spectroscopic studies of metastable ion-atom collisions: an overview of doubly excited systems

NOMI B Nuclear Instruments and Methods in Physics Research B 87 (1994) 51-57 North-Holland Beam Interactions

with Materials&Atoms

Spectroscopic studies of metastable ion-atom collisions: an overview of doubly excited systems

S. Bliman G* and M. Cornille b a LSAI, U.R.A. 775, Universite Paris Sud, 91405 Orsay Cedex, France b U.P.R. 176 CNRS, DARC, Observatoire de Paris, 92195 Meudon Cedex, France

In many isoelectronic sequences, there is a metastable state just above the ground state (viz. in the He-like and Ne-like sequences). When a low energy electron transfer collision is observed, involving a metastable projectile, photon and Auger decay spectroscopies make possible the understanding of the cascade decay of the doubly excited states populated in the transfer process as well as its dynamics. Li-like and Na-like core excited systems are considered.

1. Introduction

In recent years, many research programs have fo- cused on the study of low energy highly charged ion collisions with atoms. In the case of many electron atoms, a complicated situation arises, since multiply excited systems appear whose stabilization(s) are not simple to analyze and understand:

A4++B-,A(g-k)f+Bq’t+(q’-k) c-. (1)

Along this line, the double capture process has devel- oped:

Agf+ He + A(g-2)+(nl, n’l’) + He’+ (2)

and, more recently, collision studies involving highly charged metastable ions have opened new views into the understanding of the stabilization of core excited ions [l-3].

2. Metastable ions and charge exchange collisions

Along many different isoelectronic sequences, one currently finds, just above the ground state, a long lived metastable state. The most important situations are met along:

(i) the He isosequence: the state ls2s3S, would decay to ground lszlS, via an M, transition. For 2 I 18, that state would have a lifetime longer than 7 r 10-5 s;

(ii) the Be isosequence: there are two metastable states (1~~2s2p)~P,~ and 3P20. The decay of the former to the ground state (ls22s2)lSo is strictly forbidden

* Corresponding author.

(AJ = 0 from J = 0 to J = 0), whereas 3P20 would only decay via an M, transition (AJ= 2). Thus, at low Z (< 20) this level would have a long lifetime (for exam- ple, for C2+, 7 N 100 s);

(iii) the Ne isosequence: the situation is more or less as for the Be isosequence. Above the ground state (ls22s22p6)?i, the (ls22s22p53s)3Po,20 states are long lived (for argon, a typical lifetime of 3P20 is 7 - 2 X 10-3 s>.

A low energy charge exchange collision with an atom creates an excited system:

Ag++ B + tig-‘)+(nl) + B+. (3)

The process is state selective and the most populated level n is only dependent on the incident ion charge Q and on the target ionization potential (in atomic units); it can be expressed as:

IZ _ q3/4/11/2 (a.u.). (4)

This scaling applies irrespective of the state of the incident ion [4], ground state or metastable. Thus, considering the case of a He-like metastable ion, one obtains:

tiz-‘)+ (1~2s) 3S, + B

+ A(Z-3)+ [(ls21) 3Lnl] 2,4L + B+, (5)

whereas for a Ne-like metastable ion, one would ob- tain:

A(z-10)‘(2p53s) 3P,& + B

+tiq-l)+[(2p531) 3Ln1]2,4L+B+. (6)

The sharing among substates 1 and L is generally dependent on the collision velocity, at u < 1 a.u.

Two remarkable features are noted: in the collision process the 3L core configuration is retained and there

0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0168-583X(93)E0962-G

II. MULTI-CHARGED IONS

52 S. Bliman, M. Cornille /Nucl. Instr. and Meth. in Phys. Res. B 87 (1994) 51-57

is an excitation of the 2s electron to 2p thus giving (1~2p)~P, and/or of the 3s electron to 31 thus giving (2~~31) 3L.

3. General properties of Li-like core excited states

All these states are energetically above the first ionization limit of normal Li-like ions. Their general configuration is ls21n’l’. Careful attention should be paid to the core: it is either 1~2s or ls2p. Considering the total spin, this splits into two subconfigurations 1~2s ‘S and ls2s3S on the one hand, and (ls2p)‘P and (1~2p)~P on the other. Combining with the third electron gives (ls2srS nl)‘L, (ls2s3S nlj2L and (ls2s3S n1j4L for the 1~2s core and (ls2p ‘P nl)‘L, (ls2p3P n1)2L and (ls2p3P n1j4L for the ls2p core.

