Capture of quasifree electrons into highly charged, heavy projectiles

86

Nuclear Instruments and Methods in Physics Research B56/57 (1991) 86-91. North-Holland

Capture of quasifree electrons into highly charged, heavy projectiles Th. Stijhlker ’ and P.H. Mokler CSI-Darmstudt and Uniuerstfy Giessen, Germany

C. Kozhuharov, E.A. Livingston 2 and J. Ullrich GSI-Darmstadi, Germany

In collisions between highly charged, heavy ions and light target atoms at adiabaticity parameters of 0.5 2 u’/u’ : 0.75, resonant transfer and excitation (RTE) is an import~t charge-exchange process. Experiments for medium Z-ions are reviewed. Special emphasis is given to the competing radiative electron capture (REC) process. The general dependences of REC are discussed. In particular, we elucidate results obtained by the triple X-ray/X-ray/charge-changed particle coincidence technique for hydrogenic proJectIles up to 36Kr3” ions. With this method we could isolate a single RTE resonance, the 2s2p’Pr state, which stabilizes radiatively to the metastable ls2s’S0 state. This state can only decay via two photon emission (2El). Applying the triple coincidence technique we observed additional strong photon coincidences induced by cascades following REC into high projectile states.

1. Introduction In collisions between highly charged, heavy projectiles and low-Z target atoms, resonant transfer and excitation (RTE) and radiative electron capture (REC) are two important inner-shell processes leading to X-ray emission. Both of these processes are induced by the capture of a quasifree target electron. In the velocity regime of interest, the impulse approximation can be applied. Within the impulse approximation, RTE and REC are considered as the time reversals of fundamental atomic effects, the Auger process and the photoelectric effect. respectively. h4ainly RTE has been studied intensively for various collision systems up to 2 = 92, showing in general good agreement between experiment and theory [l-6]. REC is also a well estab~shed process in heavy-ion atom collisions [7-g]. Recently, this latter process gained increasing attention due to its importance for the present generation of accelerator facilities such as e.g. storage rings [lo].

2. Resonant transfer and excitation For RTE the interaction between a quasifree target electron and a projectile electron leads to resonant

’ Part of Ph.D. thesis project, Justus Liebig Universitat Giessen, Germany. 2 Notre Dame University, IN, USA.

capture where the energy gained by the captured target electron is used (via electron-electron interaction) to excite an electron of the incident projectile. This process forms a doubly excited intermediate state which may stabilize radiatively. For example, at the KLL resonance (Auger notation: excitation from the K to the L shell by simultaneous capture into the L shell) of hydrogenic projectiles, this process forms a doubly excited state with two electrons in the L shell and two K-shell vacancies. This state can then decay via successive emission of a I(@ hypersatellite photon and a Ku satellite X ray. The energy condition for this resonance is given in terms of the adiabaticity parameter n by q =i + (with n = u’/u’, where u denotes the projectile velocity and u the velocity of the projectile K-shell electron). Other resonances are located at co~espondingly higher collision energies up to TJ= s for the KLcc resonance limit. As an example, fig 1 shows the measured totaf projectile K X-ray yield measured in Ge”l++ II, collisions for beam energies between 11 and 20 MeV/u [ll]. The KLL-RTE resonance is located at 13 MeV/u corresponding to n = $. The width of the resonance reflects the momentum distribution (Compton profile) of the electrons in the target 15,121. Obviously, KLL-RTE is the dominant X-ray production mechanism. Also the higher RTE resonances are well enhanced compared to competing processes such as collisional electron capture, projectile K-shell excitation, and radiative electron capture into excited projectile states. By using the Xray/X-ray coincidence technique to detect the two cascade Kcr X-rays at the KLL-resonance, the other competing processes are strongly suppressed [6,13].

0168-583X/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

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Th. Stijhlker et al. / Capture of quaszfree electrons

2000

‘2

1500

% n -

1000

D”

t

may also result further radiative

32Ge3” + H2

in X-ray/X-ray stabilization.

coincidences

through

3. Radiative electron capture

I 500 1

12

14

Beam

20

18

16

Energy

(MeV/u)

Fig. 1. K X-ray cross sections measured in Ge31+ + H, collisions. The resonance positions for RTE are marked [Ill.

