Nuclear Instruments and Methods in Physics Research A262 (1987) 69-77

69North-Holland, Amsterdam

ONE- AND TWO-ELECTRON PROCESSES IN COLLISIONSOF HEAVY IONS WITH HZ AND He

Patrick RICHARD, James HALL, J.L . SHINPAUGH, J.M . SANDERS, T.N . TIPPING, T.J.M. ZOUROS,D.H . LEE and H. SCHMIDT-BÖCKINGJ.R . Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas 66506, USA

In thus paper we present a description of the apparatus and results for experiments involving one- and two-electron processes incollisions of heavy ions with H Z and He . The experiments were performed using one-electron and bare projectiles . In the first sectionwe describe the measurement of pure ionization of one-electron projectiles by H z targets and compare with previous results for Hetargets . We also present the results for one-electron capture by the projectile from H 2 targets . The energy dependence of the crosssections is compared to theoretical predictions for atomic and molecular hydrogen targets . Both experiments were performed bymeasuring only the final charge state of the projectile . In the second section we describe the measurement of partial cross sections forthe same collisions by measuring the target recoil charge state in coincidence with the projectile charge state . By this method we canmeasure pure single- and double-ionization of the target, pure single-electron transfer and transfer ionization, and pure double-elec-tron transfer . This experiment is presently being performed for bare fluorine on He ; however, absolute cross sections are notavailable at the time of thus conference .

1 . One-electron charge changing processes for fast pro-jectiles incident on molecular hydrogen

In this section, the results for one-electron loss andone-electron capture reactions by the projectile inmolecular hydrogen are presented. The measurementswere performed in a highly collimated beam line con-sisting of a differentially-pumped gas cell followed byan analyzing magnet with a 5 ° bend. The analyzedfinal projectile charge states were detected using a posi-tion sensitive surface barrier detector in the initial ex-periments. Later, a parallel plate avalanche detector(PPAD) fitted with an anode board consisting of 16vertical strips feeding 16 independent fast electroniccircuits was used . The details of the beam line areshown in fig. 1 and a sketch of the parallel plateavalanche detector is given in fig . 2. Recently, theparallel strip anode board was replaced with a one-di-mensional backgammon board which required only twofast electronic circuits feeding a sum-and-divide circuitto obtain position information . In both cases isobutaneflowed through the PPAD at a pressure of a few (- 20)mTorr. The PPAD had three grids that typically oper-ated at -500 V, ground, +500 V, respectively, and ananode which operated at +600 V. The voltage on thethree grids and their relative positions were varied tooptimize position information and the large signal-to-noise ratio .

* On leave from the University of Frankfurt, FRG.

Fig. 1. Diagram of experimental apparatus.

II . TWO-ELECTRON CORRELATED PROCESSES

70

Fig 2. Diagram showing cross sectional and end views of parallel plate avalanche detector (PPAD) with parallel strip anode board.

1 .1 . Ionization (one-electron loss) of oxygen and fluorineprojectiles incident on molecular hydrogen

In this section we describe the collision system of ahighly charged one-electron projectile incident upon amolecular target in which the projectile electron is re-moved to the continuum by the target molecule . Theionization of a tightly bound electron by a low Z atom(ton) is well described by perturbative theories ; how-ever, no procedure has been developed to account forthe ionization of a tightly bound electron by a molecule .

In this experiment we studied the ionization of one-electron oxygen and one-electron fluorine by molecularhydrogen over the projectile energy range of 0.5-2 .5MeV/amu. In previous studies, Dillingham et al . [1]measured the ionization of the same projectiles by neu-

P Richard et al. / Collisions of heavy ions with H, and He

FAST PRE-AMPLIFIER

tral He targets over the same energy range. In thepresent work, we compare the experimental results forH 2 targets to those for He targets . Using the Z2 scalingof ionization as predicted by the Born approximation[2] (where Zt represents the target atomic numbers inthis case) and the assumption that the two hydrogenatoms of Hz act as independent ionizing centers, onewould conclude that He would be twice as effective asH 2 in ionizing a one-electron projectile . Early measure-ments in our lab had indicated that the ionizing strengthof the two targets were nearly equal. However, we havefound from these later experiments that the factor oftwo obtained from the simple Zi scaling with theindependent atom approximation in fact works verywell in predicting H2 cross sections from observed Hecross sections . We also compare to the predictions of

the Born approximation for ionization by atomic hydro-gen multiplied by two to account for the two scatteringcenters in Hz .

