- Email: [email protected]
Low-energy electrons produced in ion-atom collisions
-Eig
NIIM B
Beam lnteactions with Materials 8 Atoms
ELSEVIER
Nuclear Instruments and Methods in Physics Research B 132 (1997) 24lL245
Low-energy electrons produced in ion-atom S. Swirez ‘. l, A.D. Gonzalez
collisions
‘. *
Abstract We have measured slow electron spectra arising from proton impact at intermediate energies. For the targets investigated, H2. He and Ar, the asymmetry between the emission at forward and backward directions for soft electrons is found to present a maximum as a function of the beam velocity. We investigate in detail possible contributions to the asymmetry. By comparing results for He+ and He colliding with He we found a significant concentration of electrons close to the saddle-point velocity. Another structure was found for Hz. He and Ar targets bombarded by H This structure fits well into the theoretical prediction of a low-energy binary collision peak. 0 1997 Elsevier Science B.V.
1_ Introduction Within the framework of ion-atom collisions, the study of the electron distributions for electrons emitted with energies smaller than ~10 eV has been recently taken into account by several groups [ 141. In previous articles we have reported on the asymmetry of the peak centered at the velocity I’, = 0 for He and Ne targets [ 1,5,6] and reported on two-center effects on the slow electron emission due to different projectile charges Z,, [6]. The experimental results, as well as the theoretical calculations. have shown that the peak produced by soft collisions is strongly asymmetric in the forwardbackward direction. This asymmetry was inter-
*Corresponding author. Fax: 54 944 45299: e-mail: gonzale/.ccLlcab.cne~c.edu.al ’ Also member of the Consejo National de lnvestigaciones Clentificas y T&micas (CONICET). Argentina. ’ Fax: +54 944 45299: e-mail: [email protected]. ’ Comisibn National de Energia At6mica and Universidad Nxional de Cuyo. Argentina. Olh8-583X/97/%17.00 ID 1997 Elsevier Science B.V. All rights reserved P~~SO168-583X(97)00454-0
preted as due to the effect of the receding prqjectile charge on the low-energy electron distribution. The contribution of two-center effects to the asymmetry of the soft electron peak was discussed theoretically by Fainstein et al. [7] and a parametric analysis was given by Garibotti and Cravero [8]. In this paper we study the asymmetry of the soft electron peak for different targets (H?, He and Ar) as a function of the projectile energy. for protons with energies ranging from 20 to 225 keV. We also report on preliminary experimental data which are related to possible mechanisms contributing to the emission of slow electrons. For this we investigated detailed electron spectra for He+ and He on He collisions. and Hi- colliding with H?, He and Ar as a function of the electron emission angle.
2. Experiment
and results
The experimental apparatus consists of a cylindrical mirror analyzer and has been described
elsewhere [9]. Briefly, the gas target is provided by a hypodermic needle very close to a well collimated beam, which, traversing the target region. is collected downstream in a Faraday Cup. The electron spectrometer is mounted on a rotatable design to allow measurements over 360”. Background electrons produced either by secondary collisions or by extended gas effects are carefully subtracted. Care is also taken to reduce the residual earth magnetic field to a very low value within the sensitivity range of the magnetic-field gauge. The asymmetry of the soft peak is defined as the ratio between the doubly-differential cross-sections (DDCS) for electrons emitted at H = 0” and at 0 = 180”, the DDCS being determined as explained in Bernardi et al. [5]. To correct extended target effects, the emission corresponding to a uniform gas target is subtracted. In Fig. 1 we show the asymmetry of the slow electron emission in the forward-backward direction for three different targets, HZ, He and Ar. as a function of the projectile impact energy. and for an electron emission energy E, = 1 eV. The asymmetry presents a local maximum for the three targets investigated. In particular for a He target, for which more experimental points were measured, the maximum is clearly defined, located at ~100 keV. For Ar and H2 the position of the maxima is close to 100 and 50 keV. respectively. The maxima observed are not easy to explain because of the multielectronic structure of the targets and therefore many collision channels compete. e.g., capture, transfer and ionization, multiple ionization. However, within the scheme of single ionization, a simple correlation process could tentatively explain the presence of a maximum in the asymmetry. For instance, considering a two-electron target, a charged projectile produces the ionization of one electron while the other is polarized towards the projectile Coulomb center. The lower the projectile velocity. the larger the polarization of the bound electron. Therefore, it is clear that a slow electron emitted into the forward direction is subject to a reduced target field, since the polarized electron screens the residual nuclear charge. In this case, the slow forward emission is lowered whereas the emission into the backward direction is increased. In other words. a slow electron
I
r
I
’
I
’
A
I_ 0
c
q rk -0-
T
,
2 50
100
p+Ar
n
p+He
-A-
p+H,
150
200
250
Proton Energy (keV) Fig. I. Electron asymmetry as a function of the incident proton energy for three different targets. Lines are to guide the eye.