We consider the properties of these different levels: (i) for a given nl. The energy of (1~2~‘s nl)‘L is

slightly greater than the energy of (ls2s3S nl)‘L. The energy difference is not constant at given IZ, 1 varying, and it is slightly different from AE of (1~2s) ‘S and (1~2~)~s of the helium-like ion. The same feature is observed when considering (ls2p ‘P nl) 2L and (ls2p 3P n1j2L. The energy of (ls2p ‘P n1j2L is greater than that of (ls2p3P n1)2L,

(ii) this is clearly seen in Figs. 1 and 2 which depict the energy diagram of 05+ (1~2snl)~L and 05+ (ls2pnl) 2L respectively.

Fig. 3 shows the levels of 05+ (1~21n’l’)~L [5,6]. The level structure is the same along the isoelectronic sequence. An important feature of all these levels is their total fluorescence yield, defined as:

6J r= CA,j/(CA,,+ECA,), (7)

where A,, are the transition probabilities and A, the Auger transition probabilities. They characterize in

2s *PO 2P *II 200 ‘F’ *F %

Fig. 1. Energy level diagram of 05+ (1~2snl)~L. Each level

has two different cores: ls2s3S (lowest) and ls2s’S (highest).

The fluorescence yield is shown for each level.

eV 25 zpo z,, 20 200 +O 2F 2G

Fig. 2. Energy level diagram of 05+ (ls2pnl)‘L. Each level

has two different cores: ls2p3P (lowest) and ls2p’P (highest).

Fluorescence yield: see Fig. 1.

fact the sharing between radiation and autoionization in the stabilization of each individual level. By using these data, we show hereafter that the analysis of the experimental results supports the assumption that they cascade decay. The general features of the cascade decay being two-fold, it is important to note that:

(i) the radiative branching ratio favors the fastest decay toward the lowest lying state;

(ii) the autoionization branching ratio favors decay to the energetically nearest continuum.

Figs. 4a and b show for the (ls2121’)‘L states the total fluorescence yield wr versus the atomic number [7]. To support the cascade decay of these Li-like core excited states, we consider different He-like metastable ions capturing one electron from two different gaseous targets. The capture process selectivity points to different levels for the cascade to start. We first compare the single capture collision cases involving C4+ (ls2s)3S:

C4’(ls2s) 3S + He + C3’ [ (1~21) 3L21] 2V4L + He’

(8)

eV 700 r

600

152rnl -nl

IS 2p nl -hll

Fig. 3. Energy level diagram of OS+ (1~21nl)~L. Fluorescence

yield: see Fig. 1.

S. B&an, M. Cornille /Nucl. Instr. and Meth. in Phys. Res. B 87 (1994) 51-57 53

and and

C4+(ls2s) 3S + H, -+ C 3+ [(ls21) 3L31] 2,4L -I- H; 1

(9)

06+(ls2s) 3S + H, --) 05+ ((1~21) 3L41] 2,4L + H; a

(11)

The process involving the He target populates p2 = 2 directly [(ls21j3L 21]2,4L and there is no evidence of transfer excitation. Since the fluorescence yields of all these levels are less than 0.01, no X-ray line emission is observed (Fig. 4a and b) 181. The Auger spectra, complement to the fluorescence, shows all the corresponding lines around 240 eV [2]. Switching to the Hz target, the collision ends in the population of n = 3. in the X-ray spectrum, one broad peak is seen, at 37 A, which corresponds to the transitions from C3+ [(1~2s)~S 3p]2.4Po to the ground state. None of the [(1~2s)~S 211 24L levels are seen in the X-ray range (fluorescence yield less than O.Ol), but evidence of their cascade population is provided by the VUV emissions in the transitions from C3+ [(1~21)~L31]~,~L to C3+ [(1~2s)~S 211 ‘r4L [2] followed by Auger stabilization.

