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QUASI-MOLECULAR EFFECTS IN ION-ATOM COLLISIONS AT ION ENERGIES IN THE
HUNDRED-keV REGION
E. Bøving, K. Edlund, G. Sørensen
To cite this version:
E. Bøving, K. Edlund, G. Sørensen. QUASI-MOLECULAR EFFECTS IN ION-ATOM COLLISIONS
AT ION ENERGIES IN THE HUNDRED-keV REGION. Journal de Physique Colloques, 1979, 40
(C1), pp.C1-246-C1-249. �10.1051/jphyscol:1979151�. �jpa-00218431�
JOURNAL DE PHYSIQUE Colloque C1, supp,&ment au n O 2, Tome 40, fkvrier 1979, page C1-246
QUASI-MOLECULAR EFFECTS I N ION-ATOM COLLISIONS AT I O N ENERGIES I N THE HUNDRED-keV R E G I O N
E.G. BQving, K. Edlund, and G. SQrensen
I n s t i t u t e of P h y s i c s , U n i v e r s i t y of Aarhus, DK-8000 Aarhus, Denmark
Rbsums. Les spectres de Ne I, Ne 11, Ne I11 et Ne IV ont Ctb excites dans les conditions de choc unique par impact d'ions (50 KeV-500 KeV) dans une cible de neon. Le nmi5ro atombque du pxojectile allait de 2 a 40, et 116mission optique a 6t6 enregistr6 de
I00 A 2 1000 A 2 l'aide d'un spectrometre 2 incidence rasante. Les populations des 6tats excitgs ant 6tg Btudi6es en fonction de l'bnergie du projectile. On compare les rLsul- tats de l'excitation des niveaux 3s I P et 3s 3p dans Ne I avec les mesures de sections efficaces de collision ~ e +
-
Ne. Une anomalie du rapport singulet 2 triplet a 6tC observCe dans les niveaux 2 ~ 2 ~ 5 ' P et 2 ~ 2 ~ 5 3~ de Ne III pour certains projectiles : r = 5.5 et 0.4 pour Be+-
Ne et ~ e +-
Ne B 100 KeV. La structure oscillante de la variation en fonction du nurnero atomique du projectile pourrait s'expliquer dans le cadre du modsle des orbitales mol6culaires mGme P grande Cnergie.A b s t r a c t . By means of i o n s c o l l i d i n g w i t h a neon gas d u r i n g f u l f i l m e n t of t h e s i n g l e - c o l l i s i o n requirement, atomic l e v e l s i n Ne I , Ne 11, Ne 111, and Ne I V were e x c i t e d . The atomic number of t h e p r o j e c t i l e v a r i e d b e t - ween 2 and 40, and t h e o p t i c a l emission was s t u d i e d i n t h e wavelength r e g i o n between 100 and 1000 A w i t h a g r a z i n g - i n c i d e n t s p e c t r o m e t e r , and t h e atomic-level p o p u l a t i o n s were i n v e s t i g a t e d a s a f u n c t i o n of p r o j e c - t i l e energy. For t h e e x c i t a t i o n t o t h e 3s1p and 3 s 3 p l e v e l s i n Ne I , t h e r e s u l t s were compared w i t h t h o s e d e r i v e d from previous a b s o l u t e c r o s s - s e c t i o n measurements f o r t n e Ne+-Ne c o l l i s i o n . For t h e 2s2p5 'P and 2 . ~ 2 ~ ' 3~ l e v e l s i n Ne 111, an anomalously high s i n g l e t - t o - t r i p l e t r a t i o was observed f o r c e r t a i n p r o j e c t i l e atomic numbers. Thus 100-keV
~ e + - ~ e and ~ e + - ~ e y i e l d e d a s i n g l e t - t o - t r i p l e t r a t i o of 5.5 and 0.4, r e - s p e c t i v e l y . A s y s t e m a t i c v a r i a t i o n of t h e p r o j e c t i l e atomic number r e - v e a l e d a n o s c i l l a t o r y s t r u c t u r e , t h e f e a t u r e of which could b e e x p l a i n e d w i t h i n t h e framework of t h e Molecular-Orbital model even a t t h e s e high p r o j e c t i l e e n e r g i e s .