emitted into the backward direction is subject to a higher effective residual charge. As a consequence. the asymmetry should decrease for low projectile energies. On the other hand, for higher projectile velocities, the effect of polarization is smaller and so is the electron-electron correlation. Therefore, the asymmetry of the soft electron peak decreases continuously towards the higher projectile-energy range. The case of a multielectron atom, such as Ar. is not so simple since there are multielectronic processes which also contribute to the emission of slow electrons [lo]. As seen in Fig. 1, the electron emission is asymmetric towards the forward direction. The following results will show how different
243
S. Suirez. A.D. Gonxilrr I Nucl. Insir. and Meth. in Phys. Rrs. B 132 (19971 -741-245
mechanisms which produce slow electrons can contribute to the observed asymmetry of the soft electron peak. In the first place, we consider the electron emission into the saddle potential. In order to put in evidence those electrons which are subject to the simultaneous and comparable interaction with both ions, i.e., target and projectile, we chose the symmetric collision systems He++ He and He + He. The experiment is very simple and consists in the comparison of two collision systems. One has a saddle potential in its final state (He+ + He), since for the projectile He+ the probability of electron loss is negligible for energies lower than 50 keV/u [l 11. Therefore an ejected target electron is affected by two He+-Coulomb centers. The other system (He + He) has preponderantly only one Coulomb center in the post-collisional state due to either projectile-electron loss or target ionization. The simultaneous ionization is at least three times smaller for projectile energies lower than 50 keV/u [12]. In Fig. 2 we show the comparison between the forward electron distributions resulting from the collisions He+ + He and He + He. The main difference between them stems in that a saddle potential is present in the first one, while in the second there is essentially only one center acting on an emitted electron. A dominant structure is observed close to the saddle velocity v, = q/2 [13] for the three projectile energies studied. In addition, we have studied the evolution with the angle of emission and observed that the structure is in fact concentrated in the forward direction. These results suggest that slow electrons in ion-atom ionization are strongly aifected by the two-center potential and that the asymmetry of the peak at v, = 0 is, at least partially. due to the action of a saddle potential in the post-collisional state. By studying the target dependence of the electron emission we have observed a structure which we associated tentatively to the production of lowenergy binary electrons [14]. According to Fainstein et al. [14] there is a slow binary peak which arises from the expression L’b= tip cos 8cc; COG t) - 2&)“? where up, 8 and Ei are the projectile velocity, the emission angle and the electron binding energy, respectively. This peak is superim-
+ 4
1.4
34keVh 1
1.2
z
1.0
I
0.8
+?
0.6
-
0.4
53keVlu
1 0,o
. 0.2
0.4
0.6
0.8
1
WP
Fig. 2. DDCS ratio for electron emission at zero degree, as a function of the electron velocity (u,) normalized to the cusp velocity (up), for three different beam energies.
posed on the soft collision peak and is of course responsible for a forward-backward asymmetry. In Fig. 3 we show forward electron distributions (0 N 0’) resulting from the collisions of 225 keV protons on He, Ar and H:! targets. The three spectra are arbitrarily normalized at v, N 4 a.u. We see in this figure a strong target dependence for electron velocities lower than 2 a.u. In particular, a dominant structure is peaked at around -0.5 a.u. for a Hz target and -0.6 a.u. for Ar and He (note that the velocity of the beam is close to 3 a.u.). For projectile energies lower than 60 keV such a target dependence is hardly seen and increases in importance up to the higher energy measured (225 keV). A similar target dependence was observed for the high velocity solution
244
di
225 keV H++H,, He, Ar
!25 keV H++H,
1
-A-@
60
00
0
0= 3o”
=
e= 5o”
-D-
Q= 900
-
0 0
1
2
3
4
0.0
Electron Velocity (a.~.) Fig. 3. DDCS at zero degree, as a function city. for three different gas targets.
