AUTOIONIZATION ELECTRON SPECTRA RESULTING FROM COLLISIONS BETWEEN Li2 MOLECULES AND CORE PHOTOIONIZED Li+ ATOMIC ION

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AUTOIONIZATION ELECTRON SPECTRA RESULTING FROM COLLISIONS BETWEEN Li2

MOLECULES AND CORE PHOTOIONIZED Li+

ATOMIC ION

P. Gerard, D. Cubaynes, J. Bizau, F. Wuilleumier

To cite this version:

P. Gerard, D. Cubaynes, J. Bizau, F. Wuilleumier. AUTOIONIZATION ELECTRON SPEC- TRA RESULTING FROM COLLISIONS BETWEEN Li2 MOLECULES AND CORE PHOTOION- IZED Li+ ATOMIC ION. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-719-C9-724.

�10.1051/jphyscol:19879122�. �jpa-00227231�

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JOURNAL DE PHYSIQUE

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

AUTOIONIZATION ELECTRON SPECTRA RESULTING FROM COLLISIONS BETWEEN Li, MOLECULES AND CORE PHOTOIONIZED Li+ ATOMIC ION

P. GERARD, D. CUBAYNES, J.M. BIZAU and F.J. WUILLEUMIER

Laboratoire de Spectroscopic Atomique et Ionique and LURE, Universite Paris-Sud, Bdt. 350, F-91405 Orsay Cedex, France

RESUME

Les r e s u l t a t s obtenus dans une rscente @tude de l a photoionisation du lithium atomique e t moleculaire sont prssentes. Une nouvelle i n t e r p r e t a t i o n e s t proposse.

ABSTRACT

The r e s u l t s obtained i n a recent photoemission studies of atomic and molecular

?i vapor, using electron spectrometry, a r e summarized. Suggestions for t h e explana- tion of the experimental observations are presented.

I. INTRODUCTION

In e a r l i e r studies of photoionization i n Li vapory1, using electron spectrometry, additional l i n e s t h a t could not be a t t r i b u t e d t o the photoionization of atoms, were observed. The kinettc energy of t h i s group of electrons was found t o be constant v~han the photon energy was varied between 75 eV and 150 eV. These k i n e t i c energies were measured, a t t h a t time, t o be between 48 eV and 50 eV. Their i n t e n s i t i e s were comparable t o the i n t e n s i t y of the atomic photoelectron l i n e s , a s I t can be seen i n figure 1 of reference 1. Two main reasons lead t o t h e conclusion t h a t these additio-

!?a7 lines should be a t t r i b u t e d t o Auger t r a n s i t i o n s i n a lithium molecule: i ) the corlstant value of t h e i r k i n e t i c energy a s a function of photon energy; i i ) t h e f a c t rhat Auger t r a n s i t i o n s are not possible a f t e r ionization of Li-atoms i n the 1s sub- she1 1. In addition, i n the energy range measured f o r these electrons, previous expe-

2 3

riments, involving electron impact o r ion impact ,had not revealed the existence of electron l i n e s a t energies lower than 50 eV. Only electrons emitted i n t h e autoionization of molecular excited s t a t e s had been observed ?n these experiments, above 53 eV. The f a c t t h a t no l i n e s had been observed below 50 eV was a t t r i b u t e d t o the low values of ionization crcss section by electron impact.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879122

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JOURNAL DE PHYSIQUE

However, the a t t r i b u t i o n of these l i n e s t o molecular Auger decay l e f t several questions open: F i r s t , the average f o r the lso- and lso; binding energies i n Liz had

2 9

been estimated t o be between 61.73 eV and 62.5 ev4. None of these values were com- patible w i t h the spectra observed a t t h a t time: assuming a pure Coulombic potential

4 u

curve f o r Liz

,

t h e estimated values f o r the molecular I s binding energies were i n t h e range from 65 eV t o 66.5 eV, i .e. i n the same binding energy region as the IS-electron binding energies i n atomic lithium (64.41 eV f o r the ls2s 3 S ionic s t a t e , 66.31 eV f o r the ls2s S s t a t e , according to Ref.5). These values were f a r from the 1 theoretical estimates. Second, a t the temperature used i n the photoemission experiments, t h e contribution of dimers t o the vapor pressure was supposed t o be i n the order of 1 x S 6 The i n t e n s i t y of the observed molecular s t r u c t u r e would have required molecular photoionitation cross sections considerably g r e a t e r than the atomic cross sections. No reason was found f o r a possible concentration of Liz dimer higher i n t h e source volume of the electron analyzer than i n the oven.

