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HIGH RESOLUTION LASER SPECTROSCOPY OF
THE PREDISSOCIATED b4Σg- STATE OF O+2 IN A
FAST ION BEAM
M. Horani, H. Bukow, M. Carre, M. Druetta, M. Gaillard
To cite this version:
JOURNAL DE PHYSIQUE Colloque C1, supplkment au n o 2, Tome 40, fkvrier 1979, page C1-57
HIGH RESOLUTION LASER SPFCTROSCOPY OF THE PREDISSOCIATED b41s- STATE OF 0; I N A FAST ION BEAM
M. HORANI
L a b o r a t o i r e de Photophysique M o l e c u l a i r e du CNRS, U n i v e r s i t e Paris-Sud, BStiment 213, 91405
-
Orsay, France.H.H. BUKOW
I n s t i t u t fir Experimentalphysik (111), Ruhr-Universitat, 463 Bochum, RFA. M. CARRE, M. DRUETTA and M.L. GAILLARD
L a b o r a t o i r e de Spectrom6trie Ionique e t M o l e c u l a i r e , U n i v e r s i t e Lyon 1,
69621
-
Villeurbanne, France.Resume : La technique de spectroscopie l a s e r s u r faisceau d ' i o n s r a p i d e s (FIBLAS) a
e t @
r@cemment u t i l is 6 e pour 1 'etude de l a s t r u c t u r e des ions molPculaires obtenusa
p a r t i r de s o u r c s d ' i o n s basse energie. Nous presentons i c i un premier exemple d ' u t i l i s a t i o n dans ce domaine de faisceaux d ' i o n s d'Pnergie i n t e r m e d i a i r e (en- t r e 10 e t 350 KeV). Appliquee au cas de l a p r C d i s s o c i a t i o n deo;,
b4&,
l a technique FIBLAS permet de c o n f i r m e r e t de completer l e s r e s u l t a t s recents obtenus par d ' a u t r e s groupes represent6s 2 c e t t e Conference.A b s t r a c t : The f a s t i o n beam l a s e r spectroscopy technique (FIBLAS) has r e c e n t l y been a p p l i e d t o t h e study o f t h e s t r u c t u r e o f v a r i o u s molecular i o n s u s i n g low energy i o n sources. We present here a f i r s t exemple o f t h e a p p l i c a t i o n o f medium energy beams (between 10 and 350 KeV) t o t h e same problem. I n t h e p a r t i c u l a r case o f t h e p r e d i s s o c i a t i o n o f t h e b41- s t a t e o f 0;. FIBLAS a t
9
i n t e r m e d i a t e energy enables us t o extend t h e r e s u l t s obtained simultaneously by several groups represented a t t h e Conference.
INTRODUCTION
Fast i o n beam l a s e r spectroscopy (FIBLAS) techniques have undergone a r a p i d development both i n atomic ( 1 ) and molecular physics ( 2 ) . Due t o t h e i r h i g h s e n s i t i v i t y and h i g h r e s o l u - t i o n , t h e y seem p a r t i c u l a r i l y w e l l s u i t e d f o r t h e study of molecular i o n s which r e s i s t e d so f a r d e t a i l e d spectroscopic investigation:' Such improvement i n ou? knowledge about t h e s t r u c t u - r a l p r o p e r t i e s o f molecular i o n s i s needed i n various f i e l d s o f chemistry, c o l l i s i o n physics and astrophysics ( 3 ) . Previous FIBLAS work on molecular i o n beams ( 4 - 6 ) used t h e low energy and low i n t e n s i t y beams produced by mass spec- trometers. The i m p r e ~ d i ~ v e qua1 i t y o f t h e r e s u l t s
obtained prompted us t o i n v e s t i g a t e t h e feasa- b i l i t y o f h i g h r e s o l y t i o n l a s e r spectroscopy on molecular i o n beams produced by h i g h e r ener- gy machines. This chanye o f technology presents some advantages from a p u r e l y t e c h n i c a l p o i n t o f view. such as a n a t u r a l access t o h i g h c u r - r e n t i o n sources and t o w e l l e s t a b l i s h e d beam hand1 i n g hardware. From f i r s t p r i n c i p l e s , i t way a l s o be o f i n t e r e s t i n some cases t o use t h e w i d e r Doppler t u n a b i l i t y o f f e r e d by h i g h e r v e l o c i t y i o n beams as w e l l as t h e s h o r t e r time o f f l i g h t between t h e i o n source and t h e obser- v a t i o n region. F u r t h e r tiwafi t s are expected from an o p t i m i s a t i o n o f l i n e narrowing by t h e
5
C1-58 JOURNAL DE PHYSIQUE
velocity compression e f f e c t ( 7 ) as well as from the b e t t e r handling c a p a b i l i t y f o r low mass d i s - sociation products of heavy molecules ( 8 ) .