of the electron
0.5
10
1.5
2.0
Electron Velocity (a.~.) velo-
v,, = L’~COS 0 + ( r~cos2 0 ~ 2::,)’ ’ [ 141, though it was not so strong. In addition. we found it interesting to study the angular dependence of this effect. The result is shown in Fig. 4. The data show that the intensity of the maximum in the electron distribution decreases for increasing angles. It finally disappears completely for angles larger than 70”. Even though this effect has been previously suggested from a theoretical point of view. it is not easy to confirm our observation from existing and tabulated data [15]. This difficulty is due in part to the problems involved in accurate measurements of low-energy electrons, e.g. contact potentials, remaining magnetic fields, fringing fields, extended target effects. The fluctuations produced by these undesirable effects account for measurements of different gas targets (He and Ar mainly),
Fig. 4. DDCS as a function ferent emission angles.
of the electron
velocity. for four dif-
originated from different researchers, experimental setups or simply from different runs. In our present experiment. we have switched successively to the three targets (He. Ar and HZ). one after the other. and finally we reproduced the measurements with the first one. It is clear that the peak shown in Figs. 3 and 4 alters significantly the asymmetry of the soft electron peak, especially for high projectile velocities, where the effect of the binding energy is smaller and the slow binary peak shifts towards a still lower electron velocity. 3. Conclusions We have measured electron energy distributions as a function of the beam velocity, for different ion and atom beams and a variety of gas targets. We
S. Sucirer. A.D. Gomile:
I Nucl. Insrr. uncl Meth. in Phvs. Rrs. B 132 (1997) 241-245
have investigated low-energy electrons. The asymmetry of the soft electron peak was found to present a maximum as a function of the beam velocity. for the three targets studied, Hz, He and Ar. To obtain details on the contributions leading to the observed asymmetry we have also investigated low-energy distributions for symmetric He and He’ on He collisions. A maximum placed at an electron velocity half of that of the projectile was found. Also, a peak interpreted as the low-energy part of the zero-degree binary encounter peak was found, with a position well predicted by previous theoretical approaches. This peak was investigated also for angles other than 0”. The maximum decreases in intensity as the electron emission angle increases, and disappears beyond 70”. We conclude therefore that electrons emitted into the saddle potential as well as slow binary electrons contribute to a significant asymmetry of the soft electron peak.
Acknowledgements We thank partial support from the Fundacion Antorchas, Buenos Aires, Argentina.
245
References Ill S. Suarez, CR.
Garibotti. G. Bernardi. W. Meckbach, Phys. Rev. Lett. 70 (1993) 418. PI A.B. Voitkiv, J. Phys. B 29 (1996) 5433. 131 R. Moshammer et al.. Phys. Rev. Lett. 73 (1994) 3371. [41 R. Darner et al.. Phys. Rev. Lett. 77 (1996) 4520. [51 G. Bernardi, P. Focke, S. Suarez, W. Meckbach. Phys. Rev. A 50 (1994) 5338. G. Bernardi, P. Focke. W. PI S. Sudrez. C.R. Garibotti. Meckbach, Phys. Rev. A 48 (1993) 4339. [71 P.D. Fainstein, L. Gulyas, F. Martin. A. Salin, Phys. Rev. A 53 ( 1996) 3243. PI CR. Garibotti. W. Cravero, Phys. Rev. A 48 (1993) 2012. S. Suarez, D. Fregenal, P. Focke. W. 191 G. Bernardi, Meckbach, Rev. Sci. Instrum. 67 (1996) 1761. [lOI R.D. DuBois. ST. Manson, Phys. Rev. A 35 (1987) 2007. Ull R.D. DuBois. A. Kover. Phys. Rev. A 40 (1989) 3605. t1A R.D. DuBois. in: AIP Conference Proceedings. Phys. El. At. Coll. 360 (1995) 505. [I31 R.E. Olson, T.J. Gay, H.G. Berry, E.B. Hale. V.D. Irby, Phys. Rev. Lett. 59 (1987) 36. [I41 P.D. Fainstein, V.H. Ponce, R.D. Rivarola. J. Phys. B 24 (1991) 3091. [I51 ME. Rudd, L.H. Toburen. N. Stolterfoht, Atom. Data Nucl. Data Tables 18 (1976) 413; M.E. Rudd. L.H. Toburen, N. Stolterfoht. Atom. Data Nucl. Data Tables 23 (1979) 405.
