SOFT X-RAY SPECTROSCOPY OF ARGON RECOIL IONS

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SOFT X-RAY SPECTROSCOPY OF ARGON RECOIL IONS

F. Folkmann, I. Lesteven-Vaïsse, A. Ben Sitel, M. Chantepie, D. Lecler

To cite this version:

F. Folkmann, I. Lesteven-Vaïsse, A. Ben Sitel, M. Chantepie, D. Lecler. SOFT X-RAY SPEC-

TROSCOPY OF ARGON RECOIL IONS. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-

267-C9-270. �10.1051/jphyscol:1987946�. �jpa-00227364�

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JOURNAL DE PHYSIQUE

Colloque C9, supplement au n012, Tome 48, dgcembre 1987

SOFT X-RAY SPECTROSCOPY OF ARGON RECOIL IONS

F. FOLKMANN, I. LESTEVEN-VAISSE*, A. BEN SITEL*, M. CHANTEPIE*

and D. LECLER*

Institute of Physics, University of Aarhus, DK-8000 Aarhus, Denmark

"~aboratoire de Spectroscopie Atomique, Universite de Caen, Esplanade de la Paix, F-14032 Caen Cedex, France

Abstract : Radiation from argon with wavelength 8-85 run has been measured with a grazing incidence spectrom~ter after+ excitation by 0.7 MeV/amu cll and 0.35 Mev/amu c19 and Brl

.

^{417 lines}

have been identified. With a pulsed beam the radiation has been sorted into prompt and delayed fractions. Decay time analysis for separate peaks is correlated to charge state of the recoil ion. This leads especially to good assignment of Ar VII and Ar VIII lines. Delayed spectra do not depend much on projectile charge and velocity.

1. Introduction

Spectroscopy of recoil ions after impact of fast heavy ions has alreadv shown to be a succesful method for studyinq excitation and _-

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secondary capture [1,2]. This work applies spectroscopy in the soft x-ray regime 8-85 nm to study the excitation of argon,. The present investigation extends a previous spectroscopic stu y [3] owing to better resolution and more clean spectra. The use of a

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pulsed beam 141 and sorting into delayed and prompt fractions has several perspecti- ves. The delayed spectra have no projectile contribution and are surprisingly intense with many well defined lines. The delayed spectra are compared for the different projectiles used. The delayed radiation is primary due to capture of electrons in secondary collisions. An analysis of the decay times for separate peaks is initiated and a correlation to charge states via their characteristic capture cross sections is demonstrated.

2. Experiment

The radiation of wavelength 8-85 nm was measured with a 2.2 m grazing-incidence

mono chroma tor^

McPherson 247, with a 600-lines/mm grating and a glancing angle of 3.7'. The experimental procedure is the same as described in [4], but with better spectral resolution using 100 pm slits for the pulsed measurements and 50 pm slits for an unpulsed run with 0.7 MeV/amu cll

.

An argon gas target of pressure 60 pbar or 120 pbar was bombarded by a pulsed heavy-ion beam. As beam ?as used 0.35, 0.5, and 0.7 MeV/amu c14 + and 0.35 MeV/amu ~r~ from the EN Tandem at Aarhus, bunched to 5 ns separated 250 ns, and stripped in a carbon foil in front of the target to mean charge states 9 to 14'.

The time relative to the pulsed beam was measured for each event from the spectrometer and stored in list mode with the wavelength in- formation by the distance x along the Rowland circle 141. A time spec- trum accumulated for a scan is shown in fig. 1 with indication of windows chosen for later analysis. A 12.6 ns wide PROMPT window around the beam pulse and a window DELAYED by 14.7

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188.4 ns was used for sorting. Additionally two delayed windows, delal and dela2, of equal width DT = 79.5 ns were used for analysis of an exponential decay pattern, which was observed for many lines.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987946

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JOURNAL DE PHYSIQUE

Figure 1 Time distribution for the signal from the grating spectrometer relative to the pulsed 0.7 MeV/amu c14 ' projectile, summed for a scan 9-25 nm. Time windows used 2 for sorting into PROMPT and DELAYED

wavelength scans are indicated. An

%

²⁰⁰

exponential decay is noted, for

which the windows delal and dela2

2

^$00

are used to extract the decay time.

5

DELAY TIME (ns)

3. Results and discussion

A spectroscopic investigation, based on unpulsed measurements, has led us to identify 417 lines from Ar and among those found 75 new lines, leading to 36 new energy levels of Ar IV, Ar VI and Ar VII [5].

From the pulsed measurements, the time distribution as seen in fig.