The cross sections for ionization are determinedfrom the target pressure dependence of the projectilecharge fractions, the so-called "initial growth" method .Typical results are given for 33.25 MeV F8+ + H Z infig. 3a. The slope of the 9 + fraction in this case isproportional to the one-electron loss cross section . Thecross sections obtained for the cases of 07+ + H zO8+ + H 2 + e - and for F8+ + H 2 - F9+ + H 2 + e- aregiven in fig . 4. The comparison of the F8+ + H Z crosssections with those of Fs+ + He are shown in fig . 5where an approximate factor of 2 difference is seen inthe cross sections . Fig . 6 shows the comparison withGlauber calculations [3] for bare and screened atom-ic hydrogen potentials, each multiplied by two . Thebare hydrogen ion prediction slightly underestimates(- 20%) the observed cross sections . Whether this dis-crepancy is due to the independent atom model or thetheoretical atomic cross section could be determined bytesting the theoretical atomic cross section with a mea-surement of the ionization by an atomic hydrogen target .The complete results of this experiment are to be pub-lished at a later time [4] .

1 .2 . Charge transfer (one-electron capture) to oxygen andfluorine projectiles incident on molecular hydrogen

In this section we describe the collision system of ahighly-charged bare or one-electron projectile incidentupon a molecular target in which the projectile picks upone electron from the target molecule . This processrepresents the sum of the cross sections for twoprocesses . The first process is pure one-electron transferand the second is transfer ionization in which the one-electron transfer is accompanied by additional ioniza-tion of the target in the same collision. The former givesrise to a He' and the latter to a He2+ after the collision.An experiment that measures the two separate partialcross sections for these two processes is discussed insection 2 .

As in the case of ionization, little theoretical workhas been done on electron capture from molecularhydrogen targets . The ratio of one-electron capture fromH2 targets to that from H has been measured from lowZ projectiles and is known to vary from a constantvalue of - 0.8 at low energy to -- 3 .8 at high energy [5] .Thus, for one-electron capture, molecular hydrogen can-not be simply treated as two independent hydrogenatoms as could be done above. In this experiment, weinvestigate the high energy behavior of the total crosssection for one-electron capture from H2 .

The experiments were performed exactly as de-scribed in section I .1 . Fig. 3b is a typical target pressuredependence for 33.25 MeV F9+ + Hz . The slope of the

P Richard et al / Collisions ofheavy ions with H, and He

-9-z0

IrlLwâ

w

- 8=

z0

UQWWâF

w(0aU

100

090

080

070

060

0 .50

040

a030

U

020

010

1 .00

0.90

aso

0.70

Q60

Q50

040

030

020

0.1 0

(a) 33.25 MeV F8+ + H2

0 5 10 15 20 25 30 35

PRESSURE (mTorr)

(b) 33 .25 MeV F9+ + H2

0 0

0 e+(000)

0 7+( .10)

0 5 10 15 20 25 30 35

PRESSURE (mTorr)

71

Fig . 3 . Projectile charge state pressure dependences for (a)33 .25 MeV F8+ incident on Hz and (b) 33 .25 MeV F9 +

incident on H2 .

8 + fraction in this case is proportional to the one-elec-tron capture cross section. The results of these measure-ments are shown in figs . 7, 8, and 9 . In each case themeasured cross sections are compared to the Bohr-Lindhard model [5], an empirical scaling formula due toSchlachter et al . [6] and an OBK calculation for atomichydrogen scaled to fit the data [7] . The Bohr-Lindhard

II . TWO-ELECTRON CORRELATED PROCESSES

72

NU

rUw

OtrU

ZO

QNZO

Fig. 4. Comparison of projectile ionization cross sections forF 8+ and O' 7+ incident on H 2 . The solid curves are drawn to

guide the eye

NEU

10 19UNV)

U

1020

Energy

(MeV )

Fig. 5. Comparison of projectile ionization for Fs+ incident onHe [1] and F8+ incident on H2. The solid curves are drawn to

guide the eye.

P. Richard et al. / Collisions ofheavy ions with H, and He

NEU

NOU

1018

-

- Glauber with

screened Hydrogen Potential (x2)

-Glauber with bare Hydrogen Potential (x2)

.F+e

+ H2 Projectile ionization

IF0 10 20 30 40 50 60 70

Energy

(MeV )

Fig 6. Comparison of F8+ incident on Hz data with Glaubertheory for F8+ incident on H multiplied by two. The solidcurve represents the Glauber theory with a bare hydrogenpotential . The dashed curve represents the Glauber theory witha potential which accounts for both screening and scattering by

the hydrogen electron .

