OPTOGALVANIC PHOTODETACHMENT SPECTROSCOPY

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Submitted on 1 Jan 1983

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OPTOGALVANIC PHOTODETACHMENT SPECTROSCOPY

I. Mcdermid, C. Webster

To cite this version:

I. Mcdermid, C. Webster. OPTOGALVANIC PHOTODETACHMENT SPECTROSCOPY. Journal

de Physique Colloques, 1983, 44 (C7), pp.C7-461-C7-466. �10.1051/jphyscol:1983745�. �jpa-00223302�

JOURNAL DE PHYSIQUE

Colloque C7, suppl6ment au n o l l , Tome 44, novembre 1983 page C7-461

O P T O G A L V A N I C PHOTODETACHMENT SPECTROSCOPY

I . S . McDermid and C.R. Webster*

J e t PropuZsion Laboratory, CaZifornia I n s t i t u t e o f TechnoZogy, 4800 Oak Grove Drive, Pasadena, Cazifornia 91109, U.S.A.

Resum6 - La s p e c t r o s c o p i e o p t o g a l v a n i q u e a S t S Stendue 2 une t e c h n i q u e nouvel- l e dans l a q u e l l e d e s e l e c t r o n s s o n t dEtachEs des i o n s n e g a t i f s formSs dans l a dScharge e t o b s e r v e s e n f o n c t i o n de l a longueur d'onde du l a s e r . La determina- t i o n des a f f i n i t e s e l e c t r o n i q u e s d e s i o n s atomiques I- e t Cl- e s t d S c r i t e . Les p o s s i b i l i t g s de c e t t e methode pour E t u d i e r l a s p e c t r o s c o p i e d e s i o n s n g g a t i f s m o l E c u l a i r e s s o n t a u s s i p r e s e n t e e s .

A b s t r a c t

-

A new e x t e n s i o n t o o p t o g a l v a n i c s p e c t r o s c o p y , i n which e l e c t r o n s d e t a c h e d from n e g a t i v e i o n s formed i n t h e d i s c h a r g e a r e observed a s a f u n c t i o n o f i n c i d e n t l a s e r wavelength, h a s been developed. The d e t e r m i n a t i o n of t h e e l e c t r o n a f f i n i t i e s of

r

and C l - a t o m i c i o n s i s described. The p o t e n t i a l of t h e t e c h n i q u e f o r s t u d y i n g t h e s p e c t r o s c o p y of molecular n e g a t i v e i o n s i s a l s o d i s c u s s e d .

1. I n t r o d u c t i o n

The o p t o g a l v a n i c e f f e c t h a s p r o v e n p a r t i c u l a r l y s u i t a b l e f o r t h e d e t e c t i o n o f u n s t a b l e , r a d i c a l and i o n i c , a t o m i c and m o l e c u l a r s p e c i e s /l-3/, e s p e c i a l l y s i n c e t h e s e c a n o f t e n be g e n e r a t e d by t h e d i s c h a r g e i t s e l f . I n a new e x t e n s i o n t o t h e o p t o g a l v a n i c t e c h n i q u e /4/ w e have shown t h a t t h e r a d i a t i o n i n d u c e d detachment of e l e c t r o n s from n e g a t i v e i o n s i n t h e d i s c h a r g e c a n be used t o s t u d y t h e s p e c t r o s c o p y of t h e s e n e g a t i v e i o n s , I n t h i s paper w e w i l l review t h e technique and i t s f i r s t a p p l i c a t i o n t o t h e measurement of t h e e l e c t r o n a f f i n i t y of I-. New d a t a c o n c e r n i n g t h e e l e c t r o n a f f i n i t y of Cl' w i l l a l s o be presented a s w i l l a planned a p p l i c a t i o n t o s t u d y t h e s p e c t r a of m o l e c u l a r n e g a t i v e ions.

