- Email: [email protected]
amu Oq+ (q = 2–8)
Nuclear Instruments and Methods North-Holland, Amsterdam
CROSS SECTIONS 0. HEBER,
in Physics
Research
FOR Ar RECOIL
G. SAMPOLL,
R40/41
ION PRODUCTION
R.J. MAURER,
197
(1989) 197-200
B.B. BANDONG
BY 1 MeV / amu Oq + (q = 2-8) and R.L. WATSON
~vi.lot~~n lmtistituteand Department of Chemistp. Texas A&M ~lnil~er.~i~~~, Cokge Station, TX 7784.3, U.S.4
Cross sections for the production of recoil ions of Ar” through Ar’e+ in collisions with 1 MeV/amu 02+ through OR+ ions have been measured using the time-of-flight (TOF) method. Postcollision charge state analysis of the projectile was performed to loss (q = 2-4). one-electron distinguish between events involving pure ionization ( (I = 2-S). one-electron loss (q = 2-6). two-electron capture (q = 3-8) and two-electron capture (4 = 558). The total direct ionization cross section was found to increase as y’ s. The electron capture cross sections also increased as a function of q, while at the same time the cross sections for electron loss decreased. Both of these processes exhibit a steeper q dependence than the pure ionization process. The charge state distributions are bell shaped for the electron capture process and for the electron loss process at high q. For electron loss at low q the distributions are similar to those for the pure ionization process.
In atomic collisions between fast highly-charged projectile ions and atomic gas targets, the recoil-ion charge state distribution has been found to be highly dependent on the postcollision charge of the projectile [l-3]. The processes involved in the production of recoil ions are classified according to the change in projectile charge. When the outgoing projectile has the same charge as the incoming projectile, the process is called pure ionization. When the projectile charge increases by one or more units in the collision, the process is called electron loss (of one or more electrons). In the electron capture process, the outgoing projectile has more electrons than the incoming projectile and therefore the charge state decreases. The correlation between these processes and the recoil-ion charge state distribution is usually assumed to occur because they involve different ranges of impact parameters. For projectiles with charges of about 10 + or more, the cross sections for recoil-ion production increase rapidly with increasing projectile charge. At the same time. the recoil-ion charge state distributions hardly change at all [4]. Contrary to the above behavior. the recoil-ion charge state distributions produced by projectiles having a q less than 10 change considerably as a function of q. More systematic data for recoil-ion production by low-q projectiles are needed to fully characterize the correlation between projectile q and the recoil-ion charge state distribution. The most common theoretical model for describing the multiple-vacancy production process is the independent electron approximation (IEA) [S], which predicts the experimental results for high q projectiles fairly well. For low q projectiles. however, the results of this model underestimate the experimental cross sections by 0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V
orders of magnitude compared to about a factor of two for high q projectiles [6]. as will be shown here. The aim of this work was to investigate the dependence of recoil-ion production cross sections and charge state distributions on the projectile incoming and outgoing charge states for low q projectiles such as O’+ range, in order to through 0 ‘+ in the 1 MeV/amu better characterize the collision processes. An Ar target was chosen because it is the simplest monatomic gas having two outer shell octets of electrons.
2. Experimental procedure A well defined beam of 1 MeV/amu Oy+ from the Texas A&M University Cyclotron was directed into a differentially pumped gas cell configured as the first stage of a time of flight (TOF) recoil-ion spectrometer. The postcollision projectiles were deflected by a variable magnetic field, located just outside the exit port of the gas cell system. towards a collimated silicon surface barrier particle detector. In this setup, only one outgoing charge state could be detected at a time [7]. Pressure dependence measurements were performed to verify that single collision conditions existed for both projectiles and recoil-ions. The start signal for the TOF spectrum was produced by the projectile detector. and the stop signal was generated by a chevron microchannelplate (MCP) recoil-ion detector. Since the projectile detector viewed only one charge state at a time. the total number of recoil-ions detected by the MCP (not in coincidence) was used to normalize the yields for different postcollision projectile charges to the incoming projectile intensity. Total cross sections were calculated by normalizing the recoil-ion yields to previous cross-sect. ATOMIC
PHYSICS
0. Heber et al. / Cross sections for Ar recoil ion production
198 tion measurements same system [8].
performed
with Ar13+ using the
3. Results and discussion 3.1. Cross sections In fig. 1, the total cross sections for pure ionization, one- and two-electron capture and one- and two-electron loss, as a function of the incoming projectile charge, are presented. The error associated with these measurements is estimated to be about 25%. It is apparent that pure ionization is the most probable process. Its dependence on projectile charge, q, is proportional to q’.5. One electron loss at q = 2 also has a large cross section, which implies that both processes are characterized by the same interaction for low q. The recoil-ion charge state distributions support this conclusion. As q increases, the cross sections for one- and two-electron loss decrease. This means that the average impact parameter moves closer and closer to the target nucleus, in order to provide enough energy transfer to remove additional projectile electrons.