Fig. 2 shows the measured excitation function for RTE in Ge3it+ H, collisions obtained by measuring K/K X-ray coincidences with two Si(Li) detectors, mounted 180 o apart, both at observation angles of 90 o to the incident beam. A detailed description of the setup used in this experiment is given in ref. [13]. The dashed curve in fig. 2 presents the calculated RTE cross section using the method of Brandt [12]. The Auger rates used were taken from ref. [14] and the radiative rates were calculated by using the multiconfiguration Dirac-Fock code from ref. [15]. For the Compton profile we refer to ref. [16]. Good agreement between experiment and theory is obtained for the KLL-resoname. The disagreement for the higher KLM resonances can be explained by cascades not included in the calculations. The remaining background is due to the uncorrelated electron capture by a simultaneous K-shell projectile excitation process (NTE, nonresonant transfer and excitation [17]), and to REC into the L shell which

1000

1 ’ I 32Ge3’+ -, H,

’

I ’ 1 (Coincidences) I

800 -

’

zo

In contrast to RTE, the competing REC is a nonresonant capture process, where the capture of one target electron is caused by the time varying Coulomb field of the projectile and the target electron. Here, capture of a quasifree target electron leads to an emission of one characteristic photon with an energy equivalent to the energy difference of the initial and final electron state [8]. Therefore, the centroid energy for the emitted photons is given in the CM system by i?o = E,,

-

EBI

+

&IN

9

(1)

where E,, denotes the initial electron binding energy in the target atom, E,, the final electron binding energy in the projectile, and EKIN the kinetic energy of the target electron with respect to the projectile. For bare and hydrogenic projectiles radiative electron capture populates dominantly the K-shell of the projectile. The photons emitted by this K-REC process are well separated from the projectile K radiation for our intermediate collision energies as is demonstrated by fig. 3. Here, an X-ray spectrum, measured in coincidence with projectiles having captured one electron, is shown for 7.01 MeV/u Ge3rt --) H, collisions. For the projectile energy of 7.01 MeV/u no contributions due to RTE can be expected. The most dominant X-ray contribution is the Ge Kci line caused mainly by collisional electron capture into excited projectile states. In the spectrum a strong K-REC line is located at 17.3 keV. Its width reflects, in analogy to RTE, the Compton profile of the target electrons. Additionally, the spectrum shows contributions of radiative electron capture into excited projectile states, i.e. L-REC and M-REC. An increase in

’ 8

u-l

300

t 2 0

200

100

12

14

Beam Enety

18

20

(MeV/u)

Fig. 2. Excitation function for RTE in Ge3’ + -+ H, colllisions measured via the X-ray/X-ray coincidence technique [ 111. For further details see text.

0 0

5

X-RAY

10

15

20

ENERGY (keV)

Fig. 3. X-ray spectrum measured for 7.01 MeV/u Ge3’ + + H, collisions in coincidence with ejectiles having captured one electron. I. ATOMIC/MOLECULAR

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beam energy shifts these REC contributions to higher X-ray energies as seen from eq. (1). One consequence of this energy shift is that at the KLL resonance, the L-REC photon energy is equivalent to the Kol energy, and, therefore, it is not possible to distinguish experimentally between GREC and KLL-RTE at the resonance j6]. For hydrogenic projectiles the cross section for KLL-RTE scales in the high 2 limit as Ze3 [5,18], whereas the cross section for L-REC is independent of Z at the same adiabaticity parameter [19]. For high-2 ions, both competing processes provide comparable cross sections. The scaling laws for KLL-RTE and L-REC are ~lustrated in fig. 4 (n = i). Additionally, the cross section depencences on Z for the X-ray generating processes, i.e. projectile K-shell excitation and collisional electron capture, are given in fig. 4. Cross sections for the K-REC process measured in Ge3r++ H, collisions are presented in fig. 5. For the energy region between 4.5 and 11.5 MeV/u the Xray/particle coincidene method was used (see full triangles in fig. 5 [ZO]), whereas, for 12-20 MeV/u we used only singles X-ray spectra (see full points in fig. 5 [ll]). The angular distribution of K-REC is taken into account by using the relation given in ref. [S]: J&(90”)

(2)

= $0.