I I. EXPERIMENT

Under b e t t e r experimental conditions, we repeated and we extended the photoelectron spectrometry experiments on L i , using basically the same experimental s e t up, i .e. a furnace mounted on the axis of a cylindrical mirror electron analyzer (CMA).

However, higher photon flux were available i n these new experiments, because of the routine use of new toroidal grating monochromator. Thus, data could be obtained a t b e t t e r resolution and/or a t lower atomic d e n s i t i e s . In p a r t i c u l a r , we were able t o make study of the i n t e n s i t y variation of the electron l i n e s as a function of several

parameters such as the temperature i n the lithium furnace o r the photon energy near threshold and i n t h e v i c i n i t y of atomic resonance l i n e s .

111. RESULTS

channel number

0

Q) U)

\ 8 0 0 -

-

V) ^C

0

a

F i g . l - Electron spectrum f o l - lowing ionization of a Li va-

2 por by 91.50 eV. Peak noted

( 1 ) corresponds t o photoioni-

I zation of the 2s-electrons i n

I

^{L i}atoms (Li+ l s 2 IS f i n a l

s t a t e ) . Peaks noted 2,3,4 and 5 corresponds to photoioniza-

1

t i o n of the Is-electrons i n L i

3 A atoms, the ion being l e f t i n

ls2s 3s (peak 2 ) , ls2s ( 3 ) , ls2p 3~ (4) and ls3s and ls3p

0) s t a t e s , respectively. Peaks noted A and a are o f molecular origin. (From Ref. 7)

.

200 400 600

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The r e s u l t s obtained i n t h i s new s e r i e s of experiments a r e summarized i n Figu- r e s 1 t o 4. Fig.1 shows a complete electron spectrum of Li vapor taken a t 91.50 eV photon energy. Peaks noted 1, 2, 3, 4, and 5 are electron l i n e s due to photoionization of Li atoms i n the outer- and inner-shells, as explained i n the caption. Peaks noted A and a a r e the additional l i n e s previously a t t r i b u t e d t o Auger-decay of core- ionized Li molecules. As i n the previous experiments

,'

the k i n e t i c energy of these l i n e s has been found t o be constant when the photon energy was varied over an even wider energy range, including the Is-atomic ionization thresholds around 64 eV.

However, the absolute values measured f o r t h e k i n e t i c energies of the l i n e s were d i f - ferent.7 The main s t r u c t u r e s i n the electron spectrum were observed a t 51.56 ( 6 ) eV, 52.8(1) and 53.51 eV, about 3 eV higher than i n the e a r l i e r measurements.' A reason for the discrepancy between the two s e t s of values was found i n t h e c a l i b r a t i o n of the absolute k i n e t i c energy scale. Our values a r e i n excellent agreement with t h e energies of the molecular autoionizing electrons observed by electron impact ( 51.5, 51.6, 52.9 and 53.9 eV). 2

Because of the higher photon f l u x available, we were able t o work a t a much b e t t e r resolution of the monochromator (0.3 eV compared to 0.9 eV) and t o scan the photon energy range i n which l i n e s A and a appear. F i g . 2 presents spectra obtained f o r these 1 ines i n t h e i r threshold region, around 64 eV photon energy. Below 64 eV, they could not be detected. When one increases the photon energy by small s t e p s , they appear suddenly with a large i n t e n s i t y , f l ' r s t t h e A1 l i n e s ( 51.56 eV k i n e t i c energy), and second, l i n e A2 (52.8 eV) and a , above 66 eV. The excitation function we obtained f o r t h e i r i n t e n s i t y between 64 eV and 68 eV photon energy i s shown i n Fig.3. The atomic-ionization thresholds a r e marked i n the Figure. The two arrows on the top of the figure indicate the position of some resonant atomic l i n e s involving

KINETIC ENERGY ( e V )

hr

-

^61.22^*) ^nv

-

^s.n^*r Fig.2- Electron spectra

showing the photon-energy dependence of the A-lines f o r various photon energies i n the region o f the Is- atomic i o n i z a t i o n thres- holds

.

^{From Ief}^tt o Eight and from top t o bottom, the photon energies are:

hv

-

^66.15^N 64.22eVY 64.33 eV, 66.15

eV and 66.34 eV, respecti?

v e l y

.

