- Email: [email protected]
Recent studies of simultaneous ionization and charge transfer in helium ion-atom collisions
Nuclear Instruments and Methods North-Holland. Amsterdam
RECENT
STUDIES
IN HELIUM
in Physics
Research
B24/25
OF SIMULTANEOUS
ION-ATOM
COLLISIONS
(1987) 209-213
IONIZATION
AND
209
CHARGE
TRANSFER
*
R.D. DuBOIS Pacific Northwest
Laboratory,
Richland,
WA 99352, USA
In our laboratory, we are interested in ionization of atomic and molecular targets induced by light ion impact. Generally these collisions are studied by monitoring the ejected electron spectra with the assumption that each ionizing event leads to a single secondary electron. By measuring the target ionization charge states produced by directly ionizing and charge changing collisions, we have obtained cross sections for direct ionization, pure charge transfer, and charge transfer plus ionization thus making it possible to distinguish how target ionization occurs. It was found that charge transfer plus ionization can sometimes be more probable than pure charge transfer alone. This is important because one mechanism which has until recently been disregarded as a means of producing free electrons is the possibility of capturing an electron from the target and simultaneously ionizing one or more target electrons. For ion impact velocities comparable to, or less than, the outer shell bound electron velocities, we have found that charge transfer plus ionization can be a substantial, if not principal, means of producing free electrons.
1. Introduction Ion-atom
collisions
result
in the production
of target
electrons. Information about the collisional process can be obtained by measuring the total positive (a,) and negative (a-) charge production which are related to the number of slow target ions and free electrons produced. Additional details can be obtained by measuring the electron production as a function of emission angle and energy (the doubly differential cross section). However, in both cases, multiple ionization events can lend uncertainty to the interpretation of the data. This is because u+ and u- are related to the individual ionization cross sections in the following manner: ions
and
free
(J+ = Cqu;‘; j4
(1)
u_=
(2)
C(q-i+j)uJJ. 4,i
Here superscripts i and j represent the pre- and postcollision projectile charge states respectively, and the subscript q gives the final target charge state. Thus uii with i = j is the cross section for directly ionizing the target q times, uii (j < i) is the cross section for transferring i -j electrons to the projectile and leaving the target q times ionized, and uij (j > i) is the cross section for stripping j - i electrons from the projectile and leaving the target q times ionized. Obviously only the first two processes exist for bare ion impact with the
* Supported by (OHER)
Office of Health and Environmental Research under U.S. Department of Energy Contract DEAC06-76RL0 1830.
0168-583X/87/$03.50 @ Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
third process occurring for partially stripped projectile impact. It has previously been shown [l] that multiple ionization is an important contribution to the total target ionization cross section even for light ion (H+ and He+) impact on noble gas targets. The purpose of this paper is to investigate the relative importance of direct ionization, electron capture, and electron loss in contributing to the total ion and electron production cross sections ( u + and u- respectively). Cross sections for producing target ionization via these three mechanisms will be presented for helium ion impact on various atomic targets. The relative importance of the direct ionization, the electron capture, and the electron loss terms in eqs. (1) and (2) can be investigated using He+ ions; whereas He’+ impact demonstrates the relative importance of direct ionization as compared to single and double electron capture. A brief description of the experimental procedure followed by examples of the individual cross sections and how they contribute to the total ion and electron yields will be given. Finally a closer look at the electron capture cross sections will be presented.
