Spectroscopy of few-electron slow recoil ions

Nuclear Instruments and Methods 202 (1982) 177-185 North-Holland Publishing Company

177

SPECTROSCOPY OF FEW-ELECTRON'SLOW RECOIL IONS H.F. B E Y E R 1), F. F O L K M A N N 2) a n d R. M A N N 1) /) GSI, Darmstadt, West Germany 2) Institute of Physics, University of Aarhus, Denmark

High-charge low-velocity recoil ions can be produced by fast collisions of highly stripped heavy ions on gaseous targets. The properties of this recoil ion and light source have been studied spectroscopically.We discuss some recent results on the spectroscopy of few electron states and the secondary electron capture studied with spectroscopic methods.

1. Introduction

A highly charged fast heavy ion may in a single collision remove a large number of electrons from a target atom without transferring much kinetic energy to the target atom. Such recoil ions are of great interest mainly because their electronic excitation energy (a few keV) is large compared to their kinetic energy in the laboratory (a few eV). Ions in such exotic states provide favourable conditions for spectroscopic studies and allow access to secondary collisions in the unexplored high-charge low-velocity regime. We investigated the recoil ions systematically with the aid of K X-ray and Auger-electron spectroscopy [1]. The significance of that spectral range enters through the fact that for collisions close enough to ionize the target K shell the probability for removing the outer-shell electrons becomes large. Therefore, our spectra sample the high-charge region in the recoil-charge distribution. From our spectroscopic studies the following information is obtained [2,3]: (1) the distribution of excited states produced directly by impact of fast very heavy projectiles and secondarily by a second-collision capture event; (2) through the kinematical Auger-line broadening the emitter velocity is measured obtaining the primary recoil velocity and the exothermic energy defect of the secondary quasi-resonant charge exchange; (3) total capture rates are determined by observing the secondary collisiorial excitation and deexcitation rates. The present article is aimed at providing systematics in the projectile dependence of multiple target ionization with emphasis on the feasibility of precise spectroscopic measurements when approaching the ultimate limit of very heavy and highly charged projectiles. Furthermore, total capture rates for slow recoil ions colliding with different neutrals are presented. The possibility of studying these secondary collisions spectroscopically is based on the fact that at high degrees of ionization 0167-5087/82/0000-0000/$02.75 © 1982 North-Holland

there is a large fraction of K-vacancy bearing states which are metastable. This is demonstrated in fig. 1 for K-shell ionized neon ions. At practical gas pressures the ions in states with lifetimes around 1 ns or more undergo a second collision in the target gas what may result in the production of a promptly decayit:g state.

2. E x p e r i m e n t a l

procedure

A collimated beam of heavy ions (ranging from Ar up to U) from the UNILAC at GSI traverses two different target-gas cells which can be used simultaneously. The first cell is a differentially pumped open gas cell used for target pressures of 0.02-0.3 mbar. Auger electrons can be observed with an electrostatic hemispherical electron analyzer through a small hole on one side. The instrumental line width for this analyzer is

100 Froclion of long-living states of Neq÷, "[~ lns

8O

60 Z O

c~ ~o < u_

20

I

I

I

I

I

1

2

3

4

5

I

I

I

I

1

6

7

8

9

10

IONIC CHARGE q Fig. 1. Fraction of states with lifetimes ,r ~ 1 ns in neon ions bearing a K vacancy. Statistical population of multiplet states is assumed. III. INTERACTION MECHANISMS

H.F. Beyer et aL / Few-electron slow recoil ions

178

near 1 eV. Opposite to this a 5" Rowland-circle curvedcrystal spectrometer is mounted for viewing X-rays from the same target region. At the entrance slit of a second identical X-ray spectrometer a high-pressure gas cell is mounted which is closed by 1 m g / c m 2 Ni foils for beam entrance and exit. The entrance slit is closed by a 0.8 /~m thick polypropylene foil. In the crystal spectrometers crystals of RbAP, Ge, and LiF were mounted providing in the interesting regions wavelength resolutions A/AX (fwhm) of approximately 200, 400, and 1000, respectively. With this apparatus X-ray and Auger-electron emission from gaseous targets is measured as a function of target pressure and of target composition using mixed gases. When the ion beam has passed the entrance foil of the high-pressure gas cell it is stripped to equilibrium. All projectile charge states noted below are estimated mean charges [4] when referring to data obtained with the closed cell. Due to a large number of electrons ejected in the forward direction out of the entrance foil the diagram lines from singly ionized ions are induced. This does not affect our analysis as these lines are well isolated in the spectra. When the closed cell is used at low target pressures nuclear processes as well as excitation of projectile inner shells in some cases lead to an appreciable background under the target X-rays.