Fig. 2. Experimental cross sections for Ar recoil-ion production by pure ionization in 1 MeV/amu Oq+ collisions. The numbers along the right side give the recoil-ion charge.
‘r
For one- and two-electron capture collisions, the q dependence is opposite to that of the electron loss process and goes in the same direction as the pure ionization process, but with a much steeper rate of increase. The cross sections for one-electron capture are much lower than the cross sections for pure ionization, even for fully striped oxygen. It may be concluded, therefore, that the average impact parameters for the capture process are much smaller than those for the direct ionization process. Considering the geometric cross section of the Ar L-shell and the velocity matching between the projectile and the Ar L-electrons, it is reasonable to expect electron capture to occur primarily from the Ar L-shell, as has been assumed in other reports [4].
+__QNJ- ELECTRON LOSS
WO-ELECTRON
3 i
3.2. Recoil-ion charge state distribution
I6 Fig. 1. Total cross sections for Ar recoil-ion production pure ionization, one- and two-electron loss and onetwo-electron capture in 1 MeV/amu Oq+ collisions.
by and
The partial pure-ionization cross sections for each recoil-ion charge state as a function of the oxygen projectile charge are shown in fig. 2. For low recoil-ion charges (l-4), the cross sections increase as a function of q at approximately the same rate. At higher recoil-ion charges, a new trend arises where the cross sections increase less and less with increasing q and eventually
Fig. 3. Theoretical cross by pure ionization in 1 using the CTMC model. the
sections for Ar recoil-ion production MeV/amu 04+ collisions, calculated The numbers along the right side give recoil-ion charge.
Fig. 5. Charge by two-electron
state distributions for Ar recoil-ion production loss in 1 MeV/amu 04+ collisions. The numbers give the projectile
charge.
0 3(
‘r
021
02L
2 015 F Y S w’ 0.10 cc
4 5 6
0.05
0 0
2
4 RECOIL-
Fig. 4. Charge by one-electron
state distributions for Ar recoil-ion production loss in 1 MeV/amu 04+ collisions. The numbers give the projectile
charge.
/ 6 ION
6
IO
12
CHARGE
Fig. 6. Charge state distributions for Ar recoil-ion production by one-electron capture in 1 MeV/amu 04+ collisions. The numbers give the projectile charge.
200
0. Heber et al. / Cross sections for Ar recoil ion production
reach a point where they start to decrease. This behavior is not predicted by previous theoretical calculations. In fig. 3, the results of classical trajectory Monte Carlo calculations (CTMC), similar to the calculations of Muller et al. [4] are shown. The calculated cross sections are in fair agreement with the experimental cross sections for low recoil-ion charges, but diverge rapidly from the experimental cross sections as the recoil-ion charge increases and q decreases. These discrepancies range up to several orders of magnitude. In another recent study it was found that the deviations between experiment and the predictions of the IEA model are caused by contributions to the experimental cross sections from Auger decay following L-shell ionization [9]. The recoil-ion charge state distributions for one-electron and two-electron loss are shown in figs. 4 and 5. The most probable recoil-ion charge increases with q because the average impact parameter decreases. When the average impact parameter is large (e.g. for q = 2 one-electron loss), the charge state distribution is similar to that for pure ionization, as shown in fig. 4. The distributions in fig. 5 are closer to bell shaped distributions that are characteristic of small impact parameter collisions. For large impact parameter collisions the interaction can be described by means of a point charge
model [6], but for small impact parameter collisions, the outer-shell electron screening must be taken into account
POI.
The recoil-ion charge state distribution for the electron capture process is bell shaped even for low q projectiles. as shown in fig. 6. This distribution occurs as a result of the capture of an L-shell electron along with the ejection of M-shell electrons via ionization during the collision and subsequent L-Auger decay. The mean charge of each bell shaped distribution increases as a function of q. The experimental values of the mean charge are 4.21, 4.65, 5.18, 5.50, 5.91 and 6.11 for q = 3, 4, 5, 6, 7 and 8, respectively. In the work of Muller et al. [4], the mean charge of Ar recoil-ions following oneelectron capture at 1.4 MeV/amu converged to a constant value of 7 for q > 12 + The mean charges obtained in the present experiment appear to be following this same trend. The charge state distributions for two-electron capture are shown in fig. 7. The mean charges for these distributions are 6.93, 7.21, 7.87 and 8.09 for q = 5, 6, 7 and 8. respectively. The distributions are shifted up by about two charge units compared to those for the same one-electron capture process. This shift is caused by the fact that each vacancy produced by L-capture generates an additional vacancy in the M-shell via Auger decay. The shift of the mean charge as a function of q is again towards a constant high q limit having a value around 9, as found by Muller et al. [4]. This work was supported by the Division of Chemical Sciences of the U.S. Department of Energy and the Robert A. Welch Foundation.
