statistics measurements, full symbols: results obtained from yield measurements by current integration.
II. SECONDARY EMISSION - EXPERIMENT
0. Bet&a ef al. /Nucl.
44
Insrr. and Merh. in Phys. Res. 3 I25 (1997141-44
The values pHe,, b,,, pHcZe, bHcZc and measured values b for impact of He+ ions are now used to calculate c from Eq. (4). The so-obtained results are shown in Fig. 4 and they are compared to corresponding results from yield measurements [5]. It can be seen that for low energies both methods agree reasonably and yield the lowest c value for Cu targets. The results from the statistics measurement decrease with increasing energy, whereas the values obtained from the yield measurement are almost independent of the projectile energy. A decrease of the probability c with increasing energy might be expected, because the equilibrium charge of He ions in solids obtained from stopping power measurements 191 is increasing with increasing ion velocity. The penetration depth of HP+ ions can be defined as the distance the He’ ion moves from the surface until it strips off the electron. The penetration depths will have an exponential distribution with a mean value 1. Under the assumption of an exponentially distributed escape probability of the excited electrons (mean escape depth d), the mean penetration depth I can be calculated by convolution: ,=&C I-c’ A mean electron escape depth of d = 15 A [2] gives a mean penetration depth I of about 10 A, which decreases with increasing projectile energy. This result roughly agrees with values obtained from the evaluation of yield measurements [S]. The uncertainty of the calculated values c can be hardly estimated because of the large number of assumptions made. For the convolutions envolved in the evaluation procedure the required condition, Eq. (3). is not exactly fulfilled. However, at first approximation, yields where Q is the charge and u are proportional to (Q/u)‘, the velocity of a projectile, whereas the parameters b are proportional to (u/Qj2. Therefore, the product is almost independent of u and Q, thus fulfilling Eq. (3). The experimental fact that the measured distributions for He+ impact are very close to the Polya distribution also confirms the assumptions made. Another critical approximation is made for b,,, and of the value b for the electron, which are both set equal to the value b for H+ impact at the same velocity.
5. Conclusion Results of measurements of the statistical distribution of the number of emitted electrons for impact of He+ and He2+ ions are presented. The measured distributions are very well approximated by Polya distributions. The Polya parameters for He+ impact are explained by a model which takes into account the stripping of the bound electron. The measured number distribution is assumed to be a convolution of distributions generated by unstripped and by stripped ions. These individual contributions are calculated using a previously proposed cascade model for emission statistics [6]. Using this model and the experimental data the stripping probability for He+ ions is calculated. Reasonable agreement is found with comparable results obtained from yield measurements by current integration. This agreement also confirms the cascade model for electron emission induced by light ion impact on metals with ion energies in the MeV region.
Acknowledgement This work was supported by the Austrian “Fonds zur Fijrderung der wissenschaftlichen Forschung”, Projects No. PO752 1-PHY and P09504-PHY.
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