OPTOGALVANIC DETECTION OF URANIUM HIGH-LYING LEVELS

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OPTOGALVANIC DETECTION OF URANIUM HIGH-LYING LEVELS

M. Broglia, F. Catoni, P. Zampetti

To cite this version:

M. Broglia, F. Catoni, P. Zampetti. OPTOGALVANIC DETECTION OF URANIUM HIGH-LYING LEVELS. Journal de Physique Colloques, 1983, 44 (C7), pp.C7-251-C7-259.

�10.1051/jphyscol:1983721�. �jpa-00223278�

JOURNAL DE PHYSIQUE

Colloque C7, suppl6ment au n o l l , Tome 44, novembre 1983 page C7-251

OPTOGALVANIC DETECTION OF URANIUM HIGH-LYING LEVELS

M. B r o g l i a , F. C a t o n i and P . Z a m p e t t i

ENEA, Centro Ricerche Energia Casaccia, COMB, Divisione Ingegneria SperimentaZe Arricchimento, P. 0. Box 2400, Roma A. D., I t a l y

Resume - Une e x c i t a t i o n l a s e r A p l u s i e u r s e t a g e s d a n s une lampe cornrnerciale 2 c a t h o d e c r e u s e c o u p l e e a v e c une d e t e c t i o n o p t o g a l v a n i q u e a e t 6 u t i l i s e e pour Q t u d i e r l e s e t a t s f o r t e r n e n t e x c i t e s de l ' a t o m e d ' u r a n i u m . Les r e s u l t a t s que nous avons o b t e n u s avec c e t t e s i m p l e e t p u i s s a n t e t e c h n i q u e s o n t au moins cornparables ceux d e s rnethodes de d e t e c t i o n s complexes e t c o n v e n t i o n - n e l l e s q u i u t l l i s e n t d e s j e t s atorniques.

A b s t r a c t - To i n v e s t i g a t e t h e h i g h l y e x c i t e d uranium s p e c t r u m we u s e t h e mul t i s t e p o p t o g a l v a n i c l a s e r s p e c t r o s c o p y i n a commercial h o l l o w c a t h o d e lamp.

The r e s u l t s we c b t a i n by t h i s s i m p l e and p o w e r f u l t e c h n i q u e a r e a t l e a s t corn p a r a b l e t o t h o s e o f c o n v e n t i o n a l and more complex d e t e c t i o n methods t h a t u s e a t o m i c beams.

INTRODUCTION

A t t h e p r e s e n t s t a t e o f a r t s i n uranium l a s e r i s o t o p e s e p a r a t i o n ( L I S ) t h e most p r o m i s i n g scheme seems t o be a t h r e e - s t e p e x c i t a t i o n up t o a Rydberg s t a t e , f o l l o w e d by e l e c t r i c f i e l d o r I R i o n i z a t i o n , o r up t o an a u t o i o n i z i n g s t a t e ( F i g . 1 ) . A t t h e woy k i n g t e m p e r a t u r e o f t h e p r o c e s s a f o u r t h c o l o u r is n e c e s s a r y i n o r d e r t o r e c o v e r t h e a t o m i c p c p u l a t i o n o f t h e f i r s t m e t a s t a b l e s t a t e ( a t 620 cm -1 ) , i m p r o v i n g t h e y i e l d by N 50% / l / .

F i g . 1 - Proposed scheme f o r uranium l a s e r i s o t o p e s e p a r a t i o n .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983721

C7-252 JOURNAL DE PHYSIQUE

Such scheme requires knowledge of odd levels around 33000 cm -1 , even levels above 49000 cm -1 and autoionization spectra.

Most of the information about these uranium spectral regions are not available.

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The first data on the 33000 cm region were obtained by the groups'at Los Alamos Scientific Laboratory and Laboratoire Aim6 Cotton / 2 / . They used the Fourier Trans- form Spectroscopy (FTS) in standard emission sources : owing to the small population of these levels in usual excitation conditions, very few data were obtained by this technique.

The advent of laser excitation allowed to acquire much more spectroscopic informati on, by two techniques mainly : Multistep Laser Photoionization (MSLP) and Laser In- duced Fluorescence (LIF). MSLP was used by L.R.Carlson et al. to detect levels aro- und 33000 cm -1 /3/ and by A.Coste et al. to obtain Rydberg and autoionization spec- tra / 4 / . By using LIF E.Miron et al. considerably extended the knowledge in the

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33000 cm region and detected some levels between 49500 cm and 49900 cm /5/.

All the authors we have mentioned used uranium atomic beams produced by an e-beam or an oven.

