Electron spectroscopy of foil-excited chlorine beams

NUCLEAR INSTRUMENTS AND METHODS

489-492; ©

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NORTH-HOLLAND

PUBLISHING

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ELECTRON SPECTROSCOPY OF FOIL-EXCITED CHLORINE BEAMS* D . J . P E G G t , P. M. G R I F F I N and I . A . SELLIN

University of Tennessee, Knoxville, Tennessee 37916, U.S.A. and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, U.S.A. W I N T H R O P W. S M I T H

University of Connecticut, Storrs, Connecticut 06268, U.S.A. We report on a recent spectroscopic study of autoionization electrons emitted by fast, foil-excited chlorine ion beams. The observed electrons originated in the decay of certain core-excited metastable autoionizing states in lithium-like and sodium-like chlorine. Such states are metastable since they are forbidden to autoionize via the strong Coulomb interaction but decay instead via second-order magnetic interactions (or in some cases, radiatively). Chlorine beams from the Oak Ridge tandem accelerator were passed through thin carbon foils ( ~ 15/~g/cm2) which served both to strip and excite the ions. Electrons emitted in the decay of

autoionizing states thus formed were energy analyzed after the foil by a cylindrical mirror analyzer, the position of which could be varied with respect to the foil to facilitate time-of-flight lifetime studies. Beam energies were chosen to maximize the production of the lithium-like (C114+) and sodium-like (C16+) charge states. The results of a measurement of the energy and the lifetime of the (1 s2s2p) 4P5/2 state of C114+ will be presented. A spectrum of autoionization electrons from C16+ will also be shown, but firm identification of many of the states of this system is at present difficult due to the almost complete lack of theoretical calculations of the energies and lifetimes of such states.

1. Introduction It is now well known that the foil-excitation process is effective in populating states of high electronic excitation. Hydrogenic-type transitions involving high principal quantum numbers have been observed in the beam-foil spectra of several heavy elements~). States of even higher excitation (lying above the first ionization potential of the parent system) can also be formed in processes such as multiple or core excitations2). Such states have lifetimes that are determined by the decay rates for the competing relaxation processes of both radiation and autoionization. In many cases the decay is dominated by the highly probable process of Coulomb autoionization which is induced by the interelectron electrostatic interaction. If this process is forbidden the state is said to be metastable against autoionization, but it can still autoionize via a less probable process induced by the magnetic interactions of the electrons (spin-orbit, spin-other-orbit, and spin-spin). While the rates for this type of decay are rather small for atoms and near neutral ions, they do scale quite strongly with Z and the process can become competitive with radiative decay for the more highly stripped ions of an isoelectronic sequence. Thus the lifetimes of metastable autoionization states in some highly stripped ions become amenable to study by

the beam-foil time-of-flight technique. This paper describes such a study on alkali-like chlorine ions X(n+2)+ (2Lj) GROUND STATE

I COULOMB AUTOIONIZING OR RADIATING QUARTET STATES

RADIATING SINGLET,TRIPLET ' STATES X (n+l)+ (3Lj)W LOWEST TRIPLET STATE xn*(4Lj )" TYPICAL METASTABLE QUARTET STATE

COULOMB AUTOIONIZING OR RADIATING DOUBLET STATES METASTABLE AUTOIONIZING OR RADIATING QUARTET STATES

X (n+O+ (tSo)

GROUND STATE RADIATING DOUBLET STATES

xn* (ZS~/a)

1

GROUND STATE * Research sponsored in part by the Office of Naval Research and by Union Carbide Corporation and the Oak Ridge Associated Universities under contract with the U.S. Atomic Energy Commission. t Presented the paper.

NOT TO SCALE Fig. 1. Partial energy diagram (not to scale) of a general alkalilike ion (X n+) and adjacent charge states which indicates the energy bounds on metastable autoionizing quartet states.

489 X. E L E C T R O N S P E C T R O S C O P Y ;

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490

D.J. PEGG et al.

(lithium-like and sodium-like chlorine) in which the autoionizing decay channel of metastable states was observed after foil-excitation. The metastable autoionizing states observed in this experiment are formed in core-excitation processes in which one (or possibly more than one) electron is excited from a previously closed core into an outer shell. This type of configuration can lead to quartet states in which the spin of the three active electrons are all aligned parallel. Such a state can autoionize (via magnetic interactions), emitting an electron whose kinetic energy is equal to the difference in energy between the initial quartet state energy and the final state of the system, i.e., the ground state of the ion of one charge state higher. Fig. 1 shows a partial energy level diagram of a general alkali-like ion (X "+) and adjacent charge states. A typical metastable autoionizing quartet state is shown in fig. 1, and it can be seen that such a state will be restricted in energy to the region between the ground state of the X ("+1)+ ion and the first excited state of this ion, the latter forming the series limit. Fig. 1 also shows the energy regions for allowed electric dipole radiating states and allowed autoionizing states (Coulomb autoionizing).