The dc d i s c h a r g e c e l l and a s s o c i a t e d c i r c u i t r y have been d e s c r i b e d i n a p r e v i o u s paper i n t h i s e d i t i o n (Webster and Menzies). I n t h i s s t u d y t h e c e l l windows were o f q u a r t z a n d a p u l s e d d y e l a s e r pumped by e i t h e r a n i t r o g e n o r a XeCl e x c i m e r l a s e r was u s e d u s e d a s t h e e x c i t a t i o n s o u r c e . F o r t h e s t u d i e s o f 'I a n d C l ' t h e c e l l was o p e r a t e d i n a f l o w i n g mode w i t h t h e c e l l p r e s s u r e and f l o w r a t e c o n t r o l l e d by n e e d l e v a l v e s on b o t h t h e r e a g e n t i n l e t a n d vacuum pump. T y p i c a l o p e r a t i n g p r e s s u r e w a s

-

l 0 0 mTorr w h i c h , w i t h a d c v o l t a g e o f 400

-

700 V a c r o s s t h e c e l l , g a v e d i s c h a r g e c u r r e n t s i n t h e r a n g e 1 0

-

^{40 uA. A} 0.1 U F c o u p l i n g c a p a c i t o r a l l o w e d l a s e r - i n d u c e d a c changes i n t h e d i s c h a r g e impedance t o be monitored w i t h a p r e a m p l i f i e r (PARC 113) connected t o a boxcar i n t e g r a t o r (PARC 162/165).

E l e c t r o n detachment from halogen n e g a t i v e i o n s (X") i s observed, a c c o r d i n g t o

Although t h e c o n c e n t r a t i o n of n e g a t i v e i o n s i n t h e d i s c h a r g e i s g r e a t e r t h a n t h a t o f f r e e e l e c t r o n s / 5 / t h e d i s c h a r g e c u r r e n t i s c a r r i e d a l m o s t e n t i r e l y by t h e s e e l e c t r o n s because of t h e i r much g r e a t e r m o b i l i t y . S t u d i e s of t i m e and s p a t i a l l y r e s o l v e d o p t o g a l v a n i c s i g n a l s show t h a t i t i s p o s s i b l e t o d i s t i n g u i s h very c l e a r l y

* ~ r . Webster was u n f o r t u n a t e l y u n a b l e t o come t o t h e Colloquium b e c a u s e o f a r e - s c h e d u l i n g of a n a i r b a l l o o n experiment b u t h a s s e n t t h e p a p e r h e would have p r e - s e n t e d and which i s p u b l i s h e d i n t h e P r o c e e d i n g s .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983745

JOURNAL DE PHYSIQUE

CATHODE RV (-1

CATHODE DARK SPACE NEGATIVE GLOW FAPADAY DARK SPACE QOSlTlVE COLUMN ANODE GLOH ANODE C l

Figure 1. Typical time-resolved p r o f i l e s f o r L O G s i g n a l s generated a t discharge l o c a t i o n s i d e n t i f i e d t o t h e l e f t of t h e f i g u r e . S i g n a l s h a v e b e e n n o r m a l i z e d t o t h e same peak height.

o ~ r n m a o m r m

TIME AFlER LASER PULSE (CS)

the photodetac;unent s i g n a l from o t h e r optogalvanic s i g n a l s a r i s i n g , f o r example, f r o m a b s o r p t i o n by t h e n e u t r a l d i a t o m i c h a l o g e n m o l e c u l e s . F i g u r e 1 shows t h e time-resolved p r o f i l e s of t h e LOG s i g n a l generated by a b s o r p t i o n i n t h e B-X system of I a t d i f f e r e n t l o c a t i o n s i n t h e d i s c h a r g e /6/. It s h o u l d be n o t e d t h a t e v e n f o r %he f a s t e s t of t h e s e s i g n a l s t h e r i s e t i m e i s o n t h e o r d e r o f 50 VS. By c o n t r a s t , f i e r e 2 shows t h e s i g n a l caused by photodetachment of e l e c t r o n s from I-.

The pulse corresponds t o a n i n c r e a s e i n discharge c u r r e n t and r i s e s w i t h t h e l a s e r p u l s e ( d u r a t i o n

-

^{1 0} n s ) , t h e o b s e r v e d r i s e and f a l l t i m e of t h e s i g n a l b e i n g l i m i t e d by c i r c u i t response time constants. Therefore, by c a r e f u l adjustment of t h e a p e r t u r e w i d t h and d e l a y a f t e r t h e l a s e r p u l s e i t i s p o s s i b l e t o m o n i t o r uniquely t h e s i g n a l s due t o t h e photodetachment.