2. Experimental procedure Detailed descriptions of the experimental apparatus and techniques used to measure absolute cross sections for the individual channels leading to multiple target ionization have been published [2-41. Thus only a few details will be given here. A beam of ions passed through a cell containing the target gas and then was electrostatically charge state analyzed and counted using I. ATOMIC
PHYSICS
/ RELATED
PHENOMENA
210
R. D. DuBois / Smudtoneour
ioni:ution und charge trunsfer
and krypton in figs. 1 and 2 respectively. Impact energies range from 15 to 2000 keV. Included in the figures are total cross sections for positive (0,) and negative (u _) charge production and total cross sections for electron capture (0”) and for electron loss (ul’) taken from refs. [5,7]. As can be seen, particularly for the heavier target, krypton, the contributions from multiple ionization are nonnegligible. By summing the appropriate ui cross sections, comparisons with total cross sections can be made. It is seen that the results for u+ are generally in excellent agreement except around 150-200 keV where the present sums are somewhat larger than previous work. This is primarily because the present data overestimates the electron capture cross sections above 100 keV as can be seen in figs. 1 and 2. Summing the present data to obtain u- (the free electron production) shows good agreement with the data of Rudd et al. [5], above 100 keV. The disagreement at lower impact energies is attributed to an underestimation of the direct ionization cross sections (ail) in the present work. An important feature of figs. 1 and 2 is that the total electron production cross section, obtained by adding the individual cross sections, differs considerably from the direct single ionization cross section (u:l) and, in fact, is not always well represented by the sum of the direct ionization cross sections (aqua”). The remaining contribution to u- comes from the electron loss channels (ui’) at higher energies and from the electron capture channels (u$ 1) at lower impact energies. For example, the electron capture channel (ui”) is particu-
a channeltron electron multiplier. Target ions, extracted from the collision region by means of a small electric field, were counted by a channel electron multiplier. Coincidence signals between these slow target ions and the postcollision projectile ions (Si’) where recorded for determination of the direct ionization (i =j = l), the electron capture (i = 1, j = 0), and the electron loss (i = 1, j = 2) cross sections for He+ impact, and for the direct ionization (i =j = 2), the single (i = 2, j = 1) and double capture (i = 2, j = 0) cross sections for He”+ impact. The coincidence signals (Si’) are proportional to the cross sections ui’ which are placed on an absolute scale by normalizing to single electron capture cross sections [5,6] for the He+ data below 100 keV and for all of the He’+ data. For the higher energy He+ data, normalization to the total positive ion production cross sections (a,) taken from ref. [5] was used. Due to uncertainties in detection efficiency and normalization procedures as well as poor statistics for very small cross sections the absolute a: cross sections presented here generally have overall uncertainties of approximately 25% with larger uncertainties increasing to as much as a factor of 2 for the smallest cross sections that were measured.
3. Results Cross sections for target ionization resulting from direct ionization, electron capture and electron loss by the projectile are presented for 4He+ impact on neon
I ““,
’
1 1 I ““,
Electron
4He+
100
I”“,
0
Capture
1000
-Ne
’
“I”“,
’
_
Electron
I
4He+
10
100
Loss -Ne
_ :
1000
E IkeVl 1. Cross sections contributing to direct ionization (oil), electron capture (u:“) and electron loss (IT:‘) in 4He+-Ne collisions. Present data for target ionization charge states q = 1.2.3: 0, n , A respectively. Total cross sections: 6,: ref. [5], + present data using eq. (1) in text; o_: ref. [5], - present data using eq. (2) in text: ul’: ref. [5]. v c 0,‘” using present data; ol*: ref. [5], v IX o]* using present data. Curves through present data serve only to guide the eye.
Fig.
R. D. DuBois / Simultuneoux
ionizuiion and charge transfer
Direct Ionization 4He+ - Kr
100 E IkeW
Fig. 2. Cross sections contributing to direct ionization (ui’), electron capture (u,‘“) and electron loss (0,“) in 4He+-Kr collisions. Present data are for target ionization charge states (I = 1.2.3 and 4; 0, n , A and + respectively. Total cross sections: 0,: ref. [5], + present data using eq. (1) in text; e-: ref. [S], - present data using eq. (2) in text; oi”: refs. [5,7], v X 0:” using present data; cl’: refs. [5,7], v X 0,” using present data. Curves through present data serve only to guide the eye. 10-14
5
Direct
I Single
Ionization
1 ’ I Charge Transfer He2+
-
I
1
Double
’
Charge
I
Transfe
Kr
“2’ 2 ,0-15
5
/-2 22 Ol
10-16 5
“E 0 0
/
A
2 22 O2 / m
10-l’ 5 -1
l
/
.