lithium-like chlorine (CP4+). This state can decay via autoionization induced by the magnetic interactions such as spin-orbit, spin-other-orbit, or spin-spin. In fact, there exists a differential metastability among the

ELECTRON ENERGY SPECTRUM 1 9 0 0 - t 9 9 0 eV CHLORINE BEAM 4t MeV

u) o

o3

/ -*--o

I "'°-°--o~-°--°--o

la' , ~ o - I t920 t940

1960

1980

ELECTRON ENERGY (eV)

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

method

High-energy chlorine ion beams from the Oak Ridge National Laboratory tandem accelerator were passed through thin carbon foils which served both to strip and excite the ions. Beam energies were chosen to maximize the charge states of interest (within the limits of the accelerator). Some of the emergent ions were in the metastable autoionizing quartet states under study and their autoionizing decay in flight was tracked by collecting the electrons emitted at ~ 40 ° (lab system) to the beam axis with a cylindrical mirror analyzer and channeltron combination. The relative distance between the foil and the spectrometer viewing region could be varied to facilitate time-of-flight lifetime studies.

Fig. 2. Segment of the autoionizing electron energy spectrum from 41 MeV chlorine ions undergoing decay in flight. The peak arises from the lowest three-electron quartet state, and is plotted in the ionic rest frame. I

I

:

/

I

CHLORINE BEAM 4t MeV STATE OF CIt4+ DECAY OF (ts 2s 2p) 4p IONS 5/2

~°\ °\

3

\,

(1)

3. R e s u l t s 3.1.

L I T H I U M - L I K E CHLORINE

b4 (C114+)

Fig. 2 shows a spectrum of autoionizing electrons emitted by an ~ 4 1 MeV chlorine ion beam ( v ~ 1.5 x 109 cm/s) after excitation by a carbon foil, The spectrum was taken at a position of ~ 2 cm downstream from the foil which is equivalent to a time delay of ~ 1 ns. Most of the states known to us from our work on lithium-like oxygen and fluorine a) have decayed away in this time. The single feature is associated with the decay of the (ls2s2p)4pO states of

E c

[

I

I

4

5

6

7

DISTANCE (cm)

Fig. 3. Decay in flight of the (ls2s2p)

4P°12 s t a t e

o f C114+ ions,

obtained at a tandem accelerator energy of 41 MeV.

FOIL-EXCITED

CHLORINE

J-levels of this state due to the different strengths of these magnetic interactions by which the levels couple to the continuum. The J = 1/2, 3/2 states directly autoionize via the spin-orbit, spin-other-orbit, and spin-spin interactions as well as mix with neighboring doublets of the same parity and total angular momentum. They therefore autoionize rapidly by the Coulomb interaction or radiate. The J = 5/2 level is more stable since it autoionizes only via the spin-spin interaction and is also metastable against radiative decay since it is the lowest lying state of the quartet system. Fig. 3 shows a semi-logarithmic plot of the decay of the (ls2s2p) 4p°/2 state of C1~4+. The lifetime of the state was found from the least squares best fit to the data to be 0.91 ___0.04 ns. Fig. 4 shows a partial energy level diagram (not to scale) of C114+ and adjacent ions in which the (ls2s2p)4p°/2 state is shown. The energy of 1948eV (1948+_24eV) is the energy of the autoionizing electrons (in the ionic rest frame) emitted in the decay of this state. The excitation energy of this state with respect to the C114+ ground state is thus 2 7 5 7 _ 2 4 e V and the binding energy of the state with respect to the completely stripped ion, C117+, is - 5 6 5 8 + 2 4 eV.

"~ 8 g OA

491

BEAMS

ELECTRON ENERGY SPECTRA 7 0 - 220 eV 5 MeV CHLORINE BEAM

A =3 3-cm FOIL POSITION

It

I[

70

90

t10 130 150 170 ELECTRON ENERGY (eV)

2P5 5s3P°

190

210

Fig. 5. Segment o f autoionization electron s p e c t r u m from 5 MeV chlorine ions u n d e r g o i n g decay in flight a n d plotted in the ionic rest frame. T h e figure s h o w s data for two different target positions, respectively 3 c m and 8 cm f r o m the spectrometer viewing region.

Cit6÷(ts) 2S

CIt5+ (ts2s) 3P SERIES LIMIT 3.2. SODIUM-LIKE CHLORINE (C1 +6) METASTABLE AUTOIONIZING STATE, CI t4+ 3658 eV t948 eV

CI t5+ (ts 2) IS

809.4 eV

CI t4+ ( t s 2 2 $ ) 2 S NOT TO SCALE Fig. 4. Partial energy d i a g r a m (not to scale) o f lithium-like chlorine (CP 4+) a n d adjacent ions. T h e position o f the (ls2s2p) 4pO5/2 state is s h o w n as well as the q u a r t e t state series limit.

