Negative Ion States of Polyatomic Molecules - ACS Symposium Series

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Negative Ion States of Polyatomic Molecules L. G. CHRISTOPHOROU

Downloaded by RUTGERS UNIV on May 30, 2018 | https://pubs.acs.org Publication Date: August 13, 1981 | doi: 10.1021/bk-1981-0162.ch002

Atomic, Molecular and High Voltage Physics Group, Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, T N 37830 and Department of Physics, University of Tennessee, Knoxville, T N 37916

I n t h i s p a p e r I s h a l l e l a b o r a t e on r e c e n t k n o w l e d g e c o n c e r n i n g ( i ) t h e number and e n e r g i e s , ( i i ) t h e c r o s s s e c t i o n s and d e c o m p o s i t i o n s , and ( i i i ) t h e l i f e t i m e s o f n e g a t i v e i o n s t a t e s (NISs) o f polyatomic organic molecules. These a r e formed by c a p t u r e o f an e l e c t r o n i n e i t h e r t h e f i e l d o f t h e g r o u n d o r t h e f i e l d o f an e x c i t e d e l e c t r o n i c s t a t e t h r o u g h a number o f m e c h a nisms, ( ! ) and a r e m e t a s t a b l e w i t h l i f e t i m e s r a n g i n g from ^ 1 0 to >10 sec. I s h a l l r e s t r i c t m y s e l f o n l y t o two c a s e s , n a m e l y , t h o s e N I S s w h i c h a r e formed i n t h e f i e l d o f t h e g r o u n d e l e c t r o n i c s t a t e v i a a shape r e s o n a n c e and v i a a n u c l e a r - e x c i t e d F e s h b a c h r e s o n a n c e m e c h a n i s m . These two mechanisms a r e i l l u s t r a t e d i n F i g . 1. I n the former type the e l e c t r o n i s trapped i n the a t t r a c t i v e r e g i o n o f t h e p o t e n t i a l , w h i c h r e s u l t s from t h e c o m b i n e d e f f e c t o f t h e l o n g - r a n g e p o l a r i z a t i o n p o t e n t i a l and t h e s h o r t - r a n g e r e p u l s i o n due t o t h e c e n t r i f u g a l m o t i o n o f t h e two p a r t i c l e s , and t h e NIS l i e s above t h e c o r r e s p o n d i n g p a r e n t n e u t r a l s t a t e [ i . e . , i t s e l e c t r o n a f f i n i t y , E A , i s n e g a t i v e (10 sec. Their cross

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Research sponsored by the Office of Health and Environmental Research of the US Department of Energy under contract W-7405eng-26 with Union Carbide Corporation. 0097-6156/81 /0162-0011 $06.00/0 © 1981 American Chemical Society Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTON, ELECTRON, AND ION PROBES