J OZ2 3
22@0 UA O4 v
20 r) ‘6
E/M (keV/amu)
Fig. 3. Cross sections contributing to direct ionization (OF), single capture (0,“) and double capture (~420) for He2+ -Kr collisions. Present data are for target ionization charge states 4 = 1,2,3,4,5 and 6,0, n , A, +, v and * respectively. Total cross sections: u+: ref. [6], + present data using eq. (1) in text; u-: ref. [6], - present data using eq. (2) in text; u2i: ref. [6]; a”: ref. [6], v XuF using present data. Curves through
the present
data serve only to guide the eye. I. ATOMIC
PHYSICS
/ RELATED
PHENOMENA
R. D. DuBois
212
/ Simultuneow
ionizution
and charge trunsjer
tions (Cqo+?*). From the cross sections shown in fig. 3, it is seen that the majority of the free electron production at these low energies is via the higher order electron capture channels, e.g. u,‘: 1 and u,‘“>2 for single and double electron capture respectively. In fact for low energy (10 keV/amu) He*+-Kr collisions, almost the entire free electron production is via the electron capture channels. The importance of the higher order electron capture channels (channels where capture from and ionization of the target occur simultaneously) in the production of free electrons has been shown. In addition, figs. l-3 show that these channels often contribute substantially to the total single and double capture cross sections. In certain cases, it is seen that the probability of capturing one electron from the target and ionizing an additional electron is comparable to the probability of simply capturing an electron i.e. a;.‘-’ - u;,~-~. Even more astounding is that the He*+-Kr data shows that double capture plus one or more additional ionizations can be more probable than double capture alone.
larly important in the total free electron production below 100 keV in the case of krypton. Another important feature illustrated in fig. 2 is that the probability of capturing and ionizing electrons from the target (a:‘) can be comparable to the probability of simply capturing an electron (u:“). Cross sections for ionization of krypton by He’+ impact are shown in fig. 3. For He’+ impact there is no electron loss channel but the double electron capture channel is now very important. Considerable multiple ionization of the target occurs in the charge transfer channels. Summing the cross sections for individual charge states and comparing with various total cross sections [6] shows good agreement with previous measurements of u+ and u_. However, the sum of the double electron capture channels @,z”) tends to overestimate previously measured [6] total double electron capture cross sections. Of major importance for low energy He’+ impact is that the total free electron production (u-) is much larger than the sum of the direct ionization cross sec-
1 Pure Capture
-
H+,
He+,
Two (He+,
Ionizations He*+
Data
t
5)
100 E/M
Fig. 4. Neon charge state distributions capture) and ( ) (double capture)
(keV/amu)
(j) resulting from electron capture for Ht ( -), impact. Vertical arrows indicate projectile velocities electrons. Proton data taken from refs. [2.9].
He+ (---). He*+ (.-.-.) matching those of the bound
(single target
213
R. D. DuBois / Simultuneous ionizution und churge trunsfer
4. Interpretation of data The importance of electron capture plus simultaneous ionization in helium ion-atom collisions, both in the production of free electrons and in the total electron capture cross sections, has been demonstrated. It is therefore of interest in determining the mechanism responsible for multiple ionization in the electron capture channel. It is import~t to recognize that the present capture plus ionization (C + I) process is different than the transfer ionization (TI) process that was shown [8] to be important for energies smaller than those presented here. Transfer ionization can be interpreted in terms of a molecular-orbital picture [S] in which the internal potential energy difference between the initial and final electron capture states is sufficiently large to cause additional target ionization. In the present case, the higher kinetic energy of the projectile is sufficient to cause the observed additional ionization that accompanies the capture process. It is also possible that inner shell capture followed by Auger (or X-ray) relaxation could be responsible for the additional target ionization that is observed. Inner shell capture is known [9] to play an important role in the production of higher degrees of target ionization for high energy proton-atom collisions. This is manifested by a relative decrease in the pure capture cross section and increases in the capture plus ionization cross sections for impact velocities near those of the bound inner shell electron velocities. An example for H+-Ne collisions (solid curves) is shown in fig. 4 where the capture plus ionization probabilities increase sharply for proton velocities near those of the Ne Is electrons. For projectile velocities slower than those of the inner shell but faster than those of the outer shell target electrons, velocity independent charge state distributions are found. This is an indication that the collision time is sufficiently short such that the target electrons cannot adiabatically adjust after capture from the outer shell
takes place. For projectile velocities slower than those of the outer shell target electrons, there is a monotonic decrease in the relative percentage of capture plus ionization with respect to pure capture. This velocity dependent charge state distribution occurs because the collision is slow enough to allow for some readjustment of the target electrons after the capture process. The same general features are seen to occur for helium ion impact although the relative amounts of capture plus ionization increase considerably as the projectile is changed from HC to He+ to He*+. It is interesting to note that double capture plus additional ionizations tend to exhibit the same features as are seen for single capture plus additional ionizations. This implies that the same mechanism is responsible for the additional target ionization that is observed in the single and double capture channels. A more complete understanding will require additonal experimental and theoretical studies.