These experimental results are compared with the theoretic& predictions according to Bethe and Saipeter [19] (see full line in fig.5).The predicted energy dependence of the K-REC process is in good agreement with the experimental results [ZO]. Also, within the experimental normalization uncertainty of 30% the experi-

ADIABATICITY

PARAMETER

q = 0.5

of quasifreeelectmm

3

5

7 10 20 E (MeV/U) Fig. 5. Cross section for K-REC in Ge3’+ ---)Ha collisions measured at energies between 4.5 and 11.5 MeV/u using the X-ray/particle coincidences (full triangles) [23] and for 12 up to 20 MeV/u by using only singles X-ray spectra [ll]. The K-RRC prediction according to ref. 1191multiplied by a factor of 0.7 (solid line) is given.

mental results for the absolute values agree with theory. Additionally, REC can be used as a spectroscopic tool to measure total binding energies via the linear dependence between the REC photon energy and the projectile kinetic energy (see eq. (1)). Extrapolat~g the measured K-REC centroid energies to zero projectile energy we determined the total K-shell binding energy for He-like Ge ions as 13547 + 20 eV 1211. The theoretical value given by Drake [22] is 13557 eV. For high-%: ions, like Pb, this method appears to be well suited for the study of QED effects (with a 10% accuracy). In particular, it should be possible to make direct comparison of total binding energies of hydrogenic and He-like ions.

~~._.~ 4. X-ray /X-ray /particle

10-2 L 0

20

40 2

60

80

Pro)

Fig. 4. Cross section predictions for inner-shell projectile X-ray production processes RTE, L-REC, CEC (collisional electron capture), and EXC (K-shell excitation) for Ge3t -+ H, coilisions at n = 5 f6].

coincidence spectroscopy

Applying again the X-ray/X-ray coincident method and measuring in addition the charge-changed projectiles in a triple coincidence, we studied the competing RTE and REC processes in Kr35++ H, collisions at the KLL-resonance energy of 16.7 MeV/u. The experimental setup used in this experiment is described in ref. 1231. As reported previously, the triple coincidence method enables us to isolate one single KLL-RTE subshell resonance [5,6,11]. The 2s2p’Pr level is the only one of the 10 KLL resonances which feeds predominantly the metastable 1Qs’Sa state. This state can only decay to the ground state via emission of two photons with a sum energy equivalent to the 1~2s ‘&-1s’ ‘S, transition

Th. Stiihlker et al. / Capture of quaszfree electrons

energy. Hence, observing the two-photon decay enables us to isolate the 2s2p1P, resonance. It is emphasized that this resonance can be used as a selective population mechanism for the ls2s1S, state allowing the investigation of the two-photon decay mode in He-like ions. Up to now, the experimental studies of the two-photon decay have concentrated on measuring the lifetime of the 2rSa state. No information about the spectral distribution of the emitted photons are given by these experiments. However, the photon energy distribution is very sensitive to relativistic and electron correlation effects [24,25]. Utilizing the X-ray/X-ray/particle coincidence technique information about this distribution can be extracted from the measurements [6,23]. The measured cluster plot of coincident two-photon events is shown in fig. 6 for Kr35+-+ H, at the KLL resonance energy. A broad peak is located in this plot at an X-ray energy of 13 keV/13 keV (the Ka/Ka transition energy for Kr 34+). This peak is due to the radiative stabilization of the doubly excited levels formed by the KLL-RTE process. Additionally, three prominent continuous structures (ridges) can be seen, all of them resulting from the radiative stabilization of the 2s2p’Pr level via the metastable ls2s1So state. Because three photons are involved in this process (one Ku hypersatellite and the two photons from the 2El decay) these continuous ridges are located at X-ray energies corresponding to (KHa, El), (E2, KHo!), and (E,, Ea) with El f E, = Ka. Fig. 7 shows the spectral distribution of the 2El decay of ls2s’Se. Since the two-photon decay of the 1~2s’~ state produces coincident photons whose energies sum to approximately the Kcu energy (13 kev), this sum-energy condition can be used to extract the distribution of the 2El decay from coincidence data such as shown in fig. 6. Fig. 7 shows a spectrum obtained in this manner. The theoretical distribution [24] convoluted with the response function of the X-ray detector is also shown in fig. 7 as a heavy solid line. The expected distribution is superimposed by broad X-ray lines on the high-energy side and by La, LB transition