We will discuss here the potentiality of Optogalvanic Spectroscopy (OGS) in hollow cathode lamps, as a valid alternative to techniques which use atomic beams. For re- fractory elements, like uranium, this technique associates the efficiency of sputte- ring evaporation to a straighforward and very sensitive detection tool.

EXPERIMENTAL SETUP

For the optogalvanic detection of uranium high-lying levels we have used a commercl a1 hollow cathode lamp and laser multistep pulsed excitation.

The experimental setup is standard for such measurements (Fig. 2) the only peculia- rity being in the working conditions of the lamp.

D y e L a s e r

l--=-I

Nd-YAG L a s e r D y e L a s e r 1

/ I /

U / A e H C L

t"i1

Fig. 2 - Experimental setup for multistep optogalvanic laser spectroscopy.

Owing to the complexity of the uranium spectrum it may be difficult to interpret the multistep optogalvanic spectrum.

Therefore we have found convenient to use an "old" lamp from Westinghouse Electric Corp.: in these lamps, after many working hours we have noted a particular, but ve- ry reproducible behaviour, shown in the characteristic curve (b) of Fizure 3.

Fig. 3 - Voltage-current charactsristic curves for the U/Ne hollow cathode lamps u- sed :(a) "new" lamp ; (b) "old" lamp.

,290

500

:v r

ZOO

100

In this working regime only, we can work at very low current intensity (-300 pA ) and obtain very simplified one-step optogalvanic spectra : only some buffer gas (Ne) transitions and all the uranium transitions from the fundamental state and the first metastable state are detectable (Fig. 4). These well known and identified lines sup- ply the wavelength autocalibration for the multistep optogalvanic spectra.

r

.

.

(b)

..

. -

(a) a

Fig. 4

-

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SOME ATTAINABLE RESULTS

a) Spectral region around 33000 cm -1

To investigate the 33000 cm -1 spectral region we have used two dye laser pumped by the Nd-YAG laser second harmonic. Dye laser I is tuned on a transition from

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the ground or 620 cm state to a level in the 17000 cm region; dye laser I1 is swept on the proper wavelength range.

Figure 5 shows, for instance, a portion of the spectrum we have obtained using

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the 0 cm --+ 16506 cm transition as first step. In Figure 5a) the background spectrum, i.e. the spectrum excited by laser I1 only, is reported. Figure 5b) is a two-step optogalvanic spectrum. By comparing it with the spectrum in Figure 5a) we identify the single step transitions that we used to calibrate the laser I1 wa velengxh during the scan and to calculate the high-lying levels energies.

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Fig. 5 - Two-step optogalvanic spectrum. Laser I is fixed on the 0 cm -+ 16506 cm transition, laser I1 is swept. a) Laser I1 alone (background spectrum) ; b ) laser I plus laser 11. Level wavenumbers are from ref. 151.

This technique easily allowed us to detect all the levels reported in literature, showing at least the same sensitivity as that of the other techniques, if not grea- ter. The assignement of the additional peaks in the spectrum to new levels requires

some caution. Let us consider the spectra of Figure 6 : they are two-step optogal

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vanic spectra using 0 cm -16900 cm as first step, at different laser I p2 wers. As the laser I power increases a two-photon transition up to the 33801 cm -1 level becomes more and more probable. Consequently by scanning the laser I1 wave- length we detect not only high-lying levels about 33500 cm-', but levels above the ionization limit ( I=49958 cm -1 ) too, as a consequence of excitation from the

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33801 cm level. Therefore new levels in the 33500 cm region can be unambiguou sly identified only in the low power spectrum (Fig. 6c).

Fig. 6 - Two-step optogalvanic spectra

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laser I : 0 cm ---+ 16900 cm ; laser I1 : scanning Laser I powers : a) P O ; b) 5 P c ; c) 10 -3 P O

The znergy levels thus obtained are to be compared with the data in Table I, ob- tained by E.Miron et al. using the same excitation scheme we have used and laser induced fluorescence in an uranium atomic beam /5/. As it can be seen we have de-

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tected two more levels at 33540.0 cm and 33549.6 cm

.

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Tab. I

-

OGS 33378.4 33406.6 33421.6 33474.6 33516.6 33540 .O 33549.6 33583.0 33590.0 33624.2 33645.3 33723.5 33737.5 33752.4 33800 .O 33833 .O 33838.6 33906.4

In order to understand why E.Miron et al. were not able to detect these levels although they used a buffer gas to improve the sensitivity of their method, we have carried out a measurement by simultaneously monitoring the optogalvanic si- gnal and the laser induced fluorescence in the hollow cathode lamp. The experi- mental setup is described by us elsewhere in this Colloquium.