PHOTODETACHMENT SIGNAL AT 402 nrn

F i g u r e 2. The t i m e - r e s o l v e d o p t o g a l v a n i c s i g n a l f o r p h o t o d e t a c h m e n t f r o m 1- a t 4 0 2 nm. The photodetachment s i g n a l showed no s i g n i f i c a n t s p a t i a l v a r i a t i o n , except a s m a l l delay a s s o c i a t e d w i t h e l e c t r o n mobility.

The optogalvanic-electron s i g n a l observed a s t h e l a s e r wavelength was scanned i n t h e r e g i o n 402.50 t o 406.00 nm i s shown i n f i g u r e 3. The p o t a s s i u m L I F s p e c t r u m i n c l u d e d i n t h i s t r a c e w a s used, t o g e t h e r w i t h e t a l o n f r i n g e s , t o c a l i b r a t e t h e a b s o l u t e wavelength scale.

LASER WAVELENGTH (nm:Air)

Figure 3. The i o d i n e photodetachment optogalvanic spectrum near t h e 405 nm threshold. The potassium L I F spectrum used f o r wavelength c a l i b r a t i o n i s shown i n t h e l o w e r t r a c e , which i s o f f s e t f r o m z e r o t o s e p a r a t e t h e two s p e c t r a The e t a l o n f r i n g e s have been o m i t t e d from t h i s f i g u r e s i n c e they

W e r e t o o c l o s e l y spaced f o r c l e a r r e p r e s e n t a t i o n

Near t h e photodetachment t h r e s h o l d t h e optogalvanic s i g n a l was observed t o r i s e i n accordance w i t h t h e Wigner law /7/. However, a t s h o r t e r wavelengths t h e i n c r e a s e i n t h e optogalvanic s i g n a l was s m a l l e r than expected from t h e p r e d i c t e d i n c r e a s e i n t h e detachment c r o s s s e c t i o n I n o r d e r t o i d e n t i f y t h e threshold we used t h e semi- e m p i r i c a l method of Berry e t a 1

/a/,

i.e., the point of i n f l e c t i o n of t h e observed

Figure 4. Schematic p l o t of t h e photode tachment s i g n a l near t h e t h r e s h o l d . T h e t h r e s h o l d behavior r e d i c t e d by t h e Wigner (E-Bthr)f/2 dependence i s shown by t h e c i r c l e s . The b r o k e n l i n e s i l l u s t r a t e t h e method of Berry and co-workers /8/ used t o d e t e r m i n e t h e t h r e s h o l d e n e r g y Ethr shown by t h e arrow.

404.8 405.0 405.2 405.4 LASER WAVELENGTH (nm:Air)

C7-464 JOURNAL DE PHYSIQUE

curve was taken a s t h e upper l i m i t (E, and t h e tangent t o t h e curve a t t h e point of i n f l e c t i o n defined t h e lower l i m i t The threshold energy was then taken a s t h e mean of these two l i m i t s a s shown s c h e m a t i c a l l y i n f i g u r e 4. The threshold wavelength was thus determined t o be 405.18 2 0.02 nm corresponding t o a threshold energy f o r photodetachment of 3.0591 2 0.0001 eV.

4. m e C l - Photodetachment Soectrum

Preliminary s t u d i e s of t h e photodetachment from Cl' and Br' atoms have been made.

The primary mechanism f o r t h e production of t h e atomic halogen negative i o n s i n t h e low p r e s s u r e discharge i s through d i s s o c i a t i v e attachment of t h e diatomic n e u t r a l ,

The c r o s s s e c t i o n s f o r t h i s process a r e much lower f o r C 1 and Br2 than f o r I /5/

and t h u s t h e X- c o n c e n t r a t i o n s a r e much lower. Figure 5 &ows t h e photodetaciment s p e c t r u m of C l ' r e c o r d e d a s t h e l a s e r w a v e l e n g t h w a s s c a n n e d f r o m 344.0 t o 341.5 nm. Although t h i s s p e c t r u m r e p r e s e n t s p r e l i m i n a r y d a t a i t i s c l e a r t h a t t h e signal-to-noise r a t i o i s much lower than i n t h e i o d i n e spectrum. Nonetheless, t h e s p e c t r u m i n d i c a t e s t h e o n s e t of d e t a c h m e n t a t 342.55 f 0.25 nm. These e r r o r l i m i t s a l s o r e f l e c t some u n c e r t a i n t y i n t h e a b s o l u t e w a v e l e n g t h s i n c e a f u l l c a l i b r a t i o n h a s n o t y e t been made. T h i s c o r r e s p o n d s t o a n e n e r g y t h r e s h o l d o f 3.6184 2 0.0025 eV.