I Ô1s

,Ô 17

10 20

10 15 20 25 30 35 40 45

Projectile Energy (MeV)

Fig. 7. Comparison of F9+ on H2 cross sections with variouscalculations . The dashed curve represents the Bohr-Lmdhardtheory [5] . The solid curve represents a scaled OBK calculation[7] for F9+ incident on H. The dot-dash curve represents the

empirical formula of Schlachter et al . [6] .

10 17

cn

ôU

10 19a~

âU

1 016

1 020

Fig . 8 . Comparison of F8+ on H2 cross sections with variouscalculations . The dashed curve represents the Bohr-Lindhardtheory [5] . The solid curve represents a scaled OBK calculation[7] for Fs+ incident on H. The dot-dash curve represents the

empirical formula of Schlachter et al . [6].

ôU 1p'9

1ô 20

5 10 15 20 25 30 35 40 45

Projectile Energy (MeV)

5 10 15 20 25 30 35 40 45

Projectile Energy (MeV)

Fig. 9. Comparison of O7+ on H z cross sections with variouscalculations . The dashed curve represents the Bohr-Lindhardtheory [5] . The solid curve represents a scaled OBKcalculation[7] for O7+ incident on H. The dot-dash curve represents the

empirical formula of Schlachter et al . [6] .

P. Richard et al / Collisions ofheavy ions with H, and He

[8].

73

model has a v-7 dependence, while the OBK calcula-tion and the empirical Schlachter formula both have av -9 dependence in this energy range. The data alsoshow a v -9 dependence. A comparison between experi-ment for molecular hydrogen targets and scaled OBKtheory for atomic hydrogen targets indicates that theratio of the cross section for molecular hydrogen to thatfor atomic hydrogen may be constant over this energyrange. We plan to perform the experiments with atomichydrogen at a later date for the high velocity, high Zprojectiles studied in this experiment . The one-electroncapture cross sections have been measured previouslyfor He targets [1] for the same projectiles over the sameenergy range. We have repeated some of these measure-ments as a crosscheck on our absolute cross sections,and we find excellent agreement with the previous mea-surements. Some of the results of this experiment arebeing prepared for a paper submitted to XV ICPEAC

2. Partial cross sections for one- and two-electronprocesses in collisions of bare ions with He

In the previous sections we discussed one-electroncharge changing processes measured using only the finalcharge state of the projectile ion. In this section wediscuss the method of measuring the final charge stateof the recoil He ion in coincidence with the final chargestate of the projectile ion. This technique allows us tomeasure the five possible charge-separated cross sec-tions described below. The experiment is performedusing the same apparatus used to measure the crosssections with molecular hydrogen targets with the ad-dition of the recoil ion source as shown in fig. 1. Therecoil ion source was submerged in the differential gascell and pumped from behind by a turbomolecularpump to maintain proper operating pressure for themultichannel electron multiplier plates (MCP) used todetect the recoil ions . Fig. 10 depicts the time-focusingrecoil ton source used in the experiment. The pusherplate was typically operated at +400 V, while the gridson the drift region were held at ground potential . Thecharging rings were held at -1800 V, -1200 V, -600

V, and ground potential, respectively, as required by theMCPs.

In fig. 11 a coincident projectile ion spectrum (fig .11a) and a coincident recoil ion spectrum (fig . 11b) areshown for a 2.0 mTorr He target . Because He-Hereaction cross sections are on the order of 10-15 cmz ,the pressure dependences must be performed below 1mTorr. A typical two-dimensional coincidence spectrumwith the appropriate projections is shown in fig . 12 . Thefive possible charge-separated partial cross sections arelisted and discussed below.

II . TWO-ELECTRON CORRELATED PROCESSES

74

2.1 Single ionization of He

The cross section for the reactionF9+ + He(1s2) - F9+ + He+ (ni) + e-

2 .15"

is here referred to as single ionization of He and labeleda99. The subscripts refer to the incident and scatteredprojectile charge states, respectively, and the super-scripts refer to the initial and final target charge states,respectively . For the case of 1 MeV/amu F9+ + He, weobtained a preliminary cross section of order - 10-15cm2. This process is observed as the intense peak atchannels x= 205 and y = 64 in fig . 12 .