References
PI R.D. DuBois, L.H. Toburen
and M.E. Rudd, Phys. Rev. A29 (1984) 70. PI R.D. DuBois, Phys. Rev. Lett. 52 (1984) 2348. [31 R.D. DuBois, Phys. Rev. A32 (1984) 3319. PI R.D. DuBois, Phys. Rev. A33 (1986) 1595. [51 M.E. Rudd, T.V. Goffe, A. Itoh and R.D. DuBois, Phys. Rev. A32 (1985) 829. [61 M.E. Rudd, T.V. Goffe and A. Itoh, Phys. Rev. A32 (1985) 2128. [71Y. Nakai, A. Kikuchi, T. Shirai and M. Sataka, Japan Atomic Energy Research Institute, Report No. JAERI-M 84-069 (1984). A. Miiller and E. SaIzborn, J. PI W. Groh, A.S. Schlachter, Phys. B Lett. 15 (1982) L207. [91 E. Horsdal Pedersen and L. Larsen, J. Phys. B12 (1979) 4085.
I. ATOMIC
PHYSICS
/ RELATED
PHENOMENA
RECENT
STUDIES
IN HELIUM
in Physics
Research
B24/25
OF SIMULTANEOUS
ION-ATOM
COLLISIONS
(1987) 209-213
IONIZATION
AND
209
CHARGE
TRANSFER
*
R.D. DuBOIS Pacific Northwest
Laboratory,
Richland,
WA 99352, USA
In our laboratory, we are interested in ionization of atomic and molecular targets induced by light ion impact. Generally these collisions are studied by monitoring the ejected electron spectra with the assumption that each ionizing event leads to a single secondary electron. By measuring the target ionization charge states produced by directly ionizing and charge changing collisions, we have obtained cross sections for direct ionization, pure charge transfer, and charge transfer plus ionization thus making it possible to distinguish how target ionization occurs. It was found that charge transfer plus ionization can sometimes be more probable than pure charge transfer alone. This is important because one mechanism which has until recently been disregarded as a means of producing free electrons is the possibility of capturing an electron from the target and simultaneously ionizing one or more target electrons. For ion impact velocities comparable to, or less than, the outer shell bound electron velocities, we have found that charge transfer plus ionization can be a substantial, if not principal, means of producing free electrons.
1. Introduction Ion-atom
collisions
result
in the production
of target
electrons. Information about the collisional process can be obtained by measuring the total positive (a,) and negative (a-) charge production which are related to the number of slow target ions and free electrons produced. Additional details can be obtained by measuring the electron production as a function of emission angle and energy (the doubly differential cross section). However, in both cases, multiple ionization events can lend uncertainty to the interpretation of the data. This is because u+ and u- are related to the individual ionization cross sections in the following manner: ions
and
free
(J+ = Cqu;‘; j4
(1)
u_=
(2)
C(q-i+j)uJJ. 4,i
Here superscripts i and j represent the pre- and postcollision projectile charge states respectively, and the subscript q gives the final target charge state. Thus uii with i = j is the cross section for directly ionizing the target q times, uii (j < i) is the cross section for transferring i -j electrons to the projectile and leaving the target q times ionized, and uij (j > i) is the cross section for stripping j - i electrons from the projectile and leaving the target q times ionized. Obviously only the first two processes exist for bare ion impact with the
* Supported by (OHER)
Office of Health and Environmental Research under U.S. Department of Energy Contract DEAC06-76RL0 1830.
