XUV ABSORPTION SPECTRA OF CARBON IONS

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XUV ABSORPTION SPECTRA OF CARBON IONS

E. Jannitti, P. Nicolosi, G. Tondello

To cite this version:

E. Jannitti, P. Nicolosi, G. Tondello. XUV ABSORPTION SPECTRA OF CARBON IONS. Journal

de Physique Colloques, 1988, 49 (C1), pp.C1-71-C1-73. �10.1051/jphyscol:1988113�. �jpa-00227433�

JOURNAL D E PHYSIQUE

C o l l o q u e C1, Suppl6ment au n03, Tome 49, Mars 1988

XUV

ABSORPTION SPECTRA OF CARBON IONS

E. JANNITTI, P. NICOLOSI* and G. TONDELLO*

I s t i t u t o Gas I o n i z z a t i , CNR,

1-35131

Padova, I t a l y

" ~ i p a r t i m e n t o d i E l e t t r o n i c a e I n f o r r n a t i c a , U n i v e r s i t a d i Padova,

I-35131

Padova, I t a l y

A b s t r a c t

The photoabsorption s p e c t r a of t h e carbon i o n s have been o b t a i n e d by using two laser-produced plasmas. The p h o t o i o n i z a t i o n c r o s s - s e c t i o n of t h e CV h a s been absolu- t e l y measured and t h e v a l u e a t t h r e s h o l d , o= (4.7T0.5) x lo-'' cm2, a s w e l l a s i t s behaviour a t h i g h e r e n e r g i e s a g r e e s q u i t e w e l l with t h e t h e o r e t i c a l c a l c u l a t i o n s .

The technique f o r a b s o r p t i o n spectroscopy u t i l i z i n g t w o 1 a : ; c r - p r o d u c e d plasmas h a s been a p p l i e d f o r s t u d y i n g t h e p l ~ o t o i o r ~ i z a t i o n of t h e carbon i o n s . This experimental technique h a s been a l r e a d y widely used; f o r i n s t a n c e , t h e a b s o r p t i o n s p e c t r a of t h e v a r i o u s Beryllium i o n i z a t i o n s t a g e s , from t h e n e u t r a l Be I up t o t h e H-like Be I V , have been r e c e n t l y o b t a i n e d ~ - 1 , 2 - 7 .

Carbon i s of g r e a t i n t e r e s t t o t h e p h y s i c i s t s ; i n f a c t i t i s a fundamental element i n n a t u r e . For t h i s reason a l o t of work both t h e o r e t i c a l and experimental h a s been c a r r i e d on f o r s t u d y i n g t h e atomic parameters of t h i s element.

I n t h e p r e s e n t work we r e p o r t i n p a r t i c u l a r t h e experimental o b s e r v a t i o n of t h e p h o t o i o n i z a t i o n of t h e CV and t h e measurement of t h e r e l a t i v e c r o s s - s e c t i o n . I t h a s been measured by e x t r a p o l a t i n g t o t h e continuous spectrum t h e known o s c i l l a t o r s t r e n g t h s of s u i t a b l e d i s c r e t e l i n e s . The d e r i v e d experimental v a l u e a g r e e s v e r y w e l l with t h e t h e o r e t i c a l p r e v i s i o n s of Reilman and Manson. - - The experimental t e c h n i q u e h a s been d e s c r i b e d i n d e t a i l i n a p r e v i o u s paper

L

I-/, t o which t h e r e a d e r i s r e f e r r e d . For t h i s reason o n l y t h e b a s i c p r i n c i p l e s of t h e experiment w i l l be b r i e f l y summarized h e r e . A scheme of t h e experimental a p p a r a t u s i s shown i n f i g . 1. The a b s o r p t i o n s p e c t r a a r e o b t a i n e d by using two l a s e r produced plasmas: one e m i t t i n g a background continuum r a d i a t i o n , t h e o t h e r a c t i n g a s absorbing medium. They a r e produced by s p l i t t i n g a s o l i d s t a t e l a s e r beam (8J, 15-20 n s ) i n t o two beams. Most of t h e l a s e r energy i s focused on a s o l i d p l a n e t a r g e t of r e l a t i v e l y high atomic number m a t e r i a l , e . g . Pb, i n o r d e r t o g e t a plasma e m i t t i n g a s u i t a b l y s t r o n g and smooth continuum spectrum. The remaining energy i s focused by a sphero- c y l i n d r i c a l l e n s on a p l a n e g r a p h i t e t a r g e t g e n e r a t i n g t h e absorbing plasma. Thecon- tinuum r a d i a t i o n i s e f f i c i e n t l y c o l l e c t e d by a g r a z i n g incidence t o r o i d a l m i r r o r and is focused on t h e e n t r a n c e s l i t of a g r a z i n g i n c i d e n c e spectrograph. The absorbing plasma i s g e n e r a t e d n e a r t h e e n t r a n c e s l i t which is o r i e n t e d p a r a l l e l t o t h e t a r g e t surface, I n t h i s

way

only

a

very narrow r e g i o n of t h e expanding plasma can be r a d i a - t e d by t h e backcjround continuum. An a d d i t i o n a l s l i t perpendicul.ilr attd c l o s e t o t h e former h a s been used f o r f u r t h e r l y l i m i t i n g t h e r a d i a t e d plasma r e q i o n . I t resheT?ed i n a c o n s i d e r a b l e enhancing of t h e c o n t r a s t r a t i o i n t h e d e t e c t e d s p e c t r a . Moreover t h e time d e l a y between t h e g e n e r a t i o n of t h e two plasmas can be v a r i e d by i.ntroduc- irig a s u i t a b l e o p t i c a l d e l a y between t h e two l a s e r bearas. The d e t e c t i o n system con- s i s t s of a s c i n t i l l a t o r coated f a c e - p l a t e coupled with an image i n t e n s i f i e r , a ~ ~ h o t o - d i o d e a r r a y and an o p t i c a l multichannel a n a l y z e r . The l a t t e r i s connected with a PDP computer.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988113