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s e c t i o n s a r e l a r g e ( o f t e n >10 cm ) and as a r u l e r e a c h t h e i r maximum v a l u e a t t h e r m a l e n e r g i e s (2^). We u s u a l l y l e a r n o f t h e p r o p e r t i e s o f N I S s t h r o u g h t h e i r decay c h a n n e l s . Thus k n o w l e d g e on ( i ) i s n o r m a l l y o b t a i n e d e i t h e r from t h e e n e r g y dependence o f t h e t o t a l e l e c t r o n s c a t t e r i n g c r o s s s e c t i o n , or the c r o s s s e c t i o n f o r e l a s t i c s c a t t e r i n g , or the c r o s s section for i n e l a s t i c s c a t t e r i n g i n v o l v i n g v i b r a t i o n a l and/or e l e c t r o n i c e x c i t a t i o n — a t an a n g l e o r i n t h e f o r w a r d d i r e c t i o n — v i a the decay o f the NIS by e l e c t r o n e m i s s i o n ; t h e s e cross s e c t i o n s a r e m e a s u r e d c o n v e n i e n t l y u s i n g e l e c t r o n s c a t t e r i n g and electron transmission techniques. K n o w l e d g e on ( i ) c a n be a c q u i r e d a l s o from s t u d i e s o f t h e e n e r g y dependence o f t h e c r o s s sections f o r s p e c i f i c d i s s o c i a t i v e a t t a c h m e n t fragment ions, t o t a l d i s s o c i a t i v e attachment cross s e c t i o n s or cross s e c t i o n s f o r p a r e n t n e g a t i v e i o n f o r m a t i o n ; the l a t t e r c r o s s s e c t i o n s are u s u a l l y d e t e r m i n e d ( e s p e c i a l l y a t l o w e n e r g i e s ) by t h e swarm and t h e swarm-beam t e c h n i q u e s . K n o w l e d g e on ( i i ) i s o b t a i n e d from e l e c t r o n beam, mass s p e c t r o m e t r i c and swarm t e c h n i q u e s , t h e l a s t method b e i n g most a p p r o p r i a t e f o r low e n e r g i e s and f o r n u c l e a r e x c i t e d Feshbach resonances. Our k n o w l e d g e on ( i i i ) come^ from e l e c t r o n s c a t t e r i n g methods when EA < 0 eV and T ^10 sec, from h i g h p r e s s u r e ^ l e c t r o n swarm s t u d i e s when EA > 0 eV and T i n t h e r a n g e 10 - 1 0 s e c , from t i m e - o f - f l i g h t mass s p e c t r o m e f r i c s t u d i e s when EA > 0 eV and ^ 10 s e c , and from i o n - c y c l o t r o n r e s o n a n c e t e c h n i q u e s when EA > 0 eV and ^10 sec ( s e e R e f . 2). T

a

M o l e c u l a r n e g a t i v e i o n s t a t e s are abundant; t h e i r e n e r g i e s , c r o s s s e c t i o n s , and l i f e t i m e s a r e s t r o n g l y a f f e c t e d by t h e d e t a i l s o f the m o l e c u l a r s t r u c t u r e as w e l l as t h e medium w h i c h s u r r o u n d s the m e t a s t a b l e a n i o n s . The c r o s s s e c t i o n s and l i f e t i m e s o f the N I S s a r e f u n c t i o n s o f t h e e l e c t r o n e n e r g y e. O f t e n t h e NISs a r e d e s c r i b e d — t h e i r e n e r g i e s a p p r o x i m a t e d and t h e i r numbers r a t i o n a l i z e d — i n terms o f the u n o c c u p i e d m o l e c u l a r o r b i t a l s o f t h e n e u t r a l m o l e c u l e . These a s p e c t s o f NISs w i l l be a p p a r e n t from t h e s e l e c t e d d a t a p r e s e n t e d i n t h i s p a p e r on t h e n e g a t i v e i o n shape and n u c l e a r - e x c i t e d F e s h b a c h r e s o n a n c e s o f t h r e e b a s i c groups o f p o l y a t o m i c o r g a n i c m o l e c u l e s , namely, n o n a r o m a t i c d o u b l e - b o n d e d s y s t e m s , b e n z e n e and b e n z e n e d e r i v a t i v e s , and s e l e c t e d h a l o c a r b o n s . The new k n o w l e d g e on N I S s p r o v i d e s f o r e f f e c t i v e p r o b i n g o f m o l e c u l a r s t r u c t u r e and r e a c t i o n s , and f o r the t e s t i n g o f t h e o r e t i c a l t r e a t m e n t s o f m o l e c u l a r e l e c t r o n i c structure; i t i s a l s o d i r e c t l y r e l e v a n t to a m u l t i p l i c i t y o f a p p l i e d f i e l d s and t e c h n o l o g i e s (_3, 4 ) . Nonaromatic Double-Bonded S t r u c t u r e s E f f e c t o f D o u b l e Bonds and T h e i r M u t u a l I n t e r a c t i o n on t h e E n e r g y o f N I S s Formed v i a a Shape R e s o n a n c e M e c h a n i s m . The l o w e s t NIS o f C H ^ i s l o c a t e d ( a d i a b a t i c ; 0 + 0 t r a n s i t i o n ) a t 1.55 eV above t h e g r o u n d s t a t e and i s due to e l e c t r o n c a p t u r e i n t o t h e 2

Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.