where the velocity of the target K-shell electrons matches the projectile velocity. From the K a satellite distribution the probability p L(O) for L-shell electron removal in K-shell ionizing collisions can be extracted which, in the case of highly stripped ions like Kr Is+ up to U 4°+ , is in the range of 90%. These meaurements have been extended to higher bombarding energies ranging up to 8.8 MeV/amu. Following a suggestion by Schneider et al. [5] the L-shell ionization probabilities are plotted in fig. 2 as a function of vl/ql, the impact velocity divided by the ionic charge of the projectile. Included in this graph are the data of Kauffman et al. [6] for completely stripped lighter projectiles. They lie remarkably well on the same general line as do the data for the highly but not completely stripped very heavy ions. This suggests that the electrons of the highly charged projectiles are screening the nuclear charge during the collision. This is also expected since the ionic radii of these projectiles are smaller than the mean impact parameter which is in the order of 0.3 a.u. Thus the situation is different from less highly charged lighter ions where the inner [7] but not the outer electrons [8] shield the nuclear charge. Therefore, a strong charge-state dependence must be observed which indeed is the case as demonstrated in fig. 3 for 5.9 MeV/amu krypton ions. The intensity ratio of helium-like satellites increases drastically when going from q = 16 to the mean charge q = 30 as obtained after foil stripping. An interesting question is, to what extent one can ultimately ionize a target atom and how far one can go in its atomic number. For neon bombarded with ions such as 5 MeV/amu U 65+ the K,, satellite group is only

3. Direct excitation

The multiple ionization of neon atoms by heavy ions impact was studied systematically at 1.4 MeV/amu [ 1,2] 1.0

0.6

0.~ O

--&_~

"t" 1.4 NeV/omu Ar12*, L1z:+,Ni16~ Kr18* Xe 24,+.pb 36". U40. •

5.3 MeV/amu Ti14"+

•

3 6 MeV/omu Kr 28. 5.9 MeV/orau Kr 30÷ 8.0 MeV/ornu Kr 31÷

04

0.2

O

•

5 0 MeV/omu U65. 88 MeV/omu U71.

O

15 2 2 MeV/omu C6~NT"~oS,+F9" Kouffmor~ et at 1975 KSU

I

I

o.z

I

O ~

I

o4

I

I

o.6 ~/ql

I

I

o.a

I

I

~.o

I

I

~.2

I

{au)

Fig. 2. L-shell ionization probability in K-shell ionizing collisions for bare lighter and highly stripped heavy projectiles.

179

H.F. Beyer et aL / Few-electron slow recoil ions Ne8÷ 1LOC

from 6 MeV/amu uranium impact at incident charges of 39 and 66, respectively. Only with the higher charge distinct lines are observable which then reflect the Auger decay of a pure three-electron system. The dominant lines arise from the ls2s2p 4p0 and ls2p 2 4pc states. From their kinematic broadening [3] the mean kinetic energy of the Ar 15+ recoil ions is determined to be Erec--21 eV. This corresponds to a mean impact parameter of about 5 Ar K-shell radii. For precise wavelength measurements it is important to note that the emitter velocity divided by the velocity of light Vr~/Co, which directly determines the line width, is as low as 3 × l0 -5, even for the very high charge states.

5.9 M E V / A M U KRYPTON ON NEON

120( lOOO 0=30 NI TM

8OO

/ 500

"-~.

~

J

4p

2p

400 Z 200

II

tZ 500 0=16

0

3, ;