1 has both a prompt peak and an exponential decay. An analysis of the decay times for separate lines has shown that the fall-off time is correlated to ionic charge being around 80 ns for Ar VIII, 98 ns for Ar VII, 170 ns for Ar VI, 195 ns for Ar V and 220 ns for Ar IV [6].

This is due to an effective decay time inversely proportional to capture cross section and pressure [2,4], and a systematic increase of the capture cross section with charge [I]. On the other hand, a measured decay time can be used to get a probable charge state, especially effective for Ar VII and Ar VIII. This is illustrated in fig. 2.

Another way of taking advantage of the time structure is to sort into prompt spectra, produced at the same time as the beam passes the gas cell, and delayed radiation emitted afterwards. The main results of this type of analysis is shown in figs. 3 and 4. The prompt radiation of wavelength below 39 nm is dominated by projectile lines, but above 39 nm the main contribution is seen to come from Ar 11, Ar I11 and Ar IV. However, in the two figures we have emphasizyd the delayed spectra, which are shown for excitation by ~ r ' ~ + and c19 at the same velocity 0.35 MeV/amu and by ~1"' at an energy per nucleon twice that value.

3 0 0 0

Figure 2 Decay time, calculated from sorted wavelength-scans with the time windows delal and dela2 seen in fig. 1, from division of integrals over peaks in same chan- nels: t = DT/ln[I(delal)/I(dela2)].

Results are given for wavelengths in nm: 11: 72.6, 57.7, 53.1; 111:

53.8, 49.0; IV: 68.9, 40.6; V: 44.9 26.4, 25.2; VI: 28.3, 22.0, 19.7;

VII: 16.6, 15.2, 13.5; VIII: 26.1, 18.4, 15.9, 13.8, 12.8 and 12.0.

Circles for peaks also measured with better statistics in ref. [6].

SCANDIVISION

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I1 111 I V v VI VII VIII IX X ARGON RECOIL IONIC STATE

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Two important features are observed. Firstly, there is a high intensity in the delayed spectra compared with the prompt one, with many sharp lines and high charge states dominating, e.g. Ar VIII, Ar VII and Ar VI. Secondly, the delayed emission pattern is very similar for the three shown projectiles, and thus very little sensitive to projectile velocity and nuclear and ionic charge.

This can be explained by excitation cross sections with the same slope as function of produced charge states, followed by electron capture into the created recoil ions, when they meet a neutral Ar atom.

Another essential character of this type of process, which can also be seen from figs. 3 and 4, is that the electron is captured to an excited level, with excitation energy increasing with charge. This

0.7 M e V / a m u CI"' DELAYED

I I I I I I I I I I I I I l 1 , I I I I

9 10 11 12 13 14 15 16 17 18 19 M 21 22 23 7L 25 26 i7 28 9: 30 31 32 33 34

W A V E L E N G T H (nrnl

Fi ure 3 Wavelength+scans 9-34 nm from+a 120 pbar argon gas py impact ofg0.35 MeV/amu ~ r , '0.35 MeV/amu c19 and 0.7 MeV/amu ~ 1 " ~

.

^PROMPT

and DELAYED spectra are sorted with the windows of fig. 1. Ar charge states are marked, and for identified Ar lines also the excited electron. Some prompt C1 lines are indicated with their charge state.

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C9-270 JOURNAL DE PHYSIQUE

highly selective reaction favours 6s for Ar VIII, 5d for Ar VII, 5s and 4d for Ar VI, 4d for Ar V and 4s and 3d for Ar IV.

References

[I] F. Folkmann, R. Mann and H.F. Beyer, Phys.Scripta

T3

(1983) 88 [2] F. Folkmann, K.M. Cramon, R. Mann and H.F. Beyer,

Phys-Scripta

T3

(1983) 166

131 I. Lesteven-VaTsse, M. Chantepie, J.P. Grandin, D. Hennecart, X. Husson, D. Lecler, J.P. Buchet, M.C. Buchet-Poulizac, J. Desesquelles and S. Martin, Phys.Scripta

34

(1986) 138

[ 4 ] N. H. Eisum, F. Folkmann and P.H. Mokler,

Nucl.Instr. Meth. B

23

(1987) 215

[5] I. ~esteven-~aysse, F. Folkmann, A. Ben Sitel, M. Chantepie and D. Lecler, Phys. Scripta, submitted for publication

[6] F. Folkmann, I. ~esteven-~aysse, A. Ben Sitel, M. Chantepie and D. Lecler, Nucl. Instr. Meth. B, submitted for publication

0 35 MeV/ornu ~ r " ' DELAYED

1

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WAVELENGTH l n m l

Figure 4 Wavelength scans from argon as in fig. 3 for 33-73 nm.