We now consider the collisions involving o6+ (ls2s)3S:

06+( 1~2s) 3S + He -+ OS+ [(ls21) 3L31] 2,4L+ He”

(IO)

Figs. 5a and b show typical emitted X-ray spectra, whereas Figs. 6a and b show the Auger spectra for the same collision processes [2]. The transfer process with the He target populates mostly 12 = 3 and there is a non-neg~gible fraction of transfer excitation ending in the population of (ls2p3P)31 levels. The identification of the transitions in the X-ray and Auger spectra shows the sharing between photons and autoionization. (ls2s3S)3p2Po, with a fluorescence yield of 50%, df- cays to (~s~~s)~S,,, (branching ratio = 80%) at 19.4 A, while less than 20% decays to (ls2~‘~~S~,~(ls~~S)3p 4P0, with a total fluorescence yield of 95%, dzcays to (ls22s)‘S,,, (branching ratio = 30%) at 19.5 A. From the levels (ls2~~P)3p’,~P the decay to (ls’2~)~P’ is noted at 19.5 A. This is seen in Fig. 5a. ln this X-rty spectrum, a group of lines is identified at 22.06 A: these are the transitions (ls2s3S)2p2Po decaying to 0s‘$s1~s,;, and (ls2p 3P)21’“L decaying to (ls22p>

1,2; the upper states of these transitions were eas- cade populated from [(ls21) 3L 311 2,4L. Consideration of the Auger spectrum (Fig. 6a) shows that there has

I -_... _ , : -___._=‘=--___._

: .-

-- _ _--

I 7 -z- . ::! :: j -‘I / ; / j : j -: :I !

t : i :

; __. : 1 j :. j .,. .! i ,

. . ____--r_--_ _ _ .;.._i__L : j j

D’ j ,. ‘:i ‘. j A-:. :

.:‘../ !,::_((.

I i : Z .__i 1’ : / ..: .:i ._+_:~.: t ,. :

. ., * ,.. :

2. .:.. .: :. ,. ‘.. ,._. _.... ‘_.. _,, ,.,

,. . . __ .._ =,:_*__~~ _ 7 _=,: r ..:.. 1 :I ! ..a

(4 5 6 7 8 9 III II I2 13 14 15 16 I? 18 I9 20 21 22 2

I ---

_.+..+A-_L __-

f : i’ 2

j : j : f : : .i ; ,_i j .& .i ; .-. / i ;.t :*j “1’

la-? * -’ : :

: ,.,(.__[ ;I’_;[. i :! : :._ ,-- .,“. h .., T / i i ! :

9 -.-.---_.xz_._ __.- - :

__- .: --- .-._. .-__ :y-------:

-_. - --

,. ! / 4 yi ../ !

: i : :. ./ : : :.,: :i :’ ..:.d. . . . . ( :

.” .,I

5 6 7 8 9 IO 1112131415 1617I8 19 2021$'

Fig. 4. Total fluorescence yield for 1~2121’ levels versus atomic number Z.


54 S. Bliman, M. Corniile /Nucl. Instr. and Meth. in Phys. Res. B 87 (1994) 51-57

been a fractional autoionization of O5 + [(lsZlyL 31]‘L and a cascade feed to (ls2s2)‘$,, from (ls2s3S 3p)‘P”, and to (ls2s3S)2p2Po from the (lsZ~~S3d)~D level (the 4L state decay takes place for most of them in the VUV range (Table 2 in ref. [5])). The feeds to (ls2~2p)~P’ and to (1~2p~)~P were considered: the former is populated by the radiative transitions from (1~2~3~)~s and (ls2s3d)4D; since its fluorescence yiel$ is 23%, it contributes to the broad Line at 22.4 A (branching ratio = 99%), decaying to (~&?s)~S,,,, but also to the Auger spectrum giving an intense line at nearly 416 eV. For (ls2p2)“P, the feed essentially originates from (ls2p3d)4Po and 4Do and to a lesser extent from (ls2~3p)~P* (via a two electron-one photon transition at 170 A). As for the decay of (ls2~2p)~Pe, (1~2p’)~P shares its decay between, on the one hand, pho:ons: a VUV transition to (ls2~2p)~P’ at 886 A and an X-ray transition to

17 19 21 23

h(&

lb) ’ I I I I I I

I

17 19 21 23

?&t

Fig. 5. a> X-ray spectrum emitted in low energy single electron capture: 06’ (1~2s)~S+He --f 0s’ [(ls21)3L31]2-4L+Hei; b) X-ray spectrum emitted in the transfer: 06+ (1~2s)~S+H +

05’ [(1~21)~L413~~~L+H+.

EMITTER FRAME ENERGY (eV) Fig. 6. From Ref. [2]: a) Auger spectrum fcjllowing process

(10); b) Auger spectrum following process (11).

(ls?2p)2P*,,, at 22.04 & and, on the other hand, an Auger transition at nearly 429 eV. Weaker Auger lines are identified, corresponding to the autoionization of (1~2p’)~D, ‘S (or= 0.05 at 435 and 443 eV respectively); the feed to these levels is respectively (ls2p3d>2Do and ‘F* (w, = 0.9>, while (ls2p3d)‘P” relative line intensities, both X-rays and Auger, are strongly dependent on the population sharing among the substates in the capture level.