In Figure 7 we show some fluorescence decays for a level observed by E.Miron et al.: we have not only observed all the decays reported by them, but some weaker and delayed ones as.:well.As fer as the two up mentioned levels are concerned, we only observed weak and delayed decays (Fig. 8 ) . These measurements point out that, in a lamp discharge, a d d i t i o n a l c o l l i s i o n a l p r o c e s s e s c a n c o n t r i b u t e to enhance the laser induced fluorescence by transferring atomic population from the laser excited level into some other levels with stronger radiative transiti- ons.

L I F 33378.78 33406.36 33421.12 33474.95 33516.86

33584.18 33590.90 33624.70 33645.33 33723.80 33737.46 33752.03 33801.00 33833.35 33837.82 33907.09

b) Uranium spectra above the ionization limit

Highlyexcited uranium spectra above the first ionization limit, can be obtained by either two-step ( 1 1 4 0 0 nm ) or three-step ( 1 600 nm ) excitation sche- mes. Depending on the laser powers, as we have seen, it is possible to use a two- photon plus one-stop process too.

We checked the possibility of detecting these high-lying levels by using the op- togalvanic technique, on the few spectra already reported in literature which we re recorded by multistep laser ionization in atomic beam /4/. Figure 9 compares our OG spectrum with the MSPI one by A.Coste et al., both of them obtained by the

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following excitation sequence : 0 c,m --, 16900 cm -33421 cm + levels above 50000 cm -1

.

Fig. 7

-

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0 cm -16900 cm -33379 cm in hollow cathode lamp.

a) The laser scattering signal is reported as time reference.

b),c) Fluorescence decays at 430 nm and 502 nm already observed by laser induced flu orescence in atomic beam /5/.

d) A weaker and delayed decay we detected at 384 nm in the hollow cathode lamp.

1 E - 3 V 1E - 3 U

2 0 0

Q 4

a ) 4 b

4

- 1 2 0

0 0

1 E -

Fig. 8

-

-

Wavelengths: a)laser scattering signal; b)390 nm; c1430 nm; d)456 nm.

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Z CI

m.

V V 0\ "-

Z

F i g . 9 - T h r e e - s t e p e x c i t a t i o n s p e c t r a :

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l a s e r I : 0 cm

-

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l a s e r I1 : 1 6 9 0 0 cm + 3 3 4 2 1 cm l a s ~ r I T 1 : s c a n n i n g

a ) MSPI s p e c t r u m i n a t o m i c beam / 4 / ; b) OG s p e c t r u m i n hollow cathode lamp.

CONCLUSIONS

--

From i t s d i s c o v e r y , more t h a n f i f t y y e a r s a g o , t h e OG e f f e c t h a s f o u n d a n e v e r i n - c r r a s i n g number o f a p p l i c a t i o n s i n s p e c t r o s c o p y , a n a l y t i c a l c h e m i s t r y , l a s e r t e c h - n o l o g y . I n t h i s n o t e we h a v e p o i n t e d o u t t h e power o f t h e o p t o g a l v a n i c t e c h n i q u e i n holLow c a t h o d e l a m p s , f o r h i g h - l y i n g l e v e l s d e t e c t i o n o f r e f r a c t o r y e l e m e n t s , when compared w i t h t h e a t o m i c beam t e c h n i q u e s u s e d u p t o now.

O p t o g a l v a n i c d e t e c t i o n makes a c c e s s i b l e i n a s i m p l e and s e n s i t i v e way a l a r g e amount o f d a t a o f t h e a t o m i c u r a n i u m s p e c t r u m , whose knowledge i s n e c e s s a r y t o p l a n a l a s e r i s o t o p e s e p a r a t i o n p r o c e s s .

REFERENCES

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/ 2 / STEINHAUS D . W . , RADZIEMSKI L . J . , COWAN R . D . , BLAISE J . , GUELACHVILI G . , OSMAN Z . B . a n d VERGES J . , R e p o r t LA-4501 ( 1 9 ' / 1 )

BLAISE J . a n d RADZIEIJISKI L . J . J r . , J.Opt.Soc.Am. 66 ( 1 9 / 6 ) 6 4 4

/3/ CARLSON L.R., PAISNER J.A., WORDEN E.F., JOHNSON S.A., MAY C.A. and SOLAR2 R.W., J.Opt.Soc.Am. 66 (1976) 846

/4/ COSTE A., AVRIL R., BLANCARD P., CHATELET J., LEGRE J., LIBERMAN S. and PINARD J., J.9pt.Soc.Am.

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/5/ MIRON E., DAVID R., ERE2 G., LAVI S. and LEVIN L.A., J.0pt.Soc.Am. 69 (1979) 256