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C9-722 JOURNAL DE PHYSIQUE

Fig.3- Excitation function of the A-electron lines as a func- tion of photon energy. See text for detailed comments.

PHOTON ENERGY in e V

2 2 1 2

-two-electron core.-excited s t a t e s : 1s 2s S -+(ls2p P)3s P t r a n s i t i o n a t 65.29 eV,

2 2 1 2

1s 2s S 4 ( l s 2 p P)4s P t r a n s i t i o n a t 66.44 eV. From t h i s f i g u r e , one can d e t e r - mine a t h r e s h o l d energy o f 64.4(2) eV f o r t h e A1 l i n e , which coincides

,

w i t h i n t h e e r r o r bars, w i t h t h e f i r s t 1s-atomic i o n i z a t i o n thresh01 d.

F i n a l l y , i n F i g.4, we. present some qua1 i t a t i v e r e s u l t s , showing t h e temperature dependence o f t h e A - l i n e s . The t h r e e spectra, on t h e r i g h t p a r t , a r e t h e atomic inner-she1 1 p h o t o i o n i z a t i o n 1 ines, measured a t t h r e e d i f f e r e n t temperatures i n t h e oven. T h e i r i n t e n s i t y has been normalized t o t h e same a r b i t r a r y u n i t s . The t h r e e spectra on t h e l e f t p a r t o f t h e f i g u r e a r e t h e A-lines taken a t t h e same temperatu- r e s . The r e l a t i v e v a r i a t i o n o f t h e i n t e n s i t y o f these A - l i n e s d i f f e r s g r e a t l y from t h e i n t e n s i t y of t h e group o f atomic l i n e s , which suggests t h a t t h e i r o r i g i n i s n o t due t o molecular p h o t o i o n i z a t i o n

.

I V

.

INTERPRETATION

The new sets o f r e s u l t s presented above l e a d t o a r e i n t e r p r e t a t i o n o f t h e L i data. A sumnary o f the. experimental observations can be made as follows. The observed l i n e s a r e n o t Auger l i n e s , because t h e i r a c t u a l k i n e t i c energy i s d e f i n i t e l y t o o high, and because no enhancement was found i n t h e molecular p h o t o i o n i z a t i o n cross s e c t i o n r e c e n t l y c a l ~ u l a t e d ~ ' ~ which would have helped t o e x p l a i n t h e i n t e n s i t i e s measured w i t h such a low r e l a t i v e abundance of L i p molecule ( 1 t o 2%). The p o s i t i o n o f t h e l i n e s i s now c o n s i s t e n t w i t h them being assigned t o molecular a u t o i o n i z a t i o n

3 2

l i n e s associated w i t h t h e L i z ( 1 r IT n + ) n e u t r a l e x c i t e d species

.'"

Autoioniza- t i o n l i n e s corresponding t o n @ = ll~,, 9

lo;,

^3uand l-rr have been observed a t t h e

9 9

same k i n e t i c energies under e l e c t r o n impact. (Ref. 2)

F o l l o w i n g t h e a n a l y s i s o f L a r k i n s and ~ i c h a r d s , ~ we come t o t h e conclusion t h a t t h e l i n e s A, observed i n our experiment between 50 eV and 53 eV k i n e t i c energy, a r e the consequence o f ion-molecule c o l l is i o n processes. Among. several p o s s i b l e mecha-

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nisms suggested t o account f o r t h e

observation^,^

we t h i n k t h a t t h e f o l l o w i n g se- quence o f processes e x p l a i n s reasonably we1 1 t h e experimental r e s u l t s , i n p a r t i c u l a r t h e f a c t t h a t t h e t h r e s h o l d f o r t h e A - l i n e s i s equal t o the atomic i n n e r - s h e l l t h r e s - hold.

