PHYSICAL REVIE%' A VOLUME 37, NUMBER 7

Low-energy collisions of 0 + with atoms anti molecnles

APRIL 1, 1988

M. S. Huq, * R. L. Champion, and L. D. l3overspikeDepartment ofPhysics, College of 8'illiam and Mary, W ill'iamsburg, Virginia 23lg5

(Received 28 August 1987)

Absolute total cross sections for single-electron capture have been measured for collisions of 0 +

ions with He, Ne, Ar, H„D2, N2, and 02. The relative collision energies of these experiments rangefrom approximately 3 to 400 eV. The electron-capture cross section for 0'++He collisions is foundto agree well with previous distorted-wave calculations. Capture cross sections are generally largerfor molecular targets than for the atomic targets.

INTRODUCTION

Studies of collisions between multiply charged ions andatomic and molecular species have been actively pursuedin recent years and are an area currently receiving con-siderable theoretical and experimental attention. Thesestudies have been stimulated by their relevance to high-temperature plasma behavior and the possible utility ofsuch processes as soft-x-ray laser systems. ' A largenumber of systems have been examined at collision ener-gies ranging from thermal to many MeV. Most recently,considerable efFort has been devoted to charge exchangebetween highly stripped ions and atomic and molecularhydrogen. These studies are relevant to the physicswhich governs the ionization balance at the "plasmaedge" of magnetically confined fusion devices. Hydrogenis the dominant atomic and molecular species in suchplasmas, but impurity concentrations are the largest atthe plasma boundary. Light ions such as carbon and oxy-gen get into discharges by desorption from the machinewalls. If the concentration of impurity ions is large,charge exchange involving the impurity ions can thenlead to signi6cant radiation cooling losses. It is there-fore important to understand charge exchange collisionsof incompletely stripped ions such as 0 +, C2+, etc. withatomic and molecular hydrogen.

Further interest in charge exchange collisions of multi-

ply charged ions such as 0 + stems from their relevanceto interstellar ionic physics. ' Electron capture in col-lisions of 0 + with He has been suggested' as an impor-tant mechanism for the destruction of 0 + ions in theterrestrial ionosphere and a potentially important sourceof He+ and metastable 0+ ions. However, experimentalstudies of single-electron capture in collisions of 0 +

with He have mostly been limited to measurements of to-tal cross sections at fairly high energies, ' ' the deter-mination of rate coeScients at thermal energies, ' ' andmeasurements of the kinetic energy distribution of prod-uct 0+ ions ' for collisions in the keV energy range.

Bienstock et a/. have calculated total cross sectionsfor single-electron capture for (0 + + He using a second-order distorted-wave approximation over the energyrange 0-5 keV/amu. Here we present the results of mea-surements of total cross sections for single-electron cap-

ture for collisions of 02+ with He, Ne, Ar, Hz, Dz, Nz,and 02 in the energy range extending from a few eV toseveral hundred eV relative collision energy.

Experimental apparatus and method

The apparatus used in the present experiments hasbeen described in detail previously. ' Severalmodi6cations to the original apparatus have been madeto perform the present experiments. The modificationsinclude the construction of a new ion source and the in-troduction of a high-resolution primary-beam massanalyzer. %e describe here only those experimental de-tails which are relevant to the present experiments. The6 + ions are produced in an electron impact ion sourcewhich operates with an electron energy of about 250 eVand an 02 gas pressure of roughly 10 3 torr. The 02+ isextracted and transmitted through a momentum analyzerhaving a resolving power of approximately 60. Aftermomentum analysis, the beam is focused into the col-lision region; a schematic of this portion of the apparatusis shown in Fig. 1.

The method used to measure the single charge ex-change cross sections o 2, is straightforward. In the caseof the reaction

0 ++He~He++0+ .

kinematical considerations imply that the product He+will have low kinetic energy, while the kinetic energy of0+ will be roughly equal to that of the incident 0 + ion.

Kith the geometry of the collision chamber shown inFig. 1, these kinematical constraints make it very easy tomeasure separately the slow and fast products. In one setof experiments, plate 3, cup 8, and grid I are maintainedat ground. The "slow" collision products (He+) are col-lected on elements A and 8 by suitably biasing grids IIand III and the Faraday cup. It is found that at a givenenergy, a retarding potential of approximately 5 k of thelaboratory collision energy placed between grids I and IIis sufhcient to saturate the signals on elements A and 8;o.2, is determined from these measurements.