344 0 343.5 343.0 342.5 342.0 341.5 LASER WAVELENGTH (nm)

Figure 5. The c h l o r i n e photodetachment optogalvanic spectrum.

A t f i r s t i t would seem t h a t o t h e r compounds w i t h l a r g e r e l e c t r o n attachment c r o s s s e c t i o n s , such a s halogenated polyatomic molecules, might provide b e t t e r sources of C l ' i o n s . I t h a s been n o t e d , f o r example, t h a t m u l t i p l e h a l o g e n s u b s t i t u t i o n g r e a t l y i n c r e a s e s t h e d i s s o c i a t i v e attachment c r o s s s e c t i o n and t h u s t h e l a r g e s t c r o s s s e c t i o n s a r e f o u n d f o r m o l e c u l e s s u c h a s C C 1 4 /g/. However, n e a r t h e t h r e s h o l d p h o t o d e t a c h e d e l e c t r o n s h a v e no e x c e s s k i n e t i c e n e r g y and s i n c e t h e m a g n i t u d e of t h e d i s s o c i a t i v e a t t a c h m e n t c r o s s s e c t i o n f o r C C 1 4 f o r e l e c t r o n e n e r g i e s

<

1 eV i s very l a r g e , these e l e c t r o n s a r e e f f i c i e n t l y scavenged and t h u s cannot c o n t r i b u t e t o t h e optogalvanic signal.

.

.

5. P o s s i b l e F i e l d a n d E n v i r o n m e n t a l E f f e c t s o n t h e E l e c t r o n A f f i n l k y De t e r m i n a t i o m

I n t h e l a s e r o p t o g a l v a n i c t e c h n i q u e r e p o r t e d h e r e t h e s i m p l e , compact, and i n e x p e n s i v e d i s c h a r g e t u b e r e p l a c e s t h e complex i o n g e n e r a t i o n , h a n d l i n g and d e t e c t i o n apparatus of t h e crossed-beam method. However, t h e energy r e s o l u t i o n i n

t h e optogalvanic measurements could be blurred due t o t h e i n f l u e n c e s of t h e complex d i s c h a r g e e n v i r o n m e n t . C o n t r i b u t i o n s t o t h e inhomogeneous l i n e w i d t h i n a d c d i s c h a r g e i n c l u d e b o t h t h e Doppler e f f e c t , due t o t h e m o t i o n o f t h e i o n s i n t h e e l e c t r i c f i e l d , and S t a r k b r o a d e n i n g e f f e c t s due t o t h e p r e s e n c e o f f l u c t u a t i n g microfields. I n a d d i t i o n t o broadening, t h e photodetachment t h r e s h o l d energy may be s h i f t e d by b o t h Doppler a n d f i e l d e f f e c t s . The i n f l u e n c e of t h e s e f a c t o r s i l l

determining t h e e l e c t r o n a f f i n i t y of I- i n t h i s work i s now considers,.

The e s t i m a t e d p o t e n t i a l g r a d i e n t i n t h e d i s c h a r g e , away f r o m t h e c a t n o d e f a l l r e g i o n , i s

..

^{10 V cm'l} and t h u s t h e m o b i l i t y o f I' i o n s i n a n e n v i r o n m e n t of 0.1 Torr I2 i s c a l c u l a t e d t o be o n the o r d e r o f 105 cm S-'. While t h i s d r i f t v e l o c i t y i s almost a n o r d e r of magnitude l a r g e r than t h e average molecular v e l o c i t i e s i n t h e discharge, t h e t r a n s v e r s e geometry of t h e l a s e r probe e n s u r e s t h a t t h e component of t h e d r i f t v e l o c i t y i n t h e l a s e r beam d i r e c t i o n i s very s m a l l . The Doppler e f f e c t t h e r e f o r e h a s a n e g l i g i b l e e f f e c t o n t h e p o s i t i o n of t h e t h r e s h o l d , and i t s a p p a r e n t w i d t h , which i s c a l c u l a t e d t o be