I

25

89

60 LO 20 nl 2

4

6

8

Energy

10

12

(keV)

Fig. 7. Spectral distribution of the 2El decay of Kr34+ measured by one of the detectors. The heavy solid line is a theoretical prediction [24].

lines of Kr341 at low energies. These structures and in addition the structures appearing on the second diagonal given by El + E, = 2Koi in the cluster plot (see fig. 6) can all be explained by radiative electron capture into excited projectile states followed by cascading of the captured electron. This is illustrated schematically in an energy level diagram on the right side in fig. 6. When the beam energy is at the KLL-RTE resonance (17 = t, i.e. (mJM)EKI, = ;Kol) capture into high projectile shells and cascading into the L-shell leads again to a simultaneous emission of two photons with the sum energy equivalent to Ku. Consequently, REC into high projectile shells followed by a cascade into the ground state explains the contributions along the diagonal with the sum energy El + E, = 2Ka. These contributions show a cutoff at an energy which corresponds to capture into high Rydberg states. Due to the REC contributions, the spectral distribution for the two-photon decay (compare fig. 7) can only

I

". KLL-RESONANCE

b)

,&&.,. 5

10

SiO)

15

20

25

1 (keV)

Fig. 6. X-ray/X-ray cluster plot for Kr35+ --t H, collisions measured in coincidence with projectiles having captured one electron. REC contributions are illustrated on the right side.

5

10

15

20

25

Si(Li) 1 CkeV)

Fig. 8. X-ray/X-ray

30

5

IO

15

,I

.

.

.

.

20

.

25

Si(Li) 1 (keV)

cluster plot for Kr3’+ + N, (a) and

Kr35+ --) Ar @) collisions measured in coincidence with electron capture at the IUL resonance.

I. ATOMIC/MOLECULAR

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Th. Stiihlker et al. / Capture of quasifree electrons

90

be extracted qualitatively from the measurement. However, for higher Z systems these contributions will be more clearly separated from the two-photon distribution. Moreover, REC itself can be used as a population mechanism for the 1~2s’~ state. As predicted in fig. 4, the REC cross ection stays constant for all Z at the same TJ, whereas the cross section for RTE decreases. Additionally, at lower adiabaticity parameters, aREC is enhanced and the cascading contributions move out of the diagonal. Therefore, RTE and REC are both appropriate methods for structure studies in collisions between highly charged, heavy ions and light target gases like H,. In fig. 8 the cluster plots for X-ray/X-ray/particle coincidences for Kr35 + --) N, and Kr35 + + Ar collisions at 16.7 MeV/u (17 = i) are shown. The increase in target Z enhances drastically the cross section for collisional capture (a a Z$) and also the cross section for K-shell excitation ((I a Z:). Since the KLL-RTE resonance produces only Kol/Kol X-ray coincidences the strong increase in Kol/KP particle coincidences in fig. 8 in comparison to fig. 6 shows that the uncorrelated capture and excitation process (NTE) is favored over RTE in collision systems involving higher Z targets. However, in Kr351-+ N, collisions (fig. 8a), RTE seems to be still an important X-ray production mechanism (compare the ridges caused by the 2El decay in fig. 8a). The further increase in target Z to Ar (compare fig. 8b) favors strongly the competing nonresonant electron capture processes. The Zr dependences for the various competing processes are shown in fig. 9 for Kr35+-+ ,colllisions at the KLL resonance (i.e at n = i).

5. Summary For collisions of highly charged, heavy projectiles with light target atoms it is shown that RTE and REC are dominant X-ray producing processes at intermediate collision energies. Using X-ray/X-ray and X-ray/Xray/particle coincidence techniques it was demonstrated that both processes can be used as spectroscopic tools for atomic structure studies. The use of light target atoms seems to be important in order to suppress other competing electron capture processes.

References [l] J.A. Tanis, E.M. Bernstein, W.G. Graham, M. Clark, S.M.

[2] [3]

[4]

[5]

[6]

[7]

[8] [9]

[lo] [ll]

102

[12] [13]

[14] Fig. 9. Cross section predictions for inner shell projectile X-ray production processes KLL-RTE, L-REC, CEC and EXC for Kr35+ + Z, collisions at the KLL resonance (i.e. at n = i). The curves shown were generated by connecting straight lines between points calculated at Z, = 1,2,7,18.