As a consistency check on these measurements, thesystem is biased such that the primary beam (unscattered)

Qc 1988 The American Physical Society

2350 M. S. HUQ, R. I.. CHAMPION, AND I.. D. DOVERSPIKE

6 VARORING

8e Il I'3L % W % % % 'L % Xl

I(II I I

I I I

0 +(2p Po)+He(ls 'So)~O+(2p Pi~~)

+He+(Is S&&2)

will be denoted by IyX. In what follows we will first dis-cuss the results for the rare-gas targets followed by a dis-cussion of the molecular targets.

il i

I i I

%%&IM XXX X %%%1 [

~ ~ 4

r ' ar

FIG. 1. Schematic diagram of the collision region.

is completely turned around before reaching grid II, al-lowing only the "fast" 0+ signal to be measured at theFaraday cup. This signal is also used to determine oz„thus giving an independent check on the measurement ofthe cross section and a verification that the two-electrontransfer cross section tris is negligible for the 0 ++Hesystem for our collision energies.

The laboratory energy of the primary beam is deter-mined by a retardation analysis; the energy width of thebeam varies from. about 5 eV full width at half maximum(FWHM) at the lowest energies to approximately 15 eVFWHM at the highest energies. Typical beam currentsare in the neighborhood of 0.03 nA. The cross sectionsreported here have an absolute accuracy of 210% andare reproducible to within 10%.

O~+-He collisions

The various processes that may be of interest for thepresent studies are listed in Table I. Figure 2 shows theresults of the present measurements of the total cross sec-tion for single charge transfer 0.» in collisions of 0 +

with He as a function of relative collision energy. Alsoshown in the 6gure are the low-energy measurements ofMaier and Stewart, 2s along with the theoretical calcula-tions by Bienstock et al. ' of 0.

2&, which includes only thecontributions from the IPX and IyX channels. Theagreement between the present results and those of Maierand Stewart is excellent.

One cannot rule out the possibility that metastablesafFect our measurements. However, several stud-ies' ' ' ' ' suggest that electron capture in metastable0 +-He collisions is an unlikely process in the energyrange of this study. Energy-loss and/or energy-gain stud-ies of 0+ production by Kamber et a/. have shown thatmetastables make a small contribution to charge ex-change in 6-keV, O~+-He collisions. Also, below 6 keV, aLandau-Zener- and Rapp-Francis-type calculation byHird and Ali' both indicate that oz, is small (cr ~0. 1

A ~) for collisions of metastables ('D2 and 'So) with He.Differential studies of single-electron exchange by Hastedet al. suggest that the charge exchange probability inmetastable 02+-He collisions decreases as the laboratoryenergy is decreased from 3 to 1 keV. Further indicationsof a small-capture cross section for metastables come

The ion source used in the resent experiments is cap-able not only of producing 0 + ions in the ground state( Po), but also in the metastable states ('D2 and 'So).These metastable ions have sufficiently long lifetimes tosurvive the transit time to the collision chamber and con-sequently may contribute to the scattering signal. In thepresent experiinents it was not possible to determine theexact fraction of metastable ions present in the ion beam.Just how much of a role these metastables play in our ex-periments is uncertain.

A large number of Snal-state charge exchange channelsare possible for the collison systems reported here. In or-der to identify the various processes in a systematic way,we will adopt a notation suggested by Kamber et al.Roman numerals I, II, and III are used to designate theground state ( Po) and metastable states ('D2 and 'So),respectively, of the Oi+ ions. a, P, y, 5, and e, etc.,represent the ground state and successive higher excitedstates of the O+ product ions. The ground and succes-sive higher excited states of the target product ion aredesignated by X, A, 8, C, D, etc., respectively. Thus, forexample, the reaction

l4-l2—

~0

I I I

50 IOO

E,~ (ev)

I

f50

FIG. 2. Absolute total cross section for single-electron ex-change for the helium target as a function of relative collisionenergy. The closed circles are the present results. The curve la-beled represents the results of Maier and Stewart. Thecurves labeled ———and —.—.—.are the calculations ofBienstock et a/. for single charge exchange into the 6nal chan-nels, IPX and IyX, respectively. The solid curve is the sum ofthese calculated cross sections.

37 LOW-ENERGY COLLISIONS OF O'+ %ITH ATOMS AND MOLECULES

TABLE I. Reactions for single-electron capture for 0 +-He collisions.