~I

z~O0

4. Spectroscopy of Rydberg lines

30O

The appearance of distinct spectral features in fig. 5 for wavelengths below 3.8 ,A suggests that spectroscopy of single lines might be possible at a high degree of target ionization. This aspect has been examined in more detail by measuring that wavelength region with higher instrumental resolution and using the heaviest projectiles like Pb6°+ and U 6 6 + . In fig. 7 we present the result for U 6 6 + impact. The smooth background seen in this spectrum may be attributed to uranium M X-rays as it is missing in the case of lead projectiles. The lines arise from initial states as noted in the figure. Identification of the transitions has been possible by comparing the meaured wavelengths with published theoretical calculations in case of the Ar XVIII [9] and Ar XVII [10] lines. For Ar XVI and Ar XV transition energies have been calculated using the Dirac-Fock program of Desclaux [11]. Good agreement between measured and theoretical wavelengths is obtained for all classified lines, even though the present experiment was not yet meant to provide a sensible test of the theoretical transition energies. In this connection a meaurement of the 2p --* ls transitions would be of particular interest as it may test the Lamb shift contribution to this ls binding energy [12]. For this purpose it is necessary to make sure that these lines are not blended with other transitions. Therefore, the region around the 2 P1/2.3/2 doublet was scanned in more detail and is shown as an insert in fig. 7 where the theoretical wavelength positions are marked corresponding to the states of the configurations 2p, 2s2p, and 2p2. The resolution of the two hydrogen-like components corresponds to the instrumental resolution of about 1000 whereas in the highwavelength wing of the 2PI/2 peak there is an admixture from the 2p2 IS0 state. Besides the hydrogen- and helium-like Rydberg series extending up to n = 5 there is the lithium-like series arising from configurations ls2l n p with n ~ 9. The multiplet-splitting of the corresponding helium-like core ls21 is observed and indicated in the figure.

2O0

100

' ,Lo ' ~;8 ' ~6 WAVELENGTH

' ,~

' ,;2

'

/ .m

Fig. 3. Ne K a satellite spectra for two different projectile charges.

a small fraction in the X-ray spectrum which is dominated by the hydrogen-like line series. This is observed in fig. 4 showing the measured spectrum and a fit of Gaussians to the measured data, where the smooth background, mainly due to uranium N X-rays, has already been subtracted. A large fraction of the observed intensity is due to secondary capture by bare Ne 1°+ ions as will be discussed later in this article. As the heaviest target we studied argon. There exists a similar trend in the projectile dependence as for the lighter targets. However, in order to obtain the few-electron systems higher projectile charges are necessary. Fig. 5 shows the argon X-ray spectra for Kr 3°+ , Xe 41+ and Pb6°+ impact, respectively. The lines originate from initial states as indicated in the figure. The peak Ka0 is due to the diagram K - L transition induced by electron impact. A dramatic increase in the degree of ionization is observed for increasing projectile charge. For Pb6°+ impact the K,, satellite spectrum is dominated by the two-electron satellites and there is a high intensity from the Lyman a line in Ar XVIII. In general most of the observed peaks become narrower due to the higher degree of ionization where a smaller number of multiplet states is populated and fewer line blendings occur. Fig. 6 depicts argon Auger-electron spectra arising

III. INTERACTION MECHANISMS

180

H.F. Beyer et al. / Few-electron slow recoil ions

>, 03

I

I

I

L

I

I

I

I

I

I

65+

5 MeV/amu

U

---~ Ne

t~

J 1

4

i

13

I

I

I

I

12 wavelength

11

I

t

10

I

9

/ A

Fig. 4. Measured X-ray spectrum (top) for 5 MeV/amu U 65+ ~ N e collisions and Gaussian fit to the observed X-ray lines (bottom).

5. High-charge low-velocity electron capture Population of Rydberg states in the recoil light source has been observed not only to proceed by a direct population but also by electron capture into outer shells of a long-lived recoil-ion core [2,3]. The subject has been studied carefully using light recoil ions in charge states between 4 and 10. One-electron capture was found to proceed in a highly selective way and the principal quantum number n o of the final state populated predominantly was found to be in excellent agreement with

the predictions of a classical model for all 28 charge-exchange systems studied so far [13]. The model predicts q2 n2~< 2Jn[1 + ( q - - 1 ) / ( 1 + 2 q l / 2 ) ] '

(1)

where q is the charge of the collector ion A q+ and J s is the ionization potential of the neutral collision partner B in atomic units. The geometrical capture cross section is Oc = ~'R 2 ,

H.F Beyer et al. / Few-electron slow recoil ions ENERGY 29

30

I

31

32

]

M(I

_z

=i

/ key

33

I

~

,,

o_~

35

I

3& I

Ar-K

X RAYS

36

1

I

I 42

I 41

I &O

39

K ? O + =-* A r

4.7 MeV/amu

Xe

=-.. A r

5.9 MeV/amu

P b6 0 +

=-~ A r

3.8

m

37

WAVELENGTH

•

3.6 /

3.5

37

38

m

~

I

5.9 MeV/amu

r-4

z, 3

181

3.&

I

3.3

39

I

gl.