0168-583X/87/$03.50 @ Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
third process occurring for partially stripped projectile impact. It has previously been shown [l] that multiple ionization is an important contribution to the total target ionization cross section even for light ion (H+ and He+) impact on noble gas targets. The purpose of this paper is to investigate the relative importance of direct ionization, electron capture, and electron loss in contributing to the total ion and electron production cross sections ( u + and u- respectively). Cross sections for producing target ionization via these three mechanisms will be presented for helium ion impact on various atomic targets. The relative importance of the direct ionization, the electron capture, and the electron loss terms in eqs. (1) and (2) can be investigated using He+ ions; whereas He’+ impact demonstrates the relative importance of direct ionization as compared to single and double electron capture. A brief description of the experimental procedure followed by examples of the individual cross sections and how they contribute to the total ion and electron yields will be given. Finally a closer look at the electron capture cross sections will be presented.
2. Experimental procedure Detailed descriptions of the experimental apparatus and techniques used to measure absolute cross sections for the individual channels leading to multiple target ionization have been published [2-41. Thus only a few details will be given here. A beam of ions passed through a cell containing the target gas and then was electrostatically charge state analyzed and counted using I. ATOMIC
PHYSICS
/ RELATED
PHENOMENA
210
R. D. DuBois / Smudtoneour
ioni:ution und charge trunsfer
and krypton in figs. 1 and 2 respectively. Impact energies range from 15 to 2000 keV. Included in the figures are total cross sections for positive (0,) and negative (u _) charge production and total cross sections for electron capture (0”) and for electron loss (ul’) taken from refs. [5,7]. As can be seen, particularly for the heavier target, krypton, the contributions from multiple ionization are nonnegligible. By summing the appropriate ui cross sections, comparisons with total cross sections can be made. It is seen that the results for u+ are generally in excellent agreement except around 150-200 keV where the present sums are somewhat larger than previous work. This is primarily because the present data overestimates the electron capture cross sections above 100 keV as can be seen in figs. 1 and 2. Summing the present data to obtain u- (the free electron production) shows good agreement with the data of Rudd et al. [5], above 100 keV. The disagreement at lower impact energies is attributed to an underestimation of the direct ionization cross sections (ail) in the present work. An important feature of figs. 1 and 2 is that the total electron production cross section, obtained by adding the individual cross sections, differs considerably from the direct single ionization cross section (u:l) and, in fact, is not always well represented by the sum of the direct ionization cross sections (aqua”). The remaining contribution to u- comes from the electron loss channels (ui’) at higher energies and from the electron capture channels (u$ 1) at lower impact energies. For example, the electron capture channel (ui”) is particu-
a channeltron electron multiplier. Target ions, extracted from the collision region by means of a small electric field, were counted by a channel electron multiplier. Coincidence signals between these slow target ions and the postcollision projectile ions (Si’) where recorded for determination of the direct ionization (i =j = l), the electron capture (i = 1, j = 0), and the electron loss (i = 1, j = 2) cross sections for He+ impact, and for the direct ionization (i =j = 2), the single (i = 2, j = 1) and double capture (i = 2, j = 0) cross sections for He”+ impact. The coincidence signals (Si’) are proportional to the cross sections ui’ which are placed on an absolute scale by normalizing to single electron capture cross sections [5,6] for the He+ data below 100 keV and for all of the He’+ data. For the higher energy He+ data, normalization to the total positive ion production cross sections (a,) taken from ref. [5] was used. Due to uncertainties in detection efficiency and normalization procedures as well as poor statistics for very small cross sections the absolute a: cross sections presented here generally have overall uncertainties of approximately 25% with larger uncertainties increasing to as much as a factor of 2 for the smallest cross sections that were measured.