Reactants andinitial states

0 +(2p 'I') + He

0 +(2p 'D2)+ He

02+{2p2 1g )

Products andfinal states

~O+(2p 3 S)+ He+(1s S)~O+(2p D)+ He+~O+(2p 2P) + He+

~O+(2p S) + He+~O+(2p 2D)+ He+~O+(2p 2P)+ He+

~0+(2p S) + He~O+(2p D) + He+~O+{2p P) + He+

O+(2p P) + He+~O+(2p ~D)+ He+

A„(a.u. )

2.583.764.91

2.082.793.38

1.712.162.5

25.67

+ 10.567.245.54

13.079,758.05

15.9112.5810.89

1.06—4.66

Designationof reaction

process

IaXIpxI@X

IIaXIIpXIIyX

IIIaXIIIpxIIIyXIII5XIII'

from rate coeScient calculations by Dalgarno et al. ,'

which clearly suggest that the Po state of 0 + reactswith He and the IlnX channel has a negligibly slow ratenear thermal energies. Johnson and Biondi' have point-ed out that IIISX should be the favored channel if the 'Sostate of 02+ is present. However, R„(see Table I) for theIIISXchannel is too large for eiIicient charge exchange totake place. ' Based upon these observations it seemsreasonable to assume that a» in Fig. 1 will have a negli-gible contribution from any metastable iona present inthe beam.

In a recent energy-loss study of charge exchange in the0 +-He system, Kamber et al. found that the dom-inant process at 80 eV laboratory energy is capture fromground state 0 + into the IyX channel, with no contribu-tion from the IaX channel. Thus it appears that thedominant contributions to the measured o 2& in the energyrange of the present study are from the channels IPX andI@X. These findings are consistent with the theoreticalconsiderations of Bienstock et al. ' and our measuredo.2, is in good agreement with the calculations. It is stillpossible, however, that metastables are present in thebeam and that o2, (metastables) is small. Under thesecircumstances our measurements represent a lower limiton o.2(, and any increase in the measured o2) due to suchan effect would lead to even better agreement with thecalculations.

0 +-(Ne,Ar) collisions

Flgule 3 shows the plesent 0 21 measurements for colisions of O + with Ne and Ar; the processes of interestfor these studies are listed in Table II. One noteworthyfeature exhibited by o 2, is that in both cases it is essen-tially energy independent, i.e., varies very slowly withcollision energy over the energy range of the measure-ments.

DifFerential studies of O +-Ne collisions at 1 keV byHasted et al. " indicate that the IaX channel dominatescharge exchange, with minor contributions from the IPXchannel. The IaX and IPX channels are estimated to

bl~ t6—La) l2-b

0 0,QQ00000000

~ ~ 0 0~~

00 l00 200

E„~ (ev)

FIG. 3. Absolute total cross section for single-electron ex-change as a function of collision energy. The open circles arefor the argon target and the closed circles are for the neon tar-get.

0cross the incident state at 1.06 and 1.41 A, respectively.Thus, if the single charge exchange cross section isrepresented by the gas-kinetic limit, o 2j —IIR„,the crosssections for the IaX and IpX channels would be approxi-mately in the range 3-6 A . The measured o 2, is of thisorder of magnitude over the collision energy range of thepresent studies.

It should be noted that the Landau-Zener- and Rapp-Francis-type calculations by Hird and Ali' indicatenegligibly small cross sections for charge exchange inmetastable 0 +-Ne colhsions in this energy range.

The electron-capture cross section for the Ar target isapproximately 6ve times larger than that for Ne for theenergies studied here, and is characteristic of what mightbe expected when many channels contribute to o2&.Landau-Zener calculations of o 2, in metastable 0 +-Arcollisions indicate that at about 5 keV, o z, is approxi-mately 2 A . DifFerential studies of this system by Hast-

M. S. HUQ, R. L. CHAMPION, AND L. D. DOVERSPIKE 37

TABLE II. Reactions for single-electron capture for 0 +-(Ne,Ar) collisions.