2.2 . Double ionization of He

The cross section for the reaction

F9+ + He(ls2 ) - F9+ + He2+ + 2e

is here referred to as double ionization of He and

P Richard et al / Collisions of heavy tons with H, and He

ANODE

MULTI-CHANNELELECTRON MULTIPLIERPLATES (MCP)

BEAM AXIS

0 .75"

1 .50"

Fig . 10 Diagram showing cross-sectional view of recoil ion source .

2.3. Single electron transfer

CHARGING RINGS

PUSHER PLATE

labeled a99 with the notation described above. For thecase of 1 MeV/amu F9+ + He, we obtained a pre-liminary cross section of order - 10 -16 cm2. This givesa ratio a9,2/9

9 - 0.1 in approximate agreement withthe results of Knudson et al . [9] and McGuire et al . [10] .The process labeled a99 appears in fig . 12 at x = 205and y= 96 .

The cross section for the reaction

F9+ + He(ls2 ) -> F8+ (nl) + He+(n'l')

is here referred to as single electron transfer and labeleda°s with the notation described above. The He+ ionmay or may not be in the ground state, but we do notdistinguish between the two cases . For the case of 0.7MeV/amu F9+ + He, we obtained a preliminary crosssection of order - 9 x 10-18 cm2 .

>n

Ga01GH

S00

400

300

2W

i0()

5000

4o00H

3000F-

2000E

10001-

L

P . Richard et al. / Collisions ofheavy ions with H2 and He

Projectile Ion Spectrum (ADC 2)

19 .00 MeV F9+ + He (2 .00

80 38 112 128 144PogrtiL)n

mTorr)

Recoil Ion (Time of Flight) Spectrum (ADC 1)

19 .00 MeV g9+ + lie (2 .00 iâTorr)

98 112 128 144 180

Time of Flight

1180 ^176 192 206 224 24D 258

178 192 208 224 240 258

Fig. 11 . Typical (a) projectile ion and (b) recoil ion (time-of-flight) spectra for 19 MeV F9+ incident onrnTorr.

2.4. Transfer ionization

The cross section for the reaction

F9+ + He(lsz ) -> F` (n, 1) + Hez+ + e-

is here referred to as transfer ionization and labeled

8a9with the notation described above . For the case of 0.7MeV/amu F9+ + He, we obtained a preliminary crosssection of order - 2 x 10-17 cmz . This yields a ratio oftransfer ionization to single electron transfer of about2.0 which agrees with the scaling calculation by Mc-Guire [11] the results of Tanis et al . [12] for Z= 8, and

75

He at a pressure of 2 .00

the trend of the measurements of Knudsen et al . [13]Shah and Gilbody [14], and Horsdal-Pedersen and Lar-sen [15] for Z < 3 . We will do a definitive experimentwith a reduced background contribution due to pro-jectile charge impurity .

2.5. Double electron transfer

The cross section for the reaction

F9+ + He(1sz ) -> F7+ (nl, n'l') + He"

is here referred to as double electron transfer and

11 . TWO-ELECTRON CORRELATED PROCESSES

76

P. Richard et al. / Collisions of heavy eons with H, and He

CNTS LT l SUP-RESSEO

OT 1000

4CA-95ç

1C0-3SS

40-99

10-39

1_c

tNT

Ti

GNJ-+GH

labeled a9.; with the notation described above. For thecase of 1 MeV/amu F9+ + He, we obtained a pre-liminary cross section < 10-t9 cm2. This yields a ratioof single to double electron transfer of < 50 . It shouldbe noted that the double electron transfer cross sectioncould be deduced from simply observing the final pro-jectile charge state as in section 1 if it were not for theeffects of contaminant background gas reactions, e.g.recoil ions in channels 16-60 in fig . lib reflect residualH2O, N2, 02, etc., and projectile charge state impurityentering the gas cell. For example, the cross section a9~manifests itself at channels x - 100, y - 96 in fig. 12whereas a fairly large peak due primarily to projectilecharge contaminants lies at channels x - 100, y -- 64.The effect of the projectile charge contaminants appearsin the pressure dependence shown in fig . 3b where theF7+ charge fraction pressure dependence is essentiallyflat, indicating that the double electron transfer crosssection is very small compared to the processes associ-ated with the projectile charge contaminants .