CHRISTOPHOROU

13

Negative Ion States

(b ) orbital. S i n c e to our knowledge t h i s i s the lowest NIS o f t h e i s o l a t e d m o l e c u l e , t h e EA o f C ^ H ^ i s - 1 . 5 5 e V . C o m p a r i n g t h e p o s i t i o n s o f t h e r e p o r t e d l o w e s t RLSs OF C H ^ and C H ( T a b l e 1) i t i s seen t h a t the e t h y l e n i c d o u b l e bond s t a b i l i z e s t h e NIS (makes EA l e s s n e g a t i v e ) . The p r e s e n c e o f an a d d i t i o n a l d o u b l e bond i n t h e m o l e c u l e ( T a b l e 1) l o w e r s t h e NIS e n e r g y even m o r e . T h i s o b s e r v a t i o n a p p e a r s t o be v a l i d , a l s o , f o r c y c l i c compounds where a g a i n two d o u b l e bonds l o w e r t h e n e g a t i v e i o n r e s o n a n c e e n e r g y much more t h a n does a s i n g l e d o u b l e b o n d . Furthermore, t h i s l o w e r i n g i s a f u n c t i o n o f the m u t u a l i n t e r a c t i o n o f t h e two d o u b l e b o n d s , w h i c h , i n t u r n , depends on t h e i r r e l a t i v e p o s i tions. L o c a l i z e d d o u b l e bonds c a n i n t e r a c t d i r e c t l y t h r o u g h s p a c e o r i n d i r e c t l y t h r o u g h o t h e r bonds (2)The changes i n t h e e n e r g i e s o f the NISs a p p a r e n t l y are s e n s i t i v e probes o f such interactions. The p o s i t i o n o f the N I S s i n T a b l e 1 a r e from e l e c t r o n scattering studies. On t h e b a s i s o f t h e s e d a t a , t h e EA o f t h e l i s t e d m o l e c u l e s i s n e g a t i v e and the N I S s s h o r t lived. No knowledge o f the c r o s s s e c t i o n f o r f o r m a t i o n o f these NISs i s available. g


2

2

6

E f f e c t o f CH^ G r o u p . S u b s t i t u t i o n o f CH~ f o r H l o w e r s t h e m o l e c u l a r s y m m e t r y , and t h e d a t a i n T a b l e 2 snow t h a t i t r a i s e s t h e p o s i t i o n o f t h e NIS (makes the EA s m a l l e r ) . The same e f f e c t s a r e o b s e r v e d when one o f t h e C H ^ g r o u p s o f t h e e t h y l e n e m o l e c u l e i s r e p l a c e d by a t o m i c o x y g e n , w h i c h i s t h e u n i t e d atom e q u i v a l e n t o f C H . S i m i l a r e f f e c t s are observed for aromatics ( e . g . , the p o s i t i o n o f t h e l o w e s t NIS o f a n i l i n e , m e t h y l a n i l i n e and d i m e t h y l a n i l i n e , were r e p o r t e d t o be r e s p e c t i v e l y , - 1 . 1 3 ( 1 0 ) , - 1 . 1 9 ( 1 1 ) and - 1 . 2 4 ( J j O e V ) . These d a t a were a g a i n from e l e c t r p j s c a t t e r i n g e x p e r i m e n t s ; t h e N I S s a r e v e r y s h o r t - l i v e d (•• b

Acetaldehyde

-1.3 **

Acetone

-1.5 **

b

:;>•• See

f o o t n o t e ** i n T a b l e 1.

**

Maximum o f shape resonance,

a

Ref. 6.

b

Ref. 8.

c

Ref. 9.

T a b l e 3.

E f f e c t s o f e l e c t r o n w i t h d r a w i n g groups o r atoms on the p o s i t i o n o f the f i r s t NIS o f some e t h y l e n i c - t y p e s t r u c t u r e s .

Molecule

Reported p o s i t i o n o f f i r s t NIS (eV)

Formula

Ethylene

H C=CH

(-1.55)

Formaldehyde

H C=0

b (-0.65T

2

2

Oxygen

(+0.44)

C

(+2.88)

c

Comments

Isoelectronic sequence; replace CH by u n i t e d atom e q u i v a lent, 0

a

2

°2 TetracyanoethyIene

(CN) C=C(CN)

TetrachloroethyIene

ci c=cci

2

2

2

2

(+2.12?)