NIIM B
Beam lnteactions with Materials 8 Atoms
ELSEVIER
Nuclear Instruments and Methods in Physics Research B 132 (1997) 24lL245
Low-energy electrons produced in ion-atom S. Swirez ‘. l, A.D. Gonzalez
collisions
‘. *
Abstract We have measured slow electron spectra arising from proton impact at intermediate energies. For the targets investigated, H2. He and Ar, the asymmetry between the emission at forward and backward directions for soft electrons is found to present a maximum as a function of the beam velocity. We investigate in detail possible contributions to the asymmetry. By comparing results for He+ and He colliding with He we found a significant concentration of electrons close to the saddle-point velocity. Another structure was found for Hz. He and Ar targets bombarded by H This structure fits well into the theoretical prediction of a low-energy binary collision peak. 0 1997 Elsevier Science B.V.
1_ Introduction Within the framework of ion-atom collisions, the study of the electron distributions for electrons emitted with energies smaller than ~10 eV has been recently taken into account by several groups [ 141. In previous articles we have reported on the asymmetry of the peak centered at the velocity I’, = 0 for He and Ne targets [ 1,5,6] and reported on two-center effects on the slow electron emission due to different projectile charges Z,, [6]. The experimental results, as well as the theoretical calculations. have shown that the peak produced by soft collisions is strongly asymmetric in the forwardbackward direction. This asymmetry was inter-
*Corresponding author. Fax: 54 944 45299: e-mail: gonzale/.ccLlcab.cne~c.edu.al ’ Also member of the Consejo National de lnvestigaciones Clentificas y T&micas (CONICET). Argentina. ’ Fax: +54 944 45299: e-mail: [email protected]. ’ Comisibn National de Energia At6mica and Universidad Nxional de Cuyo. Argentina. Olh8-583X/97/%17.00 ID 1997 Elsevier Science B.V. All rights reserved P~~SO168-583X(97)00454-0
preted as due to the effect of the receding prqjectile charge on the low-energy electron distribution. The contribution of two-center effects to the asymmetry of the soft electron peak was discussed theoretically by Fainstein et al. [7] and a parametric analysis was given by Garibotti and Cravero [8]. In this paper we study the asymmetry of the soft electron peak for different targets (H?, He and Ar) as a function of the projectile energy. for protons with energies ranging from 20 to 225 keV. We also report on preliminary experimental data which are related to possible mechanisms contributing to the emission of slow electrons. For this we investigated detailed electron spectra for He+ and He on He collisions. and Hi- colliding with H?, He and Ar as a function of the electron emission angle.
2. Experiment
and results
The experimental apparatus consists of a cylindrical mirror analyzer and has been described
elsewhere [9]. Briefly, the gas target is provided by a hypodermic needle very close to a well collimated beam, which, traversing the target region. is collected downstream in a Faraday Cup. The electron spectrometer is mounted on a rotatable design to allow measurements over 360”. Background electrons produced either by secondary collisions or by extended gas effects are carefully subtracted. Care is also taken to reduce the residual earth magnetic field to a very low value within the sensitivity range of the magnetic-field gauge. The asymmetry of the soft peak is defined as the ratio between the doubly-differential cross-sections (DDCS) for electrons emitted at H = 0” and at 0 = 180”, the DDCS being determined as explained in Bernardi et al. [5]. To correct extended target effects, the emission corresponding to a uniform gas target is subtracted. In Fig. 1 we show the asymmetry of the slow electron emission in the forward-backward direction for three different targets, HZ, He and Ar. as a function of the projectile impact energy. and for an electron emission energy E, = 1 eV. The asymmetry presents a local maximum for the three targets investigated. In particular for a He target, for which more experimental points were measured, the maximum is clearly defined, located at ~100 keV. For Ar and H2 the position of the maxima is close to 100 and 50 keV. respectively. The maxima observed are not easy to explain because of the multielectronic structure of the targets and therefore many collision channels compete. e.g., capture, transfer and ionization, multiple ionization. However, within the scheme of single ionization, a simple correlation process could tentatively explain the presence of a maximum in the asymmetry. For instance, considering a two-electron target, a charged projectile produces the ionization of one electron while the other is polarized towards the projectile Coulomb center. The lower the projectile velocity. the larger the polarization of the bound electron. Therefore, it is clear that a slow electron emitted into the forward direction is subject to a reduced target field, since the polarized electron screens the residual nuclear charge. In this case, the slow forward emission is lowered whereas the emission into the backward direction is increased. In other words. a slow electron
I
r
I
’
I
’
A
I_ 0
c
q rk -0-
T
,
2 50
100
p+Ar
n
p+He
-A-
p+H,
150
200
250
Proton Energy (keV) Fig. I. Electron asymmetry as a function of the incident proton energy for three different targets. Lines are to guide the eye.