A straight-forward extension of the techni- ques developped f o r the study of short lived ionic excited s t a t e s (1) i s generally impossible in the case of f a s t molecular ion beams due t o the iong radiative l i f e t i m e of most excited molecular s t a t e s : i n d i r e c t , non o p t i c a l , detec- tion of optical resonance must in general be substituted f o r fluorescence observation ( 9 ) . The s i t u a t i o n i s somewhat b e t t e r when the molecular s t a t e reached a f t e r l a s e r excita- EXPERIMENTAL TECHNIQUES
t i o n leads t o the dissociation of the molecule, thus giving the opportunity t o measure, with high s e n s i t i v i t y , the e f f e c t of optical exci- t a t i o n on the photofragment current. Previous work, a t energies below 10 KeV ( 4
-
6 ) showed t h a t the 0; molecular ion was an a t t r a c t i v e t e s t ground f o r FIBLAS techniques. Since fur- thermore, various d e t a i l s of the spectroscopy4
of the b
1-
s t a t e of 0; s t i l l remained some- 9what obscure ( l o ) , we decided t o r e i n v e s t i - gate the problem using a high energy beam of 0;.
mlrror chopper
Figure 1
-
Schematic o f FIBLAS apparatusThe experimental s e t up i s sketched on figu- the d r i f t tube potential. The available t u -
+
d i g i t a l converter which was calibrated against a
:2:0*
in200..
....
precision voltmeter. The evaluation of the abso- 1 ute beam energy i s much more d i f f i c u l t . W had e t o rely on a current measurement through a 1820 2 20 F2n r e s i s t o r connecting the accelerator terminal to ground plus a d i r e c t voltmeter rea- ding of the extraction potential. The overall accuracy of such an absolute voltage measurement
:
i
i s c e r t a i n l y no b e t t e r than but could be
improved by use of a calibrated velocity ana- ~ 0 . r
i
VLP 20135.025*
0.07 cm4 'lyser.
SPECTROSCOPIC ANALYSIS
The i n t e r p r e t a t i o n of the observed signal as indicated in figure 3 , i s r a t h e r s t r a i g h t f o r - ward on the basis of previous low energy expe- riments. Tabche-Fouaille e t a l . (11) observed a d i r e c t photo dissociation signal which was
4 4
+
assigned t o f rg+ a ru transition in 0 while Carrington e t a l . ( 6 ) showed t h a t the v' = 4 level of the b41- s t a t e was predissociated f o r rota-
9 Accclcrot~on energy
tional l e v e l s N ' > 9. Para1 l e l work by Guyon e t ( K E V ) a1
.
( 1 2 ) , using synchrotron radiation, indicatedt h a t the v ' > 5 levels of the b s t a t e were a l s o
--
Figure---
2 : Doppler tuning o f the fast 02+ i o n strongly predissociated w i t h estimated l i f e t i m e s beam i n t h e v i c i n i t y o f t h e 4965 lower than 50 nsec. A, -1- laser l i n e .-
2Figure
-- ---
3 : Panoramic scan o f t h e recorded s p e c t m between 20088 and 20205 em.
IPhe figure was obtained by juxtaposition o f t e n S kiZovoZts scans using backgrowad normaZisationC 1-60 JOURNAL DE PHYSIQUE
The recent publication by Tadjeddine e t a l . (13) extends and confirm these conclusions.