3.2

3.1

A

Fig. 5. Ar K X-ray spectra obtained with three different heavy projectiles.

6 HeV/u

with

P AP

U q÷

R--

q= 39

100-

5oJ

400

200

"'"~r'~I~ :000

6

, 2¢00 ELECTRON

2800 ENERGY

3200

[eV

q--1 q 2 / ( 2 n 2 ) --'lB"

(2)

The equations result from the assumption that the electron capture at these low collision velocities is due to an over-barrier transition in a quasi one-electron molecule as demonstrated in fig. 8. The condition for the electron transition is that the binding energy E B = J B - q / R in the field of the highly charged ion matches the energy of the Rydberg state E A and that these levels lie above the potential barrier between both centers. The predominant population of n = 6 in the hydroFig. 6. Ar Auger-electron spectra induced by 6 MeV/amu U 39+ and U 6~+. III. INTERACTION MECHANISMS

182

H.F. Beyer et aL / Few-electron slow recoil ions Energy

3.3 I

I

34 I

I

3.5 i

3.6

[

l

2000

i

/keV

3.7 ]

3.8

l

3.9

I

I

I

5.9

MeV/amu

l

LO I

U 66+

I

4.1 I

I

~

4.2 I

Ar

o')

E

m

, i, ,

.o 1500 >. o

"~ re 1000

m o.

3.75

o.

04 t/3 o,1 u9 co to

500

38

I

t

37

t

f

36

t

t

J

35

3fL.

t

I

I

3.3

Wavelength

3.2

I

I

31

I ' ~ 1 ~

3.0

29

/ A

Fig. 7. X-ray spectrum from highly excited states of argon.

gen-like sequence of fig. 4 can be attributed to the selective electron capture into Ne l°+ ions. We should also point out that in accordance to eq. (1) the presence of the line ls219p in fig. 7 can be attributed to the selective electron capture into Ar 16+ ions. One should note that there is no indication of any measurable intensity in this series for n = 10 and 8 and only little for n = 7. Let us in the following consider the particular system N e 8+ + B ---, Ne 7+ + B + +Eex o,

(3)

where the N e 8+ recoil ion of a few eV kinetic energy collides with some other neutral B. Capture rates can be measured by observing the lowering in charge after the ions have been extracted out of the primary target region by an applied voltage [14,15]. But there also exist

B

R _.!

three different ways to assess the total capture rate spectroscopically without accelerating the recoil ions: (a) to observe the production rate for the Rydberg line in Ne 7+ [3], (b) to determine the exothermic energy defect Eexo which means additional kinetic energy for the Ne 7+ ion and results in an Auger line broadening [3,16], (c) to observe the quenching of a Ne s+ line of suitable lifetime [ 17]. Let us report on method (c) as it is most precise. The ls2p ~Pl state with a lifetime of 10 -~3 s completely decays off before a second collision, even at target pressures as high as 100 mbar. The ls2p 3P t state with a lifetime of 0.18 ns, however, gets collisionally quenched by process (3). This can be observed in the pressuredependence as demonstrated in fig. 9 where the metastable quartet and triplet states are quenched at high target pressure. The singlet/triplet intensity ratio for a two-component target can be worked out easily to be [17]

EA

i ( t p ) = R 0 [ l + ~'(3p)(k¢]N1 + k¢2N2)], i(3p)

-U Fig. 8. Illustration of the charge exchange between the highly charged ion A q+ and the neutral atom B.

(4)

where R 0 denotes the ratio at zero pressure and N] and N 2 the density of the neon and the admixture, respectively, whereas k~] and kc2 are the respective capture-rate coefficients. The pressure dependence was studied experimentally with pure neon and with admixtures of He, Ar, CH4, and Xe, respectively, and the neon recoils themselves were produced by 1.4 M e V / a m u uranium impact. As an example, fig. 10 shows the pressure

183

H.F. Beyer et a L / Few-electron slow recoil ions ~p

2000-

q800-

3.6 MeV/amu

K r ~ Ne

160012 mbar ....

1400-

3p

93 mbar

1 oo2 ~000-

~

800-

2p

600-

4p

400800-

o

20

40

~o

8'0

16o

1~o

160

1~o

1~o

"T 2OO

CHANNEL NUMBER

Fig. 9. Neon K,, satellite spectra at two different target pressures [17],

e.e)

Q. vm

co

o.