3. Results Cross sections for target ionization resulting from direct ionization, electron capture and electron loss by the projectile are presented for 4He+ impact on neon
I ““,
’
1 1 I ““,
Electron
4He+
100
I”“,
0
Capture
1000
-Ne
’
“I”“,
’
_
Electron
I
4He+
10
100
Loss -Ne
_ :
1000
E IkeVl 1. Cross sections contributing to direct ionization (oil), electron capture (u:“) and electron loss (IT:‘) in 4He+-Ne collisions. Present data for target ionization charge states q = 1.2.3: 0, n , A respectively. Total cross sections: 6,: ref. [5], + present data using eq. (1) in text; o_: ref. [5], - present data using eq. (2) in text: ul’: ref. [5]. v c 0,‘” using present data; ol*: ref. [5], v IX o]* using present data. Curves through present data serve only to guide the eye.
Fig.
R. D. DuBois / Simultuneoux
ionizuiion and charge transfer
Direct Ionization 4He+ - Kr
100 E IkeW
Fig. 2. Cross sections contributing to direct ionization (ui’), electron capture (u,‘“) and electron loss (0,“) in 4He+-Kr collisions. Present data are for target ionization charge states (I = 1.2.3 and 4; 0, n , A and + respectively. Total cross sections: 0,: ref. [5], + present data using eq. (1) in text; e-: ref. [S], - present data using eq. (2) in text; oi”: refs. [5,7], v X 0:” using present data; cl’: refs. [5,7], v X 0,” using present data. Curves through present data serve only to guide the eye. 10-14
5
Direct
I Single
Ionization
1 ’ I Charge Transfer He2+
-
I
1
Double
’
Charge
I
Transfe
Kr
“2’ 2 ,0-15
5
/-2 22 Ol
10-16 5
“E 0 0
/
A
2 22 O2 / m
10-l’ 5 -1
l
/
.
J OZ2 3
22@0 UA O4 v
20 r) ‘6
E/M (keV/amu)
Fig. 3. Cross sections contributing to direct ionization (OF), single capture (0,“) and double capture (~420) for He2+ -Kr collisions. Present data are for target ionization charge states 4 = 1,2,3,4,5 and 6,0, n , A, +, v and * respectively. Total cross sections: u+: ref. [6], + present data using eq. (1) in text; u-: ref. [6], - present data using eq. (2) in text; u2i: ref. [6]; a”: ref. [6], v XuF using present data. Curves through
the present
data serve only to guide the eye. I. ATOMIC
PHYSICS
/ RELATED
PHENOMENA
R. D. DuBois
212
/ Simultuneow
ionizution
and charge trunsjer
tions (Cqo+?*). From the cross sections shown in fig. 3, it is seen that the majority of the free electron production at these low energies is via the higher order electron capture channels, e.g. u,‘: 1 and u,‘“>2 for single and double electron capture respectively. In fact for low energy (10 keV/amu) He*+-Kr collisions, almost the entire free electron production is via the electron capture channels. The importance of the higher order electron capture channels (channels where capture from and ionization of the target occur simultaneously) in the production of free electrons has been shown. In addition, figs. l-3 show that these channels often contribute substantially to the total single and double capture cross sections. In certain cases, it is seen that the probability of capturing one electron from the target and ionizing an additional electron is comparable to the probability of simply capturing an electron i.e. a;.‘-’ - u;,~-~. Even more astounding is that the He*+-Kr data shows that double capture plus one or more additional ionizations can be more probable than double capture alone.