Reactants andinitial states

0 +(2p P)+ Ne

02+(2p 2 ID )

0 +(2p 'S ) + Ne

Products and6nal states

~0+(2p3 S) + Ne+(2p' P)~O+(2p D) + Ne+~O+(2p P) + Ne+

~O+(2p' S) + Ne+

—+0+(2p S) + Ne+

8, (a.u. )

2.02.663.10

1.69

1.44

+ 13.5510.238.53

16.06

Designationof reaction

process

IaXIPXI@X

IIaX

IIIaX

0 +(2p 'P) + Ar

0 +(2p 'Dz)+ Ar0 +(2p 'So) + Ar0'+(2p' 'D, ) + Ar0 +{2p So) + Ar

~0+(2p S) + Ar+(3p' 2P)

0+(2p D) + Ar+~O+(2p' P) + Ar+

0+(2p 4P)+ Ar+

0+(2p P) + Ar+~0+(2p P) + Ar+—+0+(2p S) + Ar+~O+(2p S) + Ar+

141.691.896.0

3.882.761.241.10

19.3516.0314.334.5

7.09.85

21.8624.7

IaXI@XIyXI5X

II5XIII5XIIaXIIIaX

ed et al. at energies between 1 and 3 keV reveal thatthe single-charge-exchange probability is dominated bythe IISX and IIIeX channels with small contributionsfrom the I5X channel. The estimated crossing radii forthe IISX and IIIeX channels are 2.05 and 1.46 A, respec-tively. Assuming unit transition probability at thesecrossings leads to gas-kinetic limitiny charge exchangecross sections of approximately 7-13 A . Since our mea-sured crz, is substantially larger than these limits, it ap-pears that the 15X channel (with a crossing at 3.17 A. ) isimportant at the low collision energies reported here.

0 -(Hg, Dg,Ng, Og) Colllslolls

The number of possible exit channels increases

significantly when molecular targets are involved; s list of

these channels, which are relevant to the present studies,is presented in the paper by Kamber et al.

The present results for 0.2& for the H2 and 02 targets

are plotted in Fig. 4 along with the lowest-energy mea-surements of Phaneuf et al. for the H2 target; the resultsfor N2 and 02 are shown in Fig. 5. Although our mea-surements for the H2 target do not overlap the same ener-

gy as those of Phaneuf et al. , there appears to be s sub-stantial disagreement between the two measurementswhich is not understood at the present time.

The general shape of o 2& for these molecular targets isconsistent with the idea that exchange takes place at fair-ly large internuclear distances with the exchange proba-bilities being essentially unity. Energy-loss and/orenergy-gain studies of Kamber et ol. for 6-keV 0 + in-cident on H2 and N2 reveal that many crossings are in-

volved in these collisions; the C X„+ and D II excitedstates of Nz+ are the principal product channels for

le—0l4-

~ 0l2 +0

00()0O0&0

Og

—30—0 0 0 0 0 0 0 0 00 y Og ~ ~ 0 0

I

Km l50 XN

E~ (eV)

00

I

l00 200E ei ('~'

FIG. 4. Absolute total cross section for single-electron ex-change as a function of collision energy. The open circles arefor the D2 target and the closed circles are for the H2 target.The triangles are from Phaneuf et aI.

FIG. 5. Absolute total cross section for single-electron ex-change as a function of collision energy. The open circles arefor the 02 target and the closed circles are for the N2 target.

37 L0%-ENERGY COLLISIONS OF 0'+ %ITH ATOMS AND MOLECULES 23S3

0 +-Nz whereas Hz+(X X +) dominates the 0 +-Hz2 gsystem. Small contributions from Dt metastables werealso observed. It should be pointed out that Johnson andBiondi' have found no difrerences in the rate coeScientsfor thermal-energy reactions of ground-state and metasta-ble 0 + ions with Nz, Oz, and C02.

Absolute total cross sections for single-electron capturehave been measured for collisions of 0 + with variousatomic and molecular targets. The capture cross section

for the He target is found to agree well with thedistorted-wave calculation of Bienstock et al.

One of us {M.S.H.) would like to thank T. G. Heil, R.A. Phaneuf, and F. %. Meyer for many helpful discus™sions. He woold also like to thank E. Y. Kamber for acopy of their paper prior to publication. This work vvas

supported in part by the Division of Chemical Sciences,OSce of Basic Energy Sciences of the U.S. Departmentof Energy.

'Present address: Oak Ridge National Laboratory, Building6003, P.O. Box X, Oak Ridge, TN 37831.

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(1971).A preliminary energy-loss snd/or energy-gain study byKsmber et aL for the O~+-He system st 80 eV impact energysho~s that the metsstable states of O~+ have negligible con-tributions in charge transfer collisions.