3

{ . ~ t ~f

t

t~ / [ J

S!e

A

v m

2

t

20 I

t

40 I

TOTAL

I

60

TARGET

i

I

80 i

PRESSURE

i

i 100 /

i

I 120

I

1/,0

mbar

Fig.10. Pressure dependence of the ls2p i p / 3 p intensity ratio in Ne IX for different target composition. In the case of mixed gases the neon partial pressure amounts to 27 mbar. III. I N T E R A C T I O N M E C H A N I S M S

H. F. Beyer et al. / Few-electron

184

slow recoil ions

50,

He

i I

I

I

05

06

07

IONIZATION

I

I

I

08

09

1.0

POTENTIAL

I

au

Fig. 11. Experimental charge-exchange cross sections for Ne*+ recoil ions on various neutrals. The solid line represents the geometrical cross section as determined by eq. (2).

dependence of the singlet triplet ratio for Ne, Ne + He, and Ne + Xe targets. In the case of mixed targets the neon partial pressure was held constant to 27 mbar. Our experimental data confirm a linear pressure dependence as required by eq. (4) with different slopes for the various gas mixtures. From the slopes of linear leastsquares fits to the data the capture-rate coefficients recorded in table 1 were obtained. Using the recoil velocity of u,, = 0.8 X lo6 cm s-’ determined in an independent meaurement [3] the rates are converted into cross sections and are compared in fig. 11 with the geometrical cross section of eq. (2). Taking into account the crude theoretical treatment the agreement with the model is rather good even though an experimental confirmation of oscillations of the cross section as a function of the ionization potential is not yet clear. The experimental errors in the experiments employing methods (a) and (b) were rather large. However, we should

Table 1 Experimental

capture

rates for Ne*+ + B, u,, =0.8X

Ionization potential/a.u.

Capture k,/10e9

He Ne Ar

0.900 0.792 0.579 0.463 0.446

l.OCO.3 2.920.9 8.222.5 8.052.4 23 *7

CH4

Xe

with all three methaccuracies.

6. Conclusion The present article has discussed some results on the spectroscopy of few-electron states produced as recoil ions. Production of almost completely tripped recoil ions is possible for target atoms with an atomic number of up to at least 18 when one uses the heaviest highly charged projectiles available. The recoil source can be regarded as a quasi-stationary light source with ion energies of less than IO-20 eV for the few-electron states in neon up to argon. The present results encourage further investigation in which attention should be payed to increase the instrumental precision, extension to other spectral regions, and use of non-traditional spectroscopic methods. The study of electron capture in the high-charge low-velocity domain by spectroscopic methods is particularly successful as many aspects of the collision can be studied.

IO6 cm

s-1. Gas B

emphasize that the,results obtained ods agree within their experimental

rate cm3 s-’

We are grateful for the technical support and the assistance of the operating crew at GSI. The work was supported by a travel grant to F. Folkmann from the Danish Natural Science Research Council. References

[I] H.F. Beyer, F. Folkmann 40 (1979) Cl-17; R. Mann 40 (1979) Cl-236.

and K.-H. Schartner, and F. Folkmann,

J. Physique J. Physique

H.F. Beyer et al. / Few-electron slow recoil ions [2] H.F. Beyer, K.-H. Schartner and F. Folkmann, J. Phys. BI3 (1980) 2459; F. Folkmann, H.F. Beyer, R. M a n n and K.-H. Schartner, Nucl. Instr. and Meth. 181 (1981) 99. [3] R. Mann, F. Folkmann and H.F. Beyer, J. Phys. BI4 (1981) 1161; R. Mann, H.F. Beyer and F. Folkmann, Phys. Rev. I.ett. 46 (1981) 646. [4] V.S. Nikolaev and I.S. Dmitriev, Phys. Lett. 28A (1968) 277. [5] D. Schneider, M. Prost, P. Ziem and N. Stolterfoht, in Abstracts 7th Int. Conf. on Atomic physics, Massachusetts Institute of Technology (1980) p. 102. [6] R.L. Kauffman, C.W. Woods, K.A. Jamison and P. Richard, Phys. Rev. AI 1 (1975) 872. [7] R.L. Kauffman, C.W. Woods, K.A. Jamison and P. Richard, J. Phys. B7 (1974) 1335. [8] C.F. Moore, J. Bolger, K. Roberts, D.K. Olsen, B.M. Johnson, J.J. Mackey, L.E. Smith and D.L. Matthews, J. Phys. B7 (1974) L451. [9] G.W. Erickson, J. Phys. Chem. Ref. Data 6 (1977) 831. [10] A.M. Ermolaev and M. Jones, J, Phys. B7 (1974) 199. [11] J.P. Desclaux, Comput. Phys. Commun. 9 (1975) 31. [12] H.W. Kugel and D.E. Murnick, Rep. Prog. Phys. 40 (1977) 297 and references cited therein. [13] H.F. Beyer and R. Mann, in Progress in atomic spectroscopy, part C, eds., J. Beyer and H. Kleinpoppen (Plenum, New York, 1981) to be published. [14] C.R. Vane, M.H. Prior and R. Marrus, Phys. Rev. Lett 46 (1981) 107. [15] C . L Cocke, R. DuBois, T.J. Gray, E. Justiniano and C. Can, Phys. Rev. Lett. 46 (1981) 1671. [16] F. Folkmann, R. M a n n and H.F. Beyer, Int. Conf. on X-ray processes and inner-shell ionization, Stifling (1980). [17] H.F. Beyer, R. M a n n and F. Folkmann, J. Phys. BI4 (1981) L377; ibid B15 (1982) 1083.