I N T R O D U C T I O N
I n s p e c t r o s c o p i c s t u d i e s , s p e c t r a l - l i n e i n t e n s i t i e s a r e a t o o l i n t h e s p e c t r a l - l i n e i d e n t i f i c a t i o n and may r e v e a l t h e presence of b l e n d i n g l i n e s . M u l t i p l e t s with- i n t h e same c o n f i g u r a t i o n a r e t h u s expected t o show an i n t e n s i t y r a t i o which i s c l o s e t o t h e s t a t i s t i c a l v a l u e . However, due t o t h e f a i l u r e of t h e conventional a d i a b a t i c c r i t e r i o q i n ion-atom c o l l i s i o n s , s p e c t r o - s c o p i c s t u d i e s have p r e v i o u s l y showna com- p l e x behaviour of m u l t i p l e t r a t i o s c l o s e t o t h e t h r e s h o l d f o r t h e e x c i t a t i o n 1 ) . The p r e s e n t s t u d y d e a l s w i t h s p e c t r o s c o p i c s t u d i e s of atomic c o l l i s i o n s w i t h f u l f i l - ment of t h e s i n g l e - c o l l i s i o n c o n d i t i o n b u t w i t h i o n e n e r g i e s f a r from t h r e s h o l d i n t h e hundred-keV range. I o n s o f varying element numbers and e n e r g i e s have been used t o ex-
c i t e a neon g a s ( p r e s s u r e 10-4-10'3 t o r r ) , and m u l t i p l e t r a t i o s have been measured e x p e r i m e n t a l l y . The p r e s e n t communication d e a l s w i t h two d i f f e r e n t e x c i t a t i o n mech- anisms,
where X+ r e p r e s e n t s an i o n i c p r o j e c t i l e of v a r y i n g element number and energy. I n t h e former c a s e ( 1 )
,
t h e neon atom i s ex- c i t e d t o t h e l e v e l s I s 2 and 1.94 (Pas- chen n o t a t i o n ) w i t h no change of t h e atomic charge, w h i l e i n t h e l a t t e r c a s e( 2 )
,
two e l e c t r o n s a r e removed from t h e neon atom i n t h e c o l l i s i o n p r o c e s s , l e a v - i n g a 2s h o l e i n t h e e x c i t e d , doubly charged neon atom.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979151
EXPERIMENTAL PROCEDURE
A detailed description of the appara- tus has appeared in these proceedings2) ' In NeI, the Is2 and Is4 levels are deexcited via the 736 A and 744 A transition, and the response of the optical system is assumed to be the same for the two close lying spectral lines. The spectral lines depopu- lating the 2s 2p5 configuration in Ne I11 are 379 A for the singlet and 488 A for the triplet level. The spectral response at these two wave length was in the fol- lowing way measured to be identical within 10%. A 400-keV neon-ion beam was backscat- tered from a number of metal surfaces, and the ratio between 379 A and 488 A was measured. In all cases, the singlet-to- triplet ratio (S/T) was measured to be 0.3320.04, which is the statistical value;
hence no further calibration was performed.
Results
With a number of different projectiles and ion energies, the intensity ratio be- tween 736 & and 744 & in NeI was measured.
The wave function of the Is2 and Is4 levels can be written as linear combinations of LS wave functions, and the excitation func- tions of the Is2 is expected to be of pre- dominantly singlet character, while it is
0 C
"
.
w P CI-
Ne I S c M n w et allJ 50 VELOCITY I x lo8 mls I
Fig. Singlet-to-triplet ratio (S/T) for deexcitation of Is2 and Is4 in NeI as function of the velocity of the projectile ions.
triplet like for ?sk. It was observed that the intensity ratio of the lines depopu- lating Is2 and Is4
-
the singlet-to- triplet ratio S/T-
shows a variation be- tween 0.7 and 2.7. The solid line in the figure illustrates S/T ratios derived for the target-atom excitation in the symme- tric ~ e ++
Ne collision measured by Schartner et a1 .3).
The experimental re- sults show a strong velocity dependence as far as the S/T ratios are concerned. Thus, by varying the energy ofa
lithium pro- jectile from 100 keV to 500 keV, the S/T ratio is increased from 1.1t0.2 to 2.620.4.When the neon target is excited in a collision where two electrons have been removed, the excited levels may also occur in a singlet or a triplet state for the 2s2p5 configuration. The deexcitation of the 2 ~ 2 ~ ' I P and 2 ~ 3~ 2levels in Ne 111, ~ ~ via the 379 A and the 488 & transitions,
L I I b I I
5 10 15 20
PROJECTILE Z
Fig. 2 Singlet-to-triplet ratio (S/T) of
the deexcitation of the 2 ~ 2 ~ ~ configuration in Ne I11 shown as
function of projectile element number(4-j on lower scale and as a function of velocity as indi- cated on upper scale (--0-- 1 .