Fig. 5 shows spectra of autoionizing electrons emitted by a 5 MeV chlorine ion beam (v ~ 5 x l0 s cm/ /s) after foil-excitation. The spectra designated as 3 cm and 8 cm were taken after time delays of approximately 6 and 16 ns respectively. The beam energy was chosen to maximize the sodium-like chlorine (Cl 6+) beam fraction, and we believe that many of the spectral features are due to the decay of metastable autoionizing states of this ion. Firm identification of the spectral lines is at present prohibited by an almost complete lack of theoretical calculations of the energies and lifetimes of such states in C16+. It seems plausible however that many of the states that are observed in decay are quartet states formed from coreexcited configurations such as 2pS(nl)(n'l ') or 2s2p 6 (nl) (n' l') with n, n' _> 3, in which a single electron has been excited from a previously closed shell or subshell into an outer orbital. It is also conceivable that we are observing the decay of states of even higher multiplicity arising from the simultaneous excitation of two or more inner shell electrons. Fig. 6 shows a X. E L E C T R O N

SPECTROSCOPY;

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D.J.

P E G G et al.

partial energy diagram (not to scale) of CI 6+ and adjacent ions. Fig. 6 shows that one should expect the series limit of the quartet system to occur at an energy of 209.5 eV, the excitation energy of the (2p53s) 3pO state of C l 7 + with respect to its ground state. A rather well defined cut-off occurs in the spectrum at this energy as can be seen in fig. 5. The possibility of contamination of the spectrum from single core-excited states of adjacent ions can be excluded by the fact that the series limits for all such systems restrict maximum electron energies to well below those observed in the experiment. The absolute energies of the four most prominent features of the spectrum of fig. 5 are 90, 101, 138 and 182 eV with an estimated uncertainty in each case of + 3 eV. The energies of these lines relative to the assumed series limit are more accurately measurable. The peak at 101 eV agrees well with a recent unpublished calculation by Weiss 4) of the energies of the 4S, 4p, 4D states that are formed from the configuration 2p 53s3p of sodium-like chlorine. An estimate of the

cIS+ (2p5) 2pO

Cl7+ (2p 5 3s) 3pO SERIESLIMIT L METASTABLE ] AUTOIONIZING - - %~8.3 eV STATE,CI6+ 182 eV

209.5eV

CI7+ (2p6) IS

Permission to reproduce figs. 2 and 5 from Phys. Rev. Letters is gratefully acknowledged.

References

144. eV

C[ 6+

energies of configurations such as 2p 5 3sns (n > 4), 2p53snp ( n > 3) and 2p53p 2 can be made by using simple screening rules, and while the accuracy of this method is not sufficient for positive identification of the observed lines, the results do serve to show that many of the spectral features might account for the aforementioned configurations. A study of the decay characteristics of the main features of the presumed sodium-like chlorine spectrum of fig. 5 indicates that most of the states have a long-lived component. We can quote a lower limit of 43 ns for the lifetime of such a component associated with the peak at 182 eV. It is particularly interesting to note that a radiative decay channel which should presumably be open to the higher energy states (if they are not the lowest lying states of a given spin system) does not appreciably shorten the lifetime of the states as might be expected. While small radiative transition moments do sometimes occur, it seems more likely that angular momentum or parity selection rule violations may be primarily responsible for the small transition probabilities. The possibility remains of course that the 182 eV peak, for example, is the lowest lying, nonradiative state of a spin system with multiplicity other than four, perhaps formed from the simultaneous excitation of more than one core electron. Until suitable theoretical estimates of energies and lifetimes are available, however, the origin of these various features must remain somewhat of a mystery.

(2p 6 3s) 2S NOT TO SCALE

Fig. 6. Partial energy diagram (not to scale) o f sodium-like chlorine (C16+) and adjacent ions. The quartet state series limit is shown.

t) For example, W. N. Lennard, R. M. Sills and W. Whaling, Phys. Rev. A6 (1972) 884. 2) For example, I. A. Sellin, D. J. Pegg, P. M. Griffin and W. W. Smith, Phys. Rev. Letters 28 (1972) 1229; D. J. Pegg, I. A. Sellin, P. M. Griffin and W. W. Smith, Phys. Rev. Letters 28 (1972) 1615; H.G. Berry, I. Martinson, L. J. Curtis and L. Lundin, Phys. Rev. A3 (1971) 1934; W. S. Bickel, 1. Bergstr6m, R. Buchta, L. Lundin and I. Martinson, Phys. Rev. 178 (1969) 118; J. P. Buchet, A. Denis, J. D6sesquelles and M. Dufay, Phys. Letters 28A (1969) 529. a) Bailey Donnally, W. W. Smith, D. J. Pegg, M. Brown and i. A. Sellin, Phys. Rev. A4 (1971) 122; I. A. Sellin, D. J. Pegg, M. Brown, W. W. Smith and B. Donnally, Phys. Rev. Letters 27 (1971) 1108. 4) A. Weiss, private communication.