emitted into the backward direction is subject to a higher effective residual charge. As a consequence. the asymmetry should decrease for low projectile energies. On the other hand, for higher projectile velocities, the effect of polarization is smaller and so is the electron-electron correlation. Therefore, the asymmetry of the soft electron peak decreases continuously towards the higher projectile-energy range. The case of a multielectron atom, such as Ar. is not so simple since there are multielectronic processes which also contribute to the emission of slow electrons [lo]. As seen in Fig. 1, the electron emission is asymmetric towards the forward direction. The following results will show how different
243
S. Suirez. A.D. Gonxilrr I Nucl. Insir. and Meth. in Phys. Rrs. B 132 (19971 -741-245
mechanisms which produce slow electrons can contribute to the observed asymmetry of the soft electron peak. In the first place, we consider the electron emission into the saddle potential. In order to put in evidence those electrons which are subject to the simultaneous and comparable interaction with both ions, i.e., target and projectile, we chose the symmetric collision systems He++ He and He + He. The experiment is very simple and consists in the comparison of two collision systems. One has a saddle potential in its final state (He+ + He), since for the projectile He+ the probability of electron loss is negligible for energies lower than 50 keV/u [l 11. Therefore an ejected target electron is affected by two He+-Coulomb centers. The other system (He + He) has preponderantly only one Coulomb center in the post-collisional state due to either projectile-electron loss or target ionization. The simultaneous ionization is at least three times smaller for projectile energies lower than 50 keV/u [12]. In Fig. 2 we show the comparison between the forward electron distributions resulting from the collisions He+ + He and He + He. The main difference between them stems in that a saddle potential is present in the first one, while in the second there is essentially only one center acting on an emitted electron. A dominant structure is observed close to the saddle velocity v, = q/2 [13] for the three projectile energies studied. In addition, we have studied the evolution with the angle of emission and observed that the structure is in fact concentrated in the forward direction. These results suggest that slow electrons in ion-atom ionization are strongly aifected by the two-center potential and that the asymmetry of the peak at v, = 0 is, at least partially. due to the action of a saddle potential in the post-collisional state. By studying the target dependence of the electron emission we have observed a structure which we associated tentatively to the production of lowenergy binary electrons [14]. According to Fainstein et al. [14] there is a slow binary peak which arises from the expression L’b= tip cos 8cc; COG t) - 2&)“? where up, 8 and Ei are the projectile velocity, the emission angle and the electron binding energy, respectively. This peak is superim-
+ 4
1.4
34keVh 1
1.2
z
1.0
I
0.8
+?
0.6
-
0.4
53keVlu
1 0,o
. 0.2
0.4
0.6
0.8
1
WP
Fig. 2. DDCS ratio for electron emission at zero degree, as a function of the electron velocity (u,) normalized to the cusp velocity (up), for three different beam energies.
posed on the soft collision peak and is of course responsible for a forward-backward asymmetry. In Fig. 3 we show forward electron distributions (0 N 0’) resulting from the collisions of 225 keV protons on He, Ar and H:! targets. The three spectra are arbitrarily normalized at v, N 4 a.u. We see in this figure a strong target dependence for electron velocities lower than 2 a.u. In particular, a dominant structure is peaked at around -0.5 a.u. for a Hz target and -0.6 a.u. for Ar and He (note that the velocity of the beam is close to 3 a.u.). For projectile energies lower than 60 keV such a target dependence is hardly seen and increases in importance up to the higher energy measured (225 keV). A similar target dependence was observed for the high velocity solution
244
di
225 keV H++H,, He, Ar
!25 keV H++H,
1
-A-@
60
00
0
0= 3o”
=
e= 5o”
-D-
Q= 900
-
0 0
1
2
3
4
0.0
Electron Velocity (a.~.) Fig. 3. DDCS at zero degree, as a function city. for three different gas targets.