Nuclear-exc i t e d Feshbach C

resonance

a

See T a b l e 1

b

See T a b l e 2

c

Equated to the EA v a l u e r e p o r t e d and thus t o the p o s i t i o n o f the lowest NIS [ 0 ( R e f . 13.), C ( C N ) (Ref. 14), C C 1 (Ref. 15.)] 2

2

4

2

4


16



a

RADIAL

DISTANCE OF INCIDENT ELECTRON, p

INTERNUCLEAR

DISTANCE

Figure 1. Schematic illustration of (a) shape and (b) nuclear-excited Feshbach resonances. The symbols |0> and \R> designate, respectively, the electronic ground state of the neutral molecule and the NIS (1).

ELECTRON ENERGY (eV)

Figure 2. Negative ion autodetachment lifetime (*) and negative ion current (measured without the retarding potential difference method) (O) for C (CN)i~* as a function of electron energy (11) 2


CHRISTOPHOROU

Negative Ion States

°

S F

6

TETRACHLOROETHYLENE CI

\

/

/

c =c \


CI

0

0.5

CI CI

1.0

ELECTRON ENERGY, e (eV)

Journal of Chemical Physics

Figure 3. Comparison of the energy dependence of the C Cl/~* ion with that of SF ~*. The yields of the two ions were normalized at the peak (16). 2

6


18


instrumental. A t ^ 0 . 0 eV t h e l i f e t i m e o f C C 1 , toward a u t o detachment i s (_16) 14+3 u s e c . V e r y i n t e r e s t i n g l y the t r a n s i e n t C^Cl^ i o n c a n m u l t i p l y fragment^ y i e l d i n g l a r g e q u a n t i t i e s o f CI and a l s o s m a l l amounts o f C l ^ . The wqrk o f J o h n s o n e t a l . ( 1 6 ) has shown t h a t t h e t r a n s i e n t a n i o n s , M , o f c h l o r o e t h y I e n e s and c h l o r o e t h a n e s fragment ( e v e n when t h e s e a r e formed by the capture of e l e c t r o n s w i t h v i r t u a l l y zero_energy) through a m u l t i p l i c i t y of channels. D e p e n d i n g on M , a l l o r p a r t o f t h e d e c o m p o s i t i o n c h a n n e l s shown b e l o w were found (JJ>) t o be p o s s i b l e . 2


(*)

C)

(M-C1)+C1_ C1+(M-C1) (M-2C1)+C1 _ M ( l o n g - l i v e d ; >10 2

6

sec)

The c r o s s s e c t i o n f o r C I p r o d u c t i o n ( F i g . 4) i n d i c a t e s the e x i s t e n c e o f f i v e maxima b e l o w ^ 2 . 0 eV f o r the c h l o r o e t h y I e n e s (and a l s o f o r the c h l o r o e t h a n e s ) (16>). I t seems t h a t t h e s e a r e n o t due t o v i b r a t i o n a l e x c i t a t i o n , b u t r a t h e r due t o s e p a r a t e N I S s a s s o c i a t e d (_16) w i t h o r b i t a l s d o m i n a t e d by the p - o r b i t a l s o f the CI a t o m . T h e i r p o s i t i o n s a r e r e l a t i v e l y i n s e n s i t i v e , and t h e i r c r o s s s e c t i o n s v e r y s e n s i t i v e t o the d e t a i l s o f the m o l e c u l a r structure. A s i m i l a r b e h a v i o r i s e x h i b i t e d by a number o f p e r f l u o r o c a r b o n s t r u c t u r e s ( s e e R e f s . 18 and 19 and t h e f o l l o w i n g sections). A l t h o u g h f o r the m o l e c u l e s i n F i g . 4 _ g i s s o c i a t i v e a t t a c h m e n t p r o c e s s e s a r e e x p e c t e d t o be f a s t (

Negative Ion States of Polyatomic Molecules - ACS Symposium Series

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