CI-72 JOURNAL

DE PHYSIQUE

Each o b s e r v a t i o n h a s been made by superimposing about 2 0 exposures f o r i n c r e a s i n g t h e s i g n a l t o n o i s e r a t i o , t h e r e p r o d u c i b i l i t y of t h e a b s o r p t i o n s p e c t r a from s h o t t o s h o t b e i n g very good.

I n f i g . 2 t h e a b s o r p t i o n c o e E f i c i e n t of t h e carbon i o n s i n t h e 24-45

&

wavelength i n t e r v a l i s r e p o r t e d . The corresponding experimental parameters, i . e . a f o c a l s p o t on t h e carbon t a r g e t of 2-3 nun diameter and on o p t i c a l delay of about 4 . 5 n s , have been chosen f o r o b t a i n i n g p r e f e r e n t i a l l y t h e C V ground s t a t e p o p u l a t i o n . The C V

I ASFR

PUMP GRAZING INCIDENCE

SPECTROGRAPH

Fig. 1 Scheme of t h e experiment. T I , T2 t a r g e t s ; A , p l a n e m i r r o r ; M, t o r o i d a l mJr- r o r ; L 1 , L 2 , f o c u s i n g l e n s e s ; S , e n t r a n c e s l i t ; G ; g r a t i n g ; 1.1. image i n - t e n s i f i e r ; P D A , photodiode a r r a y .

F i g . 2 Absorption c o e f f i c i e n t o f t h e C V ion.

resonance s e r i e s l s 2 IS-lsnp 'PO with n = 2-7 i s followed by t h e p h o t o i o n i z a t i o n con- t l n u o u s d i s t r i b u t i o n and a few s a t e l l i t e l i n e s a r e c l e a r l y d i s Q i n g u l s h a b l e on t h e r e d wavelength s i d e of t h e resonance l i n e a t 40.27

i.

These l i n e s correspond t o t r a n s i t i o n s of an i n n e r e l e c t r o n i n i o n s with lower charge t h a n t h e C V one. Some fur- t h e r i n n e r s h e l l t r a n s i t i o n s of t h e same k i n d should be appearing i n between t h e o t h e r C V l i n e s b u t t h e y appear confused with t h e n o i s e . Although t h e l a t t e r i s pre- s e n t throughout t h e spectrum, it a f f e c t s i n p a r t i c u l a r only t h e weakest s i g n a l s .

I n a p r e v i o u s paper

L1-7

t h e method f o r d e r i v i n g t h e p h o t o i o n i z a t i o n c r o s s - s e c - t i o n measurement by measuring t h e 'column d e n s i t y ' h a s been d e s c r i b e d . The l a t t e r c a n be d e r i v e d from t h e a b s o r p t i o n c o e f f i c l e n t of d i s c r e t e t r a n s i t i o n s with known o s c i l -

lator strengths.

A

deconvolution procedure must be applied to the observed spectra which can be deeply distorted by the finite instrumental broadening. Briefly, a syn- thetic spectrum of Voigt profiles, in this case mostly Lorentzian, is created,then it is convoluted with the instrumental function and compared with the experimental one. The free parameters are the normalized profiles, i.e. the width of the lines, and the column density, i.e. the peak absorption coefficient of the lines.

As an example the absorption coefficient of the lines with n = 3-11 is reported in fig.

3

with a dashed curve after the convolution with the instrumental function. In the same figure the experimental absorption coefficient, for this particular case,is shown with a continuous curve for comparison. The same best-fit procedure has been applied to several independent experimental spectra. Although of the intrinsic line profiles only that one corresponding to n=7 is broader than the instrumental profile, the method results enough sensitive and precise. The CV column density was derived mostly by the lines n=4-7; those with n >

7

have been introduced for smoothing the transition between the discrete and the continuous parts of the spectrum. The photo- ionization cross-section value results o

= 4.7

x cm2 with maximum devlatlon

A 0 =

+ 0 . 5 ~ 1 0 - ~ ~ cm2 over the various independent measurements. It can be compared with

at

=

4.93 x 10-l9 cm2 reported by Reilman and Manson 13-7.

Fig.

3

Example of the best-fit of the CV absorption spectrum. Continuous curve, ex- perimental data; dashed curve, best-fit results.

References

1.

E.

an nit ti, P. Nicolosi and

G.

Tondello, Physica Scripta 36 (19871, 93.

2. E. Jannitti,

P.

Nicolosi, G. Tondello, Zheng Yongzhen and M. Mazzoni, Opt.

Commun. 63 ⁽¹⁹⁸⁷⁾ , ^37.

3 . ^{R . F .}

Reilman and S.T. Manson, Astrophys. J. Suppl. Ser. 40 (1979), 815