-

0.02 cm", i s much s m a l l e r t h a n t h e l a s e r l i n e w i d t h of 0.6 cm'l. I t i s p e r t i n e n t t o compare t h e Doppler s h i f t s e x p e c t e d i n t h e o p t o g a l v a n i c e x p e r i m e n t w i t h those observed i n t h e crossed-beam e x p e r i m e n t s i n which t h e i o n s a r e a c c e l e r a t e d t o h i g h k i n e t i c e n e r g i e s . While, l i k e t h e t r a n s v e r s e g e o m e t r y of t h e o p t o g a l v a n i c e x p e r i m e n t , t h e crossed-beam geometry minimizes the s h i f t , t h e angular divergence of t h e beams ( i o n and l a s e r ) i s u s u a l l y s u f f i c i e n t t o i n t r o d u c e s i g n i f i c a n t Doppler s h i f t s t o t h e o b s e r v e d photodetachment threshold.

I n t h e low p r e s s u r e discharge used i n t h e optogalvanic experiment t h e negative i o n s under study a r e not i n a n i d e a l f i e l d - f r e e environment. Rather, i n a d d i t i o n t o t h e discharge f i e l d discussed above, each i o n i s surrounded by an atmosphere of charged s p e c i e s described by t h e Debye-Hiickel p o t e n t i a l /TO/. The motions of t h e s e s p e c i e s c a u s e t h i s p o t e n t i a l t o f l u c t u a t e t h e r e b y c a u s i n g a b r o a d e n i n g o f t h e p h o t o d e t a c h m e n t t h r e s h o l d due t o t h e S t a r k e f f e c t . The c o n t i n u u m l i m i t i s more s e n s i t i v e t o S t a r k e f f e c t s t h a n t h e ground s t a t e . Such b r o a d e n i n g h a s been o b s e r v e d i n a b s o r p t i o n s t u d i e s of shock-produced p l a s m a s / l l / , e v i d e n c e d by t h e o b s e r v a t i o n o f a 1 5 cm" + e d t a i l n t o t h e t h r e s h o l d . The m a g n i t u d e of t h e S t a r k b r o a d e n i n g i n t h e dc d i s c h a r g e employed h e r e c a n be e s t i m a t e d f r o m a c o m p a r i s o n w i t h t h e shook-plasma experiments. The c r i t i c a l f a c t o r governing t h e i n t e n s i t y of t h e m i c r o f i e l d s i s t h e i o n number d e n s i t y n. Electron d e n s i t i e s of 1

o7

and a negative i o n t o e l e c t r o n r a t i o of 102 have been measured /12/ i n t h e Faraday dark s p a c e and p o s i t i v e column o f a n i o d i n e glow d i s c h a r g e under c o n d i t i o n s c l o s e 1 s i m i l a r t o those used i n t h i s sork. The negative i o n c o n c e n t r a t i o n of n

=

l o g cm'

3

i s s i x o r d e r s of magnitude l e s s than t h a t e s t i m a t e d f o r t h e shock-produced plasma.

The e f f e c t o f S t a r k b r o a d e n i n g i n t h e o p t o g a l v a n i c d i s c h a r g e i s t h e r e f o r e s m a l l compared t o o t h e r broadening mechanisms.

For photodetachment (photoionization) of a n e u t r a l species, f i e l d enhancement can cause l a r g e s h i f t s i n t h e threshold energy. This is because t h e photodetachment continuum i s bounded by Rydberg l e v e l s which c a n be e f f e c t i v e l y f i e l d i o n i z e d . T h i s i s n o t t h e c a s e f o r a n e g a t i v e i o n s i n c e t h e Rydberg s t a t e s a r e a b s e n t . Furthermore, t h e Debye-Hiickel p o t e n t i a l a f f e c t s a l l s t a t e s by t h e same, constant amount and t h e r e f o r e w i l l not change t h e threshold energy.