References [l] T.J. Gray, C.L. Cocke and E. Justiniano, [2]
’ [3]
[4] I-
[5] [6] [7] [8] ) .0
2
4 RECOIL-ION
6
8
IO
CHARGE
Fig. 7. Charge state distributions for Ar recoil-ion production by two-electron capture in 1 MeV/amu 04+ collisions. The numbers give the projectile charge.
[9]
[lo]
Phys. Rev. A22 (1980) 849. H. Damsgard, H.K. Haugen, P. Hvelplund and H. Knudsen, Phys. Rev. A27 (1983) 112. A. Muller, B. Schuch, W. Groh, E. Salzborn, H.F. Beyer, P.H. Mokler and R.E. Olson, Nucl. Instr. and Meth. B24/25 (1987) 111. A. Muller, B. Schuch, W. Groh, E. Salzbom, H.F. Beyer, P.H. Mokler and R.E. Olson, Phys. Rev. A33 (1986) 3010. J.H. McGuire and L. Weaver. Phys. Rev. Al6 (1977) 41. CL. Cocke, Phys. Rev. A20 (1979) 749. R.J. Maurer, C. Can and R.L. Watson, Nucl. Instr. and Meth. B27 (1987) 512. R.J. Maurer, C. Can, 0. Heber and R.L. Watson, Progress in Research (Cyclotron Institute, Texas A&M University, 1988) p. 32. 0. Heber. G. Sampoll, B.B. Bandong, R.J. Maurer. E. Moler, R.L. Watson, I. Ben-It&&, J.L. Sinpaugh. J.M. Sanders and P. Richard, submitted for publication. T. Tonuma, H. Shibata, S.H. Be, H. Kumagai, M. Kase, T. Kambara, I. Kohno, A. Ohsaki and H. Tawara, Phys. Rev. A33 (1986) 3047.
CROSS SECTIONS 0. HEBER,
in Physics
Research
FOR Ar RECOIL
G. SAMPOLL,
R40/41
ION PRODUCTION
R.J. MAURER,
197
(1989) 197-200
B.B. BANDONG
BY 1 MeV / amu Oq + (q = 2-8) and R.L. WATSON
~vi.lot~~n lmtistituteand Department of Chemistp. Texas A&M ~lnil~er.~i~~~, Cokge Station, TX 7784.3, U.S.4
Cross sections for the production of recoil ions of Ar” through Ar’e+ in collisions with 1 MeV/amu 02+ through OR+ ions have been measured using the time-of-flight (TOF) method. Postcollision charge state analysis of the projectile was performed to loss (q = 2-4). one-electron distinguish between events involving pure ionization ( (I = 2-S). one-electron loss (q = 2-6). two-electron capture (q = 3-8) and two-electron capture (4 = 558). The total direct ionization cross section was found to increase as y’ s. The electron capture cross sections also increased as a function of q, while at the same time the cross sections for electron loss decreased. Both of these processes exhibit a steeper q dependence than the pure ionization process. The charge state distributions are bell shaped for the electron capture process and for the electron loss process at high q. For electron loss at low q the distributions are similar to those for the pure ionization process.
In atomic collisions between fast highly-charged projectile ions and atomic gas targets, the recoil-ion charge state distribution has been found to be highly dependent on the postcollision charge of the projectile [l-3]. The processes involved in the production of recoil ions are classified according to the change in projectile charge. When the outgoing projectile has the same charge as the incoming projectile, the process is called pure ionization. When the projectile charge increases by one or more units in the collision, the process is called electron loss (of one or more electrons). In the electron capture process, the outgoing projectile has more electrons than the incoming projectile and therefore the charge state decreases. The correlation between these processes and the recoil-ion charge state distribution is usually assumed to occur because they involve different ranges of impact parameters. For projectiles with charges of about 10 + or more, the cross sections for recoil-ion production increase rapidly with increasing projectile charge. At the same time. the recoil-ion charge state distributions hardly change at all [4]. Contrary to the above behavior. the recoil-ion charge state distributions produced by projectiles having a q less than 10 change considerably as a function of q. More systematic data for recoil-ion production by low-q projectiles are needed to fully characterize the correlation between projectile q and the recoil-ion charge state distribution. The most common theoretical model for describing the multiple-vacancy production process is the independent electron approximation (IEA) [S], which predicts the experimental results for high q projectiles fairly well. For low q projectiles. however, the results of this model underestimate the experimental cross sections by 0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V
orders of magnitude compared to about a factor of two for high q projectiles [6]. as will be shown here. The aim of this work was to investigate the dependence of recoil-ion production cross sections and charge state distributions on the projectile incoming and outgoing charge states for low q projectiles such as O’+ range, in order to through 0 ‘+ in the 1 MeV/amu better characterize the collision processes. An Ar target was chosen because it is the simplest monatomic gas having two outer shell octets of electrons.