To further support the cascade decay statement, with at each step a sharing between X-ray (when the level is simply connected to a closed K shell level) and Auger stabilization, the transfer process involving H, is considered. The dominant process populates II = 4 in the configuration [(1~2s)~S 4112a4L, a small fraction going to [(ls2p) 3P 411 2,4L. Fig. Sb shows the X-ray with the transition’s identifications. The complementary part to this spectrum is shown in Fig. 6b where the major Auger peaks are identified: the energy range 520-530 eV results from the direct autoionization of (ls2s4s)‘S and (l.s2s4d)‘D to the only available continuum; all the other peaks, seen at lower energies, have been

S. Bliman, M. Camille /Nucl. Instr. and Meth. in Phys. Res. B 87 (1994) 51-57 55

Table 1 Table 2 Autoionization continua of the different (1~21~21) series of 05+ (ls21nl) with electron energies

Autoionization energies of the different series of Ar7+(2p5 3b@4L

[(ls2s) 3Snll~L Kls2p) spn11 *L

[(ls2s) rsn11 zL

[(ls2p) ‘Pnll zL

Energy from ground state [eVl

699.1 706.8

706.8

712.0

Continua

ls2 2

tssZs) 3s (ls2s) 3s ls2

2

&2s) rs

Energy window for series

411-569 434-569 O-7 eV 428-576 O-8 eV 434-574 O-5.5 eV

identified in the previous case and their upper levels were cascade populated. A very weak intensity contri- bution of (ls2p41)‘L levels at 535 eV and (ls2p31)‘L levels at 515 eV is observed (the 31 levels have been cascade populated). As to the (1~2121)~,~L, comparison with the spectrum of Fig. 6a shows the same lines at the same energetic positions and this supports the cascade population assumption. The difference is in the line intensity ratio of (1~2p~)~P to (ls2~2p)~P’ in the Auger spectrum and in the X-ray spectrum in the structure of the grotrp of transitions in the wavelength region 21.5 to 22.5 A, which reflects the difference in

- 3% 300- -

3p3d 3p3d 3p3d . .

3s3d 3x -

3s3d

275- 3 3 3

3s3p 3s3p 35

250-

Z

I I 1 I _I I I I I

zL [evl 4L [eVl

3sz 100 3s3p 114-116 3P2 130-132 3s3d 140-147 3p3d 154-163 3s4s 173 3s4p 182 3d2 182-185 3s4d 190-192 3s4f 194

3s3p 110-115

3P2 132-136 3s3d 140 3p3d 152-157 3s4s 173 3s4p 179-181 3d2 178-187 3s4d 189-191 3s4f 191

population sharing among the substates of the capture levels.

For the Li-like core excited states, the series limit (1~2s 3S) (nm)l, (1~2~) 3P(nm)1, (1~2s lS>n(m)l and (1~2~ rP)n(m)l end respectively at (ls2sj3S, (ls2pj3P, (1~2s) ‘S and (ls2p)‘P. The continuum to which they Auger decay is ls2 ‘So, at least for (1~2~)~s nl. For the others there is some sharing between lszlSo and (1~2~)~s for (1~2p)~P nl from a given IZ on, and (1s’) ‘So and (ls2s)rS for (ls2p)‘P nl from a given it on. Fi- nally, it should be pointed out that in the Auger spectra recorded in the energy range O-12 eV, some lines are identified as [(ls2p) 3P nil 2L with IZ 2 8 (Ryd-

eV A

350 4s , 4SO

I 4PO 4F 4D

I I I 4DO llFO “F 4G 4Go

I I I I

_ (‘4 ’ I

325- - 3s4p

300- 3p3d

275-

250- 3s3p

3Fd 3p3d _ -

3% 3p3d

-

353d 3S3d 3s3d

3

57 ==7 3P :

I I I I I I I I I I L

Fig. 7. Grotriam diagram for Ar7+ [2p531nl]2,4L with upper levels 3~41.


56 S. Bliman, M. Cornille /Nucl. I&r. and Meth. in Phys. Res. B 87 (1994) 51-57

berg states), the closest continuum being (1~2~)~s [14] (Table 1).

4. Properties of Na-like core excited states

In the case of collisions involving Ne-like metastable ions, analysis of the population mechanisms leading to Na-like core excited systems becomes very complicated.