larly important in the total free electron production below 100 keV in the case of krypton. Another important feature illustrated in fig. 2 is that the probability of capturing and ionizing electrons from the target (a:‘) can be comparable to the probability of simply capturing an electron (u:“). Cross sections for ionization of krypton by He’+ impact are shown in fig. 3. For He’+ impact there is no electron loss channel but the double electron capture channel is now very important. Considerable multiple ionization of the target occurs in the charge transfer channels. Summing the cross sections for individual charge states and comparing with various total cross sections [6] shows good agreement with previous measurements of u+ and u_. However, the sum of the double electron capture channels @,z”) tends to overestimate previously measured [6] total double electron capture cross sections. Of major importance for low energy He’+ impact is that the total free electron production (u-) is much larger than the sum of the direct ionization cross sec-
1 Pure Capture
-
H+,
He+,
Two (He+,
Ionizations He*+
Data
t
5)
100 E/M
Fig. 4. Neon charge state distributions capture) and ( ) (double capture)
(keV/amu)
(j) resulting from electron capture for Ht ( -), impact. Vertical arrows indicate projectile velocities electrons. Proton data taken from refs. [2.9].
He+ (---). He*+ (.-.-.) matching those of the bound
(single target
213
R. D. DuBois / Simultuneous ionizution und churge trunsfer
4. Interpretation of data The importance of electron capture plus simultaneous ionization in helium ion-atom collisions, both in the production of free electrons and in the total electron capture cross sections, has been demonstrated. It is therefore of interest in determining the mechanism responsible for multiple ionization in the electron capture channel. It is import~t to recognize that the present capture plus ionization (C + I) process is different than the transfer ionization (TI) process that was shown [8] to be important for energies smaller than those presented here. Transfer ionization can be interpreted in terms of a molecular-orbital picture [S] in which the internal potential energy difference between the initial and final electron capture states is sufficiently large to cause additional target ionization. In the present case, the higher kinetic energy of the projectile is sufficient to cause the observed additional ionization that accompanies the capture process. It is also possible that inner shell capture followed by Auger (or X-ray) relaxation could be responsible for the additional target ionization that is observed. Inner shell capture is known [9] to play an important role in the production of higher degrees of target ionization for high energy proton-atom collisions. This is manifested by a relative decrease in the pure capture cross section and increases in the capture plus ionization cross sections for impact velocities near those of the bound inner shell electron velocities. An example for H+-Ne collisions (solid curves) is shown in fig. 4 where the capture plus ionization probabilities increase sharply for proton velocities near those of the Ne Is electrons. For projectile velocities slower than those of the inner shell but faster than those of the outer shell target electrons, velocity independent charge state distributions are found. This is an indication that the collision time is sufficiently short such that the target electrons cannot adiabatically adjust after capture from the outer shell
takes place. For projectile velocities slower than those of the outer shell target electrons, there is a monotonic decrease in the relative percentage of capture plus ionization with respect to pure capture. This velocity dependent charge state distribution occurs because the collision is slow enough to allow for some readjustment of the target electrons after the capture process. The same general features are seen to occur for helium ion impact although the relative amounts of capture plus ionization increase considerably as the projectile is changed from HC to He+ to He*+. It is interesting to note that double capture plus additional ionizations tend to exhibit the same features as are seen for single capture plus additional ionizations. This implies that the same mechanism is responsible for the additional target ionization that is observed in the single and double capture channels. A more complete understanding will require additonal experimental and theoretical studies.
References
PI R.D. DuBois, L.H. Toburen
and M.E. Rudd, Phys. Rev. A29 (1984) 70. PI R.D. DuBois, Phys. Rev. Lett. 52 (1984) 2348. [31 R.D. DuBois, Phys. Rev. A32 (1984) 3319. PI R.D. DuBois, Phys. Rev. A33 (1986) 1595. [51 M.E. Rudd, T.V. Goffe, A. Itoh and R.D. DuBois, Phys. Rev. A32 (1985) 829. [61 M.E. Rudd, T.V. Goffe and A. Itoh, Phys. Rev. A32 (1985) 2128. [71Y. Nakai, A. Kikuchi, T. Shirai and M. Sataka, Japan Atomic Energy Research Institute, Report No. JAERI-M 84-069 (1984). A. Miiller and E. SaIzborn, J. PI W. Groh, A.S. Schlachter, Phys. B Lett. 15 (1982) L207. [91 E. Horsdal Pedersen and L. Larsen, J. Phys. B12 (1979) 4085.
I. ATOMIC
PHYSICS
/ RELATED
PHENOMENA