2. Experimental procedure A well defined beam of 1 MeV/amu Oy+ from the Texas A&M University Cyclotron was directed into a differentially pumped gas cell configured as the first stage of a time of flight (TOF) recoil-ion spectrometer. The postcollision projectiles were deflected by a variable magnetic field, located just outside the exit port of the gas cell system. towards a collimated silicon surface barrier particle detector. In this setup, only one outgoing charge state could be detected at a time [7]. Pressure dependence measurements were performed to verify that single collision conditions existed for both projectiles and recoil-ions. The start signal for the TOF spectrum was produced by the projectile detector. and the stop signal was generated by a chevron microchannelplate (MCP) recoil-ion detector. Since the projectile detector viewed only one charge state at a time. the total number of recoil-ions detected by the MCP (not in coincidence) was used to normalize the yields for different postcollision projectile charges to the incoming projectile intensity. Total cross sections were calculated by normalizing the recoil-ion yields to previous cross-sect. ATOMIC
PHYSICS
0. Heber et al. / Cross sections for Ar recoil ion production
198 tion measurements same system [8].
performed
with Ar13+ using the
3. Results and discussion 3.1. Cross sections In fig. 1, the total cross sections for pure ionization, one- and two-electron capture and one- and two-electron loss, as a function of the incoming projectile charge, are presented. The error associated with these measurements is estimated to be about 25%. It is apparent that pure ionization is the most probable process. Its dependence on projectile charge, q, is proportional to q’.5. One electron loss at q = 2 also has a large cross section, which implies that both processes are characterized by the same interaction for low q. The recoil-ion charge state distributions support this conclusion. As q increases, the cross sections for one- and two-electron loss decrease. This means that the average impact parameter moves closer and closer to the target nucleus, in order to provide enough energy transfer to remove additional projectile electrons.
Fig. 2. Experimental cross sections for Ar recoil-ion production by pure ionization in 1 MeV/amu Oq+ collisions. The numbers along the right side give the recoil-ion charge.
‘r
For one- and two-electron capture collisions, the q dependence is opposite to that of the electron loss process and goes in the same direction as the pure ionization process, but with a much steeper rate of increase. The cross sections for one-electron capture are much lower than the cross sections for pure ionization, even for fully striped oxygen. It may be concluded, therefore, that the average impact parameters for the capture process are much smaller than those for the direct ionization process. Considering the geometric cross section of the Ar L-shell and the velocity matching between the projectile and the Ar L-electrons, it is reasonable to expect electron capture to occur primarily from the Ar L-shell, as has been assumed in other reports [4].
+__QNJ- ELECTRON LOSS
WO-ELECTRON
3 i
3.2. Recoil-ion charge state distribution
I6 Fig. 1. Total cross sections for Ar recoil-ion production pure ionization, one- and two-electron loss and onetwo-electron capture in 1 MeV/amu Oq+ collisions.
by and
The partial pure-ionization cross sections for each recoil-ion charge state as a function of the oxygen projectile charge are shown in fig. 2. For low recoil-ion charges (l-4), the cross sections increase as a function of q at approximately the same rate. At higher recoil-ion charges, a new trend arises where the cross sections increase less and less with increasing q and eventually
Fig. 3. Theoretical cross by pure ionization in 1 using the CTMC model. the
sections for Ar recoil-ion production MeV/amu 04+ collisions, calculated The numbers along the right side give recoil-ion charge.
Fig. 5. Charge by two-electron
state distributions for Ar recoil-ion production loss in 1 MeV/amu 04+ collisions. The numbers give the projectile
charge.
0 3(
‘r
021
02L
2 015 F Y S w’ 0.10 cc
4 5 6
0.05
0 0
2
4 RECOIL-
Fig. 4. Charge by one-electron
state distributions for Ar recoil-ion production loss in 1 MeV/amu 04+ collisions. The numbers give the projectile
charge.