1. Atomic p h o t o i o n i z a t i o n : ~i ( l s 2 2 s )

+

h J

-+

^Li'(lsn1)

+

e-

*

2. on-mlecule c o l l i s i o n : ~ i + ( l m l ) + ~ i ~ ( 1 r ~ 2 ~ ~ ) + ~ i + ( l s ~ ) + ~ i ~ ( l 2 2 % ~ n +

*

³ ² ⁴

3. Molecular a u t o i o n i z a t i o n : L i z ( l o - 2 v nf ) -4 Li; ( l o - 20- ) + e-

9 9

Since t h e molecular s t r u c t u r e has been found t o resonate a t some photon ener- g i e s corresponding t o t h e e x c i t a t i o n energies o f two-electron atomic t r a n s i t i o n s t o l s n l n ' l

'

e x c i t e d s t a t e s o f atomic l i t h i u m , c o l l i s i o n process 2 c o u l d a l s o i n v o l v e core-exci t e d atomic 1 i t h i um. 9

A q u a l i t a t i v e a n a l y s i s o f t h i s phenomenon would r e q u i r e accurate measurements of t h e t e a E r a t u r e i n t h e i n t e r a c t i o n volume. However, when one takes i n t o account t h e r e l a t i v s abundance o f L i + i o n s produced by p h o t o i o n i z a t i o n t o t h e r e l a t i v e abundance o f Li, molecule ( I t o 2%) and t h e d e n s i t y o f atomic L i i n t h e

KINETIC E N ~ R G Y ( e ~ ) 3

beam (1312 t o 1013 atomslcm ) , one

;en ends up w i t h c r o s s s e c t i o n f o r p r o -

cess 2 t h a t should have values i n t h e

boa 10-15to cm2 range t o e x p l a i n

t h e data. This does n o t seem t o be un-

zrr reasonable, s i n c e i t i s e s t a b l i s h e d

t h a t ion-atom cross s e c t i o n s f o r i n - n e r - s h e l l e x c i t a t i o n and i o n i z a t i o n can be g r e a t e r than photodonization ,, cross s e c t i o n s by several orders o f

magnitude. 1 0 , l l

W 2w

158

a ^2w 1-

Fig.4- Electron spectra following

sr ionization of a Li vapor at 81 eV photon energy, for three different temperatures in the oven. The left colunm shows the intensity of the A and a electron lines. The right

2% column presents the intensity of the atomic photoionization lines to which all spectra have been normali-

I zed. The spectra are not corrected for the energy transmission of the

I I ' tw CMA.

I : I I , ' , I I so

I I C%.&,c:'i" 't' '111t%,,,

BINDING ENERGY (eV)

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(39-724 JOURNAL DE PHYSIQUE

One should note t h a t tl.lis i n t e r p r e t a t i o n of the spectrun~ does not change much the values of t h e p a r t i a l photoionization cross sections t h a t have been measured f o r atomic l i t h i ~ m . ~ In t h e analysis of the d a t a , only t h e s p e c t r a have been kept t h a t shows a r e l a t i v e i n t e n s i t y f o r t h e i n t e n s i t y of the A-lines lower than l o % , i n com- parison t o the i n t e n s i t y of the atomic l i n e s . As f o r t h e Auger l i n e s , t h e i r i n t e n s i - ty under our experimental conditions,is probably t o weak t o allow t h e i r detection.

V . ACKOWLEDGEMENTS

The authors a r e extremely grateful t o F. Larkins f o r helpful discussion and suggestions in t h e i n t e r p r e t a t i o n of the data.

1. S . Krummacher, V . Schmidt, J.M. Bizau, D.L. Ederer, P. Dhez and F.J. Wuilleumier, J . Phys. B

-

15,4363(1982).

2. W.H.E. Schwarz, W. Butscher, D.L. Ederer, T.B. Lucatorto, B. Ziegenbein, W. Meh- lhorn and H . Prompeler, J . Phys. B - 11,591(1978;.

3. P. liem, R. Bruch and N . S t o l t e r f o h t , J . Phys. B

8,

L480(1975).

4. W.H.E. Schwarz and T.C. Chang, I n t . J . Quantum Chem. =,91(1976).

5. C. Moore, Table of Energy Levels, National Bureau of Standards, Washington,DC, 1971.

6. A.N. Nesmeyanov, in:"Vapor Pressure of t h e Chemical Elements", Elsevier, Amster- dam, 1963, p.122.

7 .P .Gerard,Thesis 3Sme Cycle, University Paris-Sud, Orsay, 1984 (unpublished).

8. G.B. Backsay, G. Bryant and N.S. Hush, 1 n t . J . Quant. Chem.,1987.

9. F.P. Larkins and J.A. Richards, Aust. J . Phys. 39,1(1987).

-

10. U. Fano and W . Lichten, Phys. Rev. L e t t . %,627(1965).

11. F.P. Larkins, J . Phys. B - 5,571(1972).