I t was thus q u i t e natural t o t r y t o assign t h e spectral features in figure 3 (which appear on a strong d i r e c t photodissociation current) t o reso- nant excitation of predissociating l e v e l s corres- ponding t o various A v = 3 bands of the f i r s t
+
negative system of 0
.
Since the l a s e r wave- length i s about 49658,
the observed rovibronic bands should be ( 3 , 0 ) , ( 4 , 1 ) , ( 5 , 2 ) , ( 6 , 3 ) .. .
The i d e n t i f i c a t i o n of the l i n e s was made on the basis of the molecular constants of A1 bri tton e t a1
.
(14) with.. proper extrapola- tion f o r the v' > 4 levels of b4&. Due t o the d i f f i c u l t y of measuring with good accuracy the absolute beam energy (necessary t o c a l c u l a t e the absolute Doppler s h i f t e d wavelength), the reduction of the data was by no mans s t r a i g h t - forward. This d i f f i c u l t y precludes the d i r e c t approach of f i t t i n g the frequencies af the ob-served l i n e s t o the differences of the eigen- values of appropriate Hamil tonian matrices. The
4
method we adopted was t o t r e a t the b
1-
s t a t e 9 separagly since the lower s t a t e s parameters were already known with a reasonable accuracy from optical spectroscopy. For the b s t a t e , we used the usual Hamiltonian matrix, as in pre- vious analysis by Albritton e t a1.(14) and Carrington e t a1.
( 6 ) . The parameters involved are the rotational constant B, i t s f i r s t c e n t r i - fugal d i s t o r t i o n correction D and the spin-spin and spi n-rotation interaction parameters, X and y. These l a s t two constants were determined with good accuracy from a non l i n e a r l e a s t square f i t t o spin combination differences F t 2 ( N 1 ) - F t l ( N ' ) and Ft3(N')-
FI4(N1) thus relying only ( t o f i r s t order) on beam energy difference measurementsThey are collected i n Table I . In figure 4, we have plotted the observed spin s p l i t t i n g s f o r the v1=4 level against N ' and f o r comparison have given the values calculated from the constants of table 1 a s f u l l 1 ine curves.
The e n t i r e spectrum was put on an absolute energy scale by making such energy interval measurements across the complete spectrum both f o r the parallel and anti parallel beam geometries, relying, of course, on good l i n e a r i t y in the terminal voltage measurement device. We consider t h i s s c a l e t o be accurate a t the 3 x level which imp1 i e s an absolute energy measurement with an e r r o r bar 20.07 cm-I a t 20100 cm-I.
Figure 4 : Differences AF(F2-FI and F3-F41 between the energies of the spin
4
-
components of the b
lg
u r = 4 level of 0; plotted as a function of 1'. The f u l l curves are caZeuZated from the constants of Table I. The points represent the experimental data. An overall t h r e e parameters f i t on the band o r i g i n , and the B and D constants (using the values f o r X and y determined i n the f i r s t s t e p of the analysis) leads t o an agreement between computed and measured frequencies with a rms deviation of 20 mK over 150 new 1 ines i d e n t i f i e d i n the (4-1) and ( 5-
2) bands. In t h i s f i t , the4
Table 1 b41,
Rotational Constants f o r the b41- ( v a = 4 and 5) levels of 0; (cm-l) 9
B 1.18646 -
+
11 1.16353 -+
16D 0.00000656 -
+
12 0.00000703 -+
39 Y -0.001256-
+
65 -0.00131 -+
24X
0.2086-
+
6 0.2038 -+
15Note : Uncertainties (la) are i n u n i t s o f t h e l a s t s i g n i f i c a n t figure
.
The T o ' s are for the (4-1) and (5-2) bands.LINE WIDTH
One a t t r a c t i v e feature of the FIBLAS tech- nique i s of course the very narrow instrumental linewidth achievable through the velocity com- pression e f f e c t ( 7 ) .Since the short term s t a b i l i - t y of the Sames a c c e l e r a t e r i s i n the
!%loe5
range, one expects a Doppler width due t o longitudinal beam velocity d i s t r i b u t i o n of the order of 50MW f o r 100 KeV 0; ions a t 49658.