/ 6 ION
6
IO
12
CHARGE
Fig. 6. Charge state distributions for Ar recoil-ion production by one-electron capture in 1 MeV/amu 04+ collisions. The numbers give the projectile charge.
200
0. Heber et al. / Cross sections for Ar recoil ion production
reach a point where they start to decrease. This behavior is not predicted by previous theoretical calculations. In fig. 3, the results of classical trajectory Monte Carlo calculations (CTMC), similar to the calculations of Muller et al. [4] are shown. The calculated cross sections are in fair agreement with the experimental cross sections for low recoil-ion charges, but diverge rapidly from the experimental cross sections as the recoil-ion charge increases and q decreases. These discrepancies range up to several orders of magnitude. In another recent study it was found that the deviations between experiment and the predictions of the IEA model are caused by contributions to the experimental cross sections from Auger decay following L-shell ionization [9]. The recoil-ion charge state distributions for one-electron and two-electron loss are shown in figs. 4 and 5. The most probable recoil-ion charge increases with q because the average impact parameter decreases. When the average impact parameter is large (e.g. for q = 2 one-electron loss), the charge state distribution is similar to that for pure ionization, as shown in fig. 4. The distributions in fig. 5 are closer to bell shaped distributions that are characteristic of small impact parameter collisions. For large impact parameter collisions the interaction can be described by means of a point charge
model [6], but for small impact parameter collisions, the outer-shell electron screening must be taken into account
POI.
The recoil-ion charge state distribution for the electron capture process is bell shaped even for low q projectiles. as shown in fig. 6. This distribution occurs as a result of the capture of an L-shell electron along with the ejection of M-shell electrons via ionization during the collision and subsequent L-Auger decay. The mean charge of each bell shaped distribution increases as a function of q. The experimental values of the mean charge are 4.21, 4.65, 5.18, 5.50, 5.91 and 6.11 for q = 3, 4, 5, 6, 7 and 8, respectively. In the work of Muller et al. [4], the mean charge of Ar recoil-ions following oneelectron capture at 1.4 MeV/amu converged to a constant value of 7 for q > 12 + The mean charges obtained in the present experiment appear to be following this same trend. The charge state distributions for two-electron capture are shown in fig. 7. The mean charges for these distributions are 6.93, 7.21, 7.87 and 8.09 for q = 5, 6, 7 and 8. respectively. The distributions are shifted up by about two charge units compared to those for the same one-electron capture process. This shift is caused by the fact that each vacancy produced by L-capture generates an additional vacancy in the M-shell via Auger decay. The shift of the mean charge as a function of q is again towards a constant high q limit having a value around 9, as found by Muller et al. [4]. This work was supported by the Division of Chemical Sciences of the U.S. Department of Energy and the Robert A. Welch Foundation.
References [l] T.J. Gray, C.L. Cocke and E. Justiniano, [2]
’ [3]
[4] I-
[5] [6] [7] [8] ) .0
2
4 RECOIL-ION
6
8
IO
CHARGE
Fig. 7. Charge state distributions for Ar recoil-ion production by two-electron capture in 1 MeV/amu 04+ collisions. The numbers give the projectile charge.
[9]
[lo]
Phys. Rev. A22 (1980) 849. H. Damsgard, H.K. Haugen, P. Hvelplund and H. Knudsen, Phys. Rev. A27 (1983) 112. A. Muller, B. Schuch, W. Groh, E. Salzborn, H.F. Beyer, P.H. Mokler and R.E. Olson, Nucl. Instr. and Meth. B24/25 (1987) 111. A. Muller, B. Schuch, W. Groh, E. Salzbom, H.F. Beyer, P.H. Mokler and R.E. Olson, Phys. Rev. A33 (1986) 3010. J.H. McGuire and L. Weaver. Phys. Rev. Al6 (1977) 41. CL. Cocke, Phys. Rev. A20 (1979) 749. R.J. Maurer, C. Can and R.L. Watson, Nucl. Instr. and Meth. B27 (1987) 512. R.J. Maurer, C. Can, 0. Heber and R.L. Watson, Progress in Research (Cyclotron Institute, Texas A&M University, 1988) p. 32. 0. Heber. G. Sampoll, B.B. Bandong, R.J. Maurer. E. Moler, R.L. Watson, I. Ben-It&&, J.L. Sinpaugh. J.M. Sanders and P. Richard, submitted for publication. T. Tonuma, H. Shibata, S.H. Be, H. Kumagai, M. Kase, T. Kambara, I. Kohno, A. Ohsaki and H. Tawara, Phys. Rev. A33 (1986) 3047.