The method of a n a l y s i s employed i n c l u d e s t h e wavelength c a l i b r a t i o n uncertainty, and t h e (semiempirical) procedure used f o r t h e t h r e s h o l d d e t e r m i n a t i o n i n c o r p o r a t e s a l l b r o a d e n i n g f a c t o r s . The s h a r p r i s e o b s e r v e d i n t h e p h o t o d e t a c h m e n t c r o s s s e c t i o n (see Figs. 3 & 5) a t t e s t s t o t h e absence of s i g n i f i c a n t broadening effects.

The e l e c t r o n a f f i n i t y of i o d i n e was t h e r e f o r e determined t o be,

and f o r c h l o r i n e we f i n d

C7-466 JOURNAL D€ PHYSIQUE

The agreement between t h e s e v a l u e s and those from previous s t u d i e s /9,13/ s u g g e s t s t h a t t h e m a g n i t u d e o f t h e e r r o r s o u r c e s i n t h e o p t o g a l v a n i c method have been a s s e s s e d correctly.

6. Photodetac&ncnt SDectroscoDv f o r t h e Studv of Molecular Negative Ion9

I n a r e c e n t p a p e r /14/ S c h u l z e t a 1 have d e s c r i b e d t h e a p p l i c a t i o n o f t h r e s h o l d photodetachment spectroscopy t o t h e study of ON- and OD-. We suggest t h a t t h e i o n beam s o u r c e i n t h i s t y p e o f e x p e r i m e n t c o u l d be r e p l a c e d by a n o p t o g a l v a n i c d i s c h a r g e c e l l . I n t h e s t u d i e s of OH' t h e p h o t o d e t a c h m e n t w a s a s i n g l e p h o t o n p r o c e s s . The s p e c t r u m a p p e a r s a s a s e r i e s o f a b r u p t r i s e s o n t o p o f a g e n e r a l l y r i s i n g s i g n a l and r e f l e c t s t h e i n c r e a s e s i n t h e photodetachment c r o s s s e c t i o n w i t h both i n c r e a s i n g photon energy and t h e opening of new detachment channels. Apart from the c o n s t a n t l y r i s i n g background, t h e d i f f e r e n t i a l of t h e observed spectrum c o r r e s p o n d s t o t h e a b s o r p t i o n s p e c t r u m o f t h e n e g a t i v e i o n and i n f o r m a t i o n r e g a r d i n g t h e r o t a t i o n a l and v i b r a t i o n a l s t r u c t u r e can be e x t r a c t e d i n t h e normal way.

Many molecular negative i o n s have s t a b l e and a c c e s s i b l e e l e c t r o n i c s t a t e s below t h e photodetachment threshold. For example t h e 0-0 band of t h e

~~c ^{- x2z+}

t r a n s i t i o n of C: l i e s n e a r 18450 cm". A s e c o n d p h o t o n w i t h t h i s e n e r g y i s s u $ f i c i e n t t o p h o t o d e t a c h t h e e l e c t r o n f r o m t h e B s t a t e a n d t h u s t h e B-X t r a n s i t i o n c a n be s t u d i e d by means of two-photon resonant detachment /15/. I n t h i s case t h e spectrum w i l l appear a s a normal s e r i e s of peaks r a t h e r than s t e p s a s i n t h e s i n g l e photon p r o c e s s . Also, i n a n a n a l o g o u s t e c h n i q u e t o m u l t i - c o l o u r r e s o n a n t i o n i z a t i o n spectroscopy two o r more photons of d i f f e r e n t e n e r g i e s can be used.

S i n c e p h o t o d e t a c h e d e l e c t r o n s c a n be m o n i t o r e d unambiguously i n a n optogalvanic e x p e r i m e n t , a n d s i n c e t h e l o w p r e s s u r e d c d i s c h a r g e can produce s i g n i f i c a n t c o n c e n t r a t i o n s of n e g a t i v e i o n s , o p t o g a l v a n i c photodetachment spectroscopy holds g r e a t p r o m i s e f o r t h e s p e c t r o s c o p i c s t u d y o f both a t o m i c and m o l e c u l a r n e g a t i v e ions.

The r e s e a r c h d e s c r i b e d i n t h i s p a p e r w a s p e r f o r m e d a t t h e J e t P r o p u l s i o n Laboratory, C a l i f o r n i a I n s t i t u t e of Technology, under c o n t r a c t w i t h t h e National Aeronautics and Space Administration.

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