32 64 66 128 i6C 192 224

Position

3. Current status of experiments

2Z;6

224

192

L160 00

" .4.-iCu

128

64

32

0

WO

NE

Fig . 12 . Typical two-dimensional spectrum coincidences between projectile ions (ADC 2; cf. fig . lla) and recoil ions (ADC 1 ; cf . fig.lib) for 19 MeV F9+ incident on He at a pressure of 2.00 mTorr. The sizes of the data points represent the relative coincident

intensities

We have measured total cross sections for one-elec-tron capture from molecular hydrogen as reported insection 1 .2. We plan to build a do discharge tube atomichydrogen source to enable us to investigate one-electroncapture from atomic hydrogen .

Test spectra have been obtained for recoil ion-pro-jectile ion coincidences for high velocity bare ions onHe targets as reported in section 2, but no definitivecross sections have been determined at the time of thismeeting. A PPAD utilizing a "backgammon board"anode has been developed for this work and is perfor-ming very well in the coincidence mode. From theobserved spectra it is obvious that the experiment canbe improved by reducing the background from contami-nant projectile charge states . The beam line backgroundgas pressure is _ 10-7 nun Hg and cannot be easilyimproved, but based on our calculations and observa-tions this is not the problem. The upstream slits whichdefine the beam are thought to be the primary source ofprojectile charge impurity. We plan to insert a magnet

just before the gas cell to remove the impurity compo-nents from all upstream sources. Murphy's law shouldcontinue to apply as we improve on the experiment andfurther reduce the errors .

Acknowledgement

This work was supported by the Division of Chem-ical Sciences, U.S. Department of Energy.

References

[1] T.R. Dillingham, J.R . Macdonald and Patrick Richard,Phys . Rev. A24 (1981) 1237 .

[2] G. Basbas, W. Brandt, and R. Laubert, Phys . Rev. A7(1973) 983 ; G. Basbas, W. Brandt, and R. Laubert, Phys.Rev. A17 (1978) 1655 .

[3] J.E . Golden and J.H . McGuire, Phys . Rev. Lett . 32 (1974)1218 ; J.H . McGuire, Phys . Rev. A26 (1982) 143; J.H.McGuire, N. Stolterfoht and P.R . Simony, Phys. Rev. A24(1981) 97 .

[4] T.N . Tipping, Master's Thesis, Kansas State University(1986) unpublished; and T.N . Tipping et al., to be sub-mitted for publication.

[5] H. Knudsen, H.K . Haugen and P. Hvelplund, Phys . Rev.

P. Richard et al. / Collisions ofheavy ions with HZ and He 77

A24 (1981) 2287 ; H. Knudsen, H.K . Haugen and P.Hvelplund, Phys. Rev. A23 (1981) 597.

[6] A.S . Schlachter, K.H . Berkner, H.F . Beyer, W.G . Graham,W. Groh, R. Mann, A. Müller, R.E. Olson, R.V . Pyle,J.W . Stearns and J.A . Tanis, Phys . Scripta T3 (1983) 153;A.S . Schlachter, J.W . Stearns, W.G . Graham, K.H .Berkner, R.V . Pyle andJ.A. Tanis, Phys . Rev. A27 (1983)3372.

[71 M.R.C . McDowell and J.P. Coleman, Introduction to theTheory of Ion-Atom Collisions (North-Holland,Amsterdam, 1970) pp . 375-381.

[8] J.M . Sanders, T.N. Tipping, James Hall, J.L . Shmpaugh,D.H . Lee and Patrick Richard, Abstract for XV ICPEAC,Brighton, England, 1987, p. 679.

[91 H. Knudsen, L.H . Andersen, P. Hvelplund, G. Astner, H.Cederquist, H. Danared, L. L1leby and K.G . Rensfelt, J.Phys . B17 (1984) 3545 .

[101 J.H . McGuire, A. Müller, B. Schuch, W. Groh and E.Salzbom, Phys . Rev. A35 (1987) 2479 .

[111 J.H . McGuire, to be published.[12] J.A. Tanis, M.W. Clark, R. Price, S.M . Ferguson and R.E .

Olson, Cord. on Physics of Multiply Charged Ions,Groningen, The Netherlands (Sept. 1986) Nucl . Instr. andMeth . B23 (1987) 167.

[13] H. Knudsen, L.H. Andersen, P. Hvelplund, J. Sorensenand D. Ciric, J. Phys. B20 (1987) L253 .

[14] M.B . Shah and H.B. Gilbody, J. Phys . B187 (1985) 899.[15] E. Horsdal-Pedersen and L. Larsen, J. Phys. B12 (1979)

4085 .

II . TWO-ELECTRON CORRELATED PROCESSES