We consider the collision:

Ar8+ (2~~3s) 3P,,, + B

+ Ar7+ [(2p531) 3Lnl] 2,4L + B+. (12)

The simplest straightforward case is the single electron transfer case ending in Ar7+ [(2~~3s>~S nl]2,4L (with B = He atom, the populated level has II = 4) [9]. There, again, the stabilization of these doubly excited states occurs via sharing between both radiation and autoionization. As in the Li-like case, the fluorescence yield is

4.0 42 4.4 4.6 4.8 h (nm)

11 46 LB sa 52

Wavelength (1)

Fig. 8. a) X-ray spectrum for the stabilization of Ar7+[(2p5 3~41)]‘,~L resulting from collision process (12) (B = He); b) X-ray spectrum for the stabilization of Ar7+[(2p53s51)]2,4L

resulting from collision process (12) (B = HJ.

510 20 LO 60 80 100 120 140 EemkVI la) '1' * z ‘I

, ?p,Z? , 417 r 6p , Bp , l?O 1zO l40 GmleVl

Fig. 9. From Ref. Ill]: a) Auger spectrum for process (12) - see caption of Fig. 8a; b) Auger spectrum for process (12) -

see caption of Fig. 8b.

the main quantity for explaining the sharing among photon and Auger stabilization. Given the level structure (Figs. 7a and b), the cascade decay takes place via a certain number of two electron-one photon transitions [lo] (this reflects configuration interactions) and autoionization to the 2p6)?S, single continuum. Most of the substates in the capture level, that is (3s41j4L, have fluorescence yields > 0.2. It should, be noted, however, that there is not a systematic large wT associ- ated with a large J value, as in Li-like states. In the ‘L, the diversity of wr should be underlined. Considera- tion of the Auger spectrum (Fig. 8a) shows that the high intensity peak at 190-192 eV is mostly due to the autoionization of [(2p53s13S 4d12L, a small fraction of (3s4f)‘L being superimposed on the high energy wing at 194 eV (the agreement with calculation is good). The weak intensity peaks are explained only if the WV cascade is considered. Table 2 gives the energy range for the autoionization of each group of levels and the identifications fit with the calculations. Finally in the ‘L, the core excited lowest level is (2~~3s’) and it is only populated by cascade; it share its stabilisation between an X photon and an Auger electron. Since in photon decay either the inner or the outer electron may close the 2p5 subshell, it is thus possible to observe two satellite series either from ns or nd electrons (Fig. 8a) (around 49 A in the former case, around 42 A in the nd case). The Auger decay spectrum underlines the existence of a cascade decay from the (2~~3~41) levels to intermediate states 2~~3131’ (Fig. 9a> Ill].

Switching to H, as the target in process (12), the capture level changes from it = 4 to II = 5 and then the observed stabilization in the photon and in the Auger channel support the cascade decay assumption (Fig. 8b and 9b). In Figs. 8a and b, the electron line (line a> can

S. Bliman, M. Cornille /Nucl. Instr. and Meth. in Phys. Res. B 87 (1994) 51-57 57

16 17 18 19 20 21 22 23 24 25 26 z

Fig. 10. Fluorescence yield of the state (2p53s2)‘P&,,,,, versus atomic number.

be seen due to autoionization of Ar’+(2p’3~~)~P&,~ to the single available continuum Ar8+(2p6) (this level is populated by cascade).

Cascade decay has been observed in the system Fe17++ He -+ Fe15’+ He*’ where the autoionization doublet (at an electron energy of 235 eV) due to Fe15f(2p53s2)2Pl,~,,,~o is unambiguously identified c121.

Finally, for the Na-Iike sequence, we show in Fig. 10 the total fluorescence yield of the (2~~3s’)~P’ state versus the atomic number. It is clear that, with increas- ing Z, the spectroscopy will show X-ray satellites to the 2~~3s + 2p6 Ne-like transitions [13].

5. Conclusion

The importance of electron capture to metastable ions ending in doubly excited ionic states is underlined. This collision process is by far simpler to observe and diagnose than the dielectronic recombination populating the like levels.

Mention should be made here of the double electron transfer process populating doubly excited states.

This process raises the question and observability of the spin flip,

AQ’(2p’) f He -+ A(Q-2~f(2p5nIn’l’) + He”,

(13)

thus violating the Wigner spin conservation rule.

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spectroscopic studies of metastable ion-atom collisions: an overview of doubly excited systems

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