Z + Ne
-
~ e 1 1 1 ~ ( 2s 2p5 ) M L O U T Y ( xlo8
cmk)0.5 1.0 1.5 2.0
6
I I
- 100 keV -
T --0-- VELOCITY
C 1-248 JOURNAL DE PHYSIQUE
respectively, has previously been reported to depend strongly on the atomic structure of the projectile4). Hence, for certain collision systems such as 100-keV Be +Net
+
an anomaleously high S/T ratio of 5.5k0.6 was measured. Further, the measured S/T ratios show an oscillatory structure as a fucntion of element number of the projec- tile. As shown in Fig. 2, the S/T ratio for the symmetric Ne +Ne collision is close
+
to the statistical value, while high values are observed for element numbers -4 and -15, respectively.
The S/T ratio shows an energy depen- dence which, for the collision system P +Net
+
is shown in Fig. 3 in the ehergy range
1 1 < , 1 1 , 1 t 1 1 J
YX) 500 m
ENERGY IN KEV
Fig. 3 Singlet-to-triplet ratio (S/T) in Ne I11 for the collision system P +Ne as a function of the ion 4-
energy.
100-800 keV. This also includes results ob- tained with P++, but the collision process does not seem to depend on the projectile charge. It thus appears that a statistical S/T ratio is reached only at projectile energies considerably higher than 1 MeV.
When the energy dependence of the spectral line intensities is investigated, it appears that the intensity of the 379
A
line is al- most constant in the energy range studied while the 488 A line shows an increasing trend tawards higher energies. The spectral intensities of the transition depopulating the singleta~dtriplet level are shown in Fig. 4.Fig. 4. The spectral line intensity of the 379 A transition ( 2 ~ 'P) 2 ~and ~ the 488 A transition (2s2p5 3 ~ ) as a function of ion energy.
Discussion
Concerning the singlet and triplet excitation in Ne I, it has previously been suggested that excitation of the triplet
(IsI,) level occurs via an electron ex- change process5! and consequently higher projectile energies will reduce the popula- tion of this level, a trend which is in agreement with the present data (Fig. 1).
Work is in progess with respect to evalu- ating absolute cross sections in order to compare them with theoretical estimates of the ion-impact excitation of neon to the 2p5 3s configuration.
Regarding the 2s 2p5 configuration in Ne 111, the anomaly of the S/T ratio has been explained qualitatively on basis of the molecular-orbital picture41. In prin- ciple, the explanation deals with the twofold ionization occurring via two chan- nels or via one channel only. With refe- rence to Fig. 5, showing the MO correlation diagram for slightly asymmetric collisions, both 3pon and 2so vacancies lead to 2s
g
vacancies in either projectile or target.
Consequently, a 2 p electron has to be removed from the 4fon orbital, which can be regarded as a second channel. The ab- sence of correlation between the electron spin therefore will result in population of 2s 2p5 3 ~ .Occa~ionally, the 3d o
g orbital will be doubly ionized and this
the 3da orbital at small internuclear g
distances. For Z > 10, singlets are po-
-
u pulated by double ionization from the 4faorbital, which is a frequent process at the projectile energies investigated. Ac- cording to Eichler et a1.6)
,
the 4fo or-bital is radially coupled to 3po. Because
W the 4fo orbital correlates to 2p in neon
and the 3po orbital to 2s, the observed high S/T ratio for Z > 10 is an experi- mental evidence of the new correlation
0 OD
INTERNUCLEAR DISTANCE rules of Eichler et al. 6)
.
Fig. 5. Correlation diagram for the asym- CONCLUSION
metric X 4- +Ne collision system. The ex~erimental S/T ratios presented here demonstrate that this parameter yield channel process leads to a simultaneous new information on the atomic-collision ionization of 2s and 2p, which results in models. Measurements of multiplet inten- population of a singlet state. For Ne +Ne,
+
sities can thus be regarded not only as a the removal of the two electrons is uncor- tool for the spectral-line-identification related, resulting in an almost statisti- process, but it may be used to study the cal multiplet population. For projectile complexity of the collision process itself, numbers Z < 10, the main process of excit- i.e., the relative importance of the dif- ing singlet is the double ionization from ferent quasimolecular couplings.BIBLIOGRAPHY
1 1 1 S.Dworetsky, R.Novick, W.W.Smith, and 141 R.G.B@vinq and G-Sarensen, Phys.Rev.
N.Tolk, Phys.~ev.Letters
18,
939 (1967) Letters40,
315 (1978)121 G.S@rensen, these proceedings [51 G.W.York, J.T.Park, V-Pol, and P.H.
[31 K.H.Schartner, R.Hippler, and H.F.Beyer, Grandall, Phys.Rev.E 1497 (1972) J . P h y s . B m 93 (1977) [61 J-Eichler, U.Wille, B.Fastrup, and K
Taulbjerg, Phys-Rev. 707 (1975)