of the electron
0.5
10
1.5
2.0
Electron Velocity (a.~.) velo-
v,, = L’~COS 0 + ( r~cos2 0 ~ 2::,)’ ’ [ 141, though it was not so strong. In addition. we found it interesting to study the angular dependence of this effect. The result is shown in Fig. 4. The data show that the intensity of the maximum in the electron distribution decreases for increasing angles. It finally disappears completely for angles larger than 70”. Even though this effect has been previously suggested from a theoretical point of view. it is not easy to confirm our observation from existing and tabulated data [15]. This difficulty is due in part to the problems involved in accurate measurements of low-energy electrons, e.g. contact potentials, remaining magnetic fields, fringing fields, extended target effects. The fluctuations produced by these undesirable effects account for measurements of different gas targets (He and Ar mainly),
Fig. 4. DDCS as a function ferent emission angles.
of the electron
velocity. for four dif-
originated from different researchers, experimental setups or simply from different runs. In our present experiment. we have switched successively to the three targets (He. Ar and HZ). one after the other. and finally we reproduced the measurements with the first one. It is clear that the peak shown in Figs. 3 and 4 alters significantly the asymmetry of the soft electron peak, especially for high projectile velocities, where the effect of the binding energy is smaller and the slow binary peak shifts towards a still lower electron velocity. 3. Conclusions We have measured electron energy distributions as a function of the beam velocity, for different ion and atom beams and a variety of gas targets. We
S. Sucirer. A.D. Gomile:
I Nucl. Insrr. uncl Meth. in Phvs. Rrs. B 132 (1997) 241-245
have investigated low-energy electrons. The asymmetry of the soft electron peak was found to present a maximum as a function of the beam velocity. for the three targets studied, Hz, He and Ar. To obtain details on the contributions leading to the observed asymmetry we have also investigated low-energy distributions for symmetric He and He’ on He collisions. A maximum placed at an electron velocity half of that of the projectile was found. Also, a peak interpreted as the low-energy part of the zero-degree binary encounter peak was found, with a position well predicted by previous theoretical approaches. This peak was investigated also for angles other than 0”. The maximum decreases in intensity as the electron emission angle increases, and disappears beyond 70”. We conclude therefore that electrons emitted into the saddle potential as well as slow binary electrons contribute to a significant asymmetry of the soft electron peak.
Acknowledgements We thank partial support from the Fundacion Antorchas, Buenos Aires, Argentina.
245
References Ill S. Suarez, CR.
Garibotti. G. Bernardi. W. Meckbach, Phys. Rev. Lett. 70 (1993) 418. PI A.B. Voitkiv, J. Phys. B 29 (1996) 5433. 131 R. Moshammer et al.. Phys. Rev. Lett. 73 (1994) 3371. [41 R. Darner et al.. Phys. Rev. Lett. 77 (1996) 4520. [51 G. Bernardi, P. Focke, S. Suarez, W. Meckbach. Phys. Rev. A 50 (1994) 5338. G. Bernardi, P. Focke. W. PI S. Sudrez. C.R. Garibotti. Meckbach, Phys. Rev. A 48 (1993) 4339. [71 P.D. Fainstein, L. Gulyas, F. Martin. A. Salin, Phys. Rev. A 53 ( 1996) 3243. PI CR. Garibotti. W. Cravero, Phys. Rev. A 48 (1993) 2012. S. Suarez, D. Fregenal, P. Focke. W. 191 G. Bernardi, Meckbach, Rev. Sci. Instrum. 67 (1996) 1761. [lOI R.D. DuBois. ST. Manson, Phys. Rev. A 35 (1987) 2007. Ull R.D. DuBois. A. Kover. Phys. Rev. A 40 (1989) 3605. t1A R.D. DuBois. in: AIP Conference Proceedings. Phys. El. At. Coll. 360 (1995) 505. [I31 R.E. Olson, T.J. Gay, H.G. Berry, E.B. Hale. V.D. Irby, Phys. Rev. Lett. 59 (1987) 36. [I41 P.D. Fainstein, V.H. Ponce, R.D. Rivarola. J. Phys. B 24 (1991) 3091. [I51 ME. Rudd, L.H. Toburen. N. Stolterfoht, Atom. Data Nucl. Data Tables 18 (1976) 413; M.E. Rudd. L.H. Toburen, N. Stolterfoht. Atom. Data Nucl. Data Tables 23 (1979) 405.