[15] [16] [17]

Shafroth B.M. Johnson, K.W. Jones and M. Meron, Phys. Rev. Lett. 49 (1982) 1325. J.A. Tanis, Nucl. Instr. and Meth. A262 (1987) 52. W.G. Graham, K.H. Berkner, E.M. Bernstein, M.W. Clark, B. Feinberg, M.A. McMahan, T.J. Morgan, W. Rathbun. A.S. Schlachter and J.A. Tanis, Phys. Rev. Lett. 65 (1990) 2773. R. Schuch, E. Justiniano, M. Schulz, P.H. MokIer, S. Reusch, S. Datz, P.F. Dittner, J. Giese, P.D. Miller, H. Schiine, T. Kambara, A. Mtller, Z. Stachura, R. Vane, A. Warczak and G. Wintermeyer, 16th ICPEAC , New York, 1989, p. 562. P.H. Mokler, S. Reusch, Th. Stohlker, R. Schuch, M. Schulz, G. Wintermeyer, Z. Stachura, A. Warczak, A. Miller, Y. Awaya and T. Kambara, Radiat. Eff. and Defects in Solids 110 (1989) 39. P.H. MokIer, Tn. Stiihlker, Ch. Kozhuharov, J. Ullrich, S. Reusch, Z. Stachura, A. Warczak, A. Mtiller, R. Schuch, E.A. Livingston, M. Schulz, Y. Awaya and T. Kambara, invited paper to X-90, Conf. on X-Ray and Inner Shell Processes, Knoxville, 1990. H.W. Schnopper, H.D. Betz, J.P. Delvaille, K. KaIata, A.R. Schwab, K.W. Jones and H.G. Wagner, Phys. Rev. Lett. 29 (1972) 898. E. Spindler, H.-D. Betz and F. Bell, Phys. Rev. Lett. 42 (1979) 832. R. Anholt, S.A. Andriamonje, E. Morenzoni, Ch. Stoller, J.D. Molitoris, W.E. Meyerhof, H. Bowman, J.S. Xu, Z.Z. Xu, J.O. Rasmussen and D.H.H. Hoffmann, Phys. Rev. Lett. 53 (1984) 234. D. Liesen, Phys. Scripta 36 (1987) 723. S. Reusch, thesis, Universitat Giessen (1988) and report GSI-88-19 (1988) ISSN 0171-4546. D. Brand& Phys. Rev. A27 (1983) 1314. S. Reusch, P.H. Mokler, A. Warczak, Z. Stachura, T. Kambara, R. Schuch, M. Schulz, G. Wintermeyer and A. Mtller, 16th ICPEAC (New York, 1989) p. 614 L.A. Vainshtein and U.I. Safronova, Atom. Data Nucl. Data Tables 25 (1980) 311. I.P. Grant, B.J. McKenzie, P.H. Norrington, D.F. Mayers and N.C. Pyper, Comp. Phys. Commun. 21 (1980) 207. P. Eisenberger, Phys. Rev. A2 (1970) 1678. M. Clark, D. Brandt, J.K. Swenson and S.M. Shafroth, Phys. Rev. Lett. 54 (1985) 544.

Th. Stiihlker et al. / Capture of quasifree electrons [18] P.H. Mokler and S. Reusch, Z. Phys. D8 (1988) 393. [19] H.A. Bethe and E. Salpeter, Handbuch der Physik 35 (Springer, 1957) p. 88. [20] Th. Stijhlker et al., to be published. [21] P.H. Mokler, Th. Stahlker, C. Kozhuharov, Z. Stachura and A. Warczak, to be published. [22] G.W. Drake, Can. J. Phys. 66 (1988) 586. [23] Th. Stiihlker, C. Kozhuharov, A.E. Livingston, P.H.

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Mokler, J. Ulhich and B. Fricke, suppl. Z. Phys. D, in press (contribution to the HCI V Conf., Giessen, 1990). [24] G.W.F. Drake, Phys. Rev. A34 (1986) 2871 and private communication (1989). [25] K. Ilakovac, G. Jerbic-Zorc and N. Ilakovac, invited paper to X-90, Conf. on X-ray and Inner Shell Processes, Knoxville, 1990.

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