Further contribu- tions a r i s i n g from angular divergences are be- low 10 MHz (which i s a l s o the same order of magnitudes as the l a s e r short term s t a b i l i t y ) . The observed l i n e s i n figure 3 have widely va- rying widths between 170 MHz and several giga-hertz, thus c l e a r l y displaying the e f f e c t of predissociation on the natural l i n e width, i n good agreement with previous observations (6,13). Careful analysis of the p r o f i l e of l i n e s f r e e of blending showed no evidence of l a s e r power broadening as indicated in figure 5.
A preliminary analysis of the data clear- l y indicates a systematic variation according t o the upper s t a t e f i n e s t r u c t u r e level as pre- viously noted by Carrington e t a1
.
( 6 ) thusgiving f u r t h e r support f o r t h e i r i n t e r p r e t a t i o n of the predissociation mechanism via a coup1 ing t o the 41- s t a t e .
9
Figure 5 : Two examples o f l e a s t square f i t o f a Lorentzian p r o f i l e t o t h e observed l i n e for
two d i f f e r e n t Zaser power d i f f e r i n g by a factor o f t e n . The power was measured a t
-
C1-62 JOURNAL DE PHYSIQUE
CONCLUSION
The powerful spectroscopic tools which have J.T., Ozenne, J.B., Pernot, C., and Tadjeddine, been developped in f a s t ion beam l a s e r spectros- M . , Chem.Phys.
-
17 (1976) 81.copy a r e c l e a r l y able to bring new information i n the f i e l d of molecular ion physics. Their i n t r i n s i c high resolution c a p a b i l i t i e s unravel f i n e r d e t a i l s of the intramolecular forces whi- l e t h e i r high s e n s i t i v i t y will permit the study of r a r e species otherwise too unstable f o r con- ventional precision spectroscopie investigation. In t h i s context, we have shown t h a t the use of heavy ion accelerators i s i n some casesadvanta- geous and does not lead t o unsurmamtable techni- cal d i f f i c u l t i e s .
Acknowledgements : The authors a r e i n debt t o
Dr. S. Leach and Prof. J . Durup f o r numerous enlightening discussions and continuous support during the course of t h i s work. One of us (H,.H.B) thanks the Deutsche Akademische Aus- landsdienst f o r financial support. We a l s o thank Dr. D.L. Albritton f o r kindly supplying the computer program used in t h i s work. REFERENCES
1/ Dufay, M. and Gaillard, M.L.
"Laser Spectroscopy I I I U e d . by Hall J.L. and Cal sten J . L . (Springer Verlag, Berlin) 1977, p.231.
2/ Wing, W . H . , same reference as above, p.69 3/ Leach, S. "Optical Spectroscopy of Molecular
ions" i n :"Spectroscopy of the excited s t a t e " ed. di Bartolo, B. (Plenum Press, New York) 1976.
41 Huber, B.A., Miller, T.k,l., Cosby, P.C., Zeman, H . D . , Leon, R.L., Moseley, J.T. and Peterson, J.R., Rev. S c i . 1 n s t r u m . s (1977) 1306.
51 Carrington, A . , Roberts, P.G. and Sarre, P.J., Mol. Phys.
2
(1977) 291.6/ Carrington, A., Roberts, P.G. and Sarre, P.J., Mol. Phys.
-
35 (1978) 1523.7/ Kaufman, S.L. 0pt.Commun. 17 (1976) 309. 8/ Fournier, P.,private communication.
9/ Carri ngton, A. and Sarre,.P;'J. ;These proceeding
12/ Guyon, P.M., Baer, T., Ferreira, L.F.A. Nenner I . , Tabche-Fouail l & , A . , Botter, R. and Govers, T.R., J . Phys. B,
-
11, (1978) L141.13/ Tadjeddine, M., Abouaf, R . , Cosby, P.C., Huber, B.A., and Moseley, J.T., J . Chem., Phys.
69
(1978) 710.14/ Albritton, D.L., Schmeltckopf, A.L., Harrop, W.J., Zare, R . N . , and Czarny, J . , J . Mol. Spectrosc.,
67
(1977) 157.151 Albritton, D.L.