Discussion Andr~i: Could you comment on the problem of the lifetime or the time of existence of a specific charge state or specific level with the capture rates you just discussed? Doesn't this time limit the final resolution of the method itself? Can you even say that 60 eV recoil energy is due to the velocity or just the time of existence of these levels? Beyer: No, the whole method is based on the fact that we produce long-lived states in charge states 7 to 10. These states now can undergo a second collision within their lifetime. [For example], if you have a Ne I°+ [ion] that's no problem, and if you have a neon ls2s that's also no problem, and if you have an ion as we studied here, for instance the 3P I in the two-electron system, that decays off partially before the second collision, and this gives us a means of studying the capture rate via the quenching. Does that answer your question? Andrii: Well, my question was more or less pointing to the ultimate resolution which you can obtain due to the short time of existence of these levels.

185

Beyer: Ah, I see, by the uncertainty - but that is inherent in every system. Andrii: I'm not talking about the free lifetime of some level. I'm talking exactly about the time of existence of such a state in such a high-pressure environment. Beyer: Oh, those are a few nanoseconds. Man'us: In the spectra that you showed of the argon bombarded by the uranium, as you pointed out you have very intensive lines from the hydrogen-like argon, and it was my impression actually that they were more intense than the lines from the lower charge states. Does this imply that when you bombard with uranium, you produce relatively large amounts of hydrogen-like compared to the other charge states? Beyer: You must know that these are K X-ray transitions and in order to see any intensity you have to ionise the K shell. That samples a certain impact parameter region for the collision and thereby also [produces] the high charges in the recoil ion charge distribution. So if you make a K shell vacancy you also remove the outer electrons. Marrus: Oh sure, but what I'm asking is, what is the relative production, say of the hydrogen-like argon compared, say, to the lithium-like? Beyer: That's quite high if you go to these highly charged and very heavy ions. Murniek: There's been a lot of research in the last several years in possible ultraviolet and soft X-ray lasers via recombination and you have a technique here in which you can populate possible metastable states of highly ionised species, and the question is, what are the densities you think you can reach with the beams available? Beyer: At these gas pressures we used, it's nearly one-to-one: one primary particle will produce approximately one highly charged recoil ion and this has a probability of nearly one to undergo a capture. Murniek: What is the beam current? Beyer: What you make with these accelerators - it's in the order of one hundred particle-nanoamperes. Bashkin: What is the shape of the pulse for the beam? Beyer: The time structure of the accelerator? It's a microstructure of 27 M H z and there are beam bunches of 2 ns. So you have a bunch of 2 ns and the period is 37 ns. Bashkin: I was just wondering if there were any possibilities of measuring lifetimes in your gas target by looking at some transition as a function of time after the pulse? Beyer: We've applied such methods to discriminate between the prompt excitation and the secondary excitation and there was also a study together with Sellin and M a n n who studied the lifetimes of the three-electron systems of lighter target atoms like neon, oxygen and so on using that method. Martinson: I should like to rephrase Jurgen Andr~i's question a bit. Would it be possible to find cases where you could measure very short lifetimes from line shapes? Beyer: I'm not quite sure about what yields you'd get. You certainly have to go to a low density target in order to get rid of the secondary excitation.

III. I N T E R A C T I O N M E C H A N I S M S