Mass Spectrometry in Inorganic Chemistry

1 Mass Spectrometric Studies of Chemionization by Reaction of Electronically Excited Species J. L .

FRANKLIN

Downloaded by 80.82.77.83 on May 17, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0072.ch001

Rice University, Houston, Tex.

During

the last few

has been carried formation

years an increasing

number

out on a class of reactions

of ions in gases by reactions

or molecules.

The

t y p i c a l . Recently

reaction

Ar* +

of

of two neutral N

2

-->

ArN + 2

progress has been made toward

ing the rate constants be in the order of 10

-9

studies

involving

the atoms

+

e is

determin-

of these reactions,

which

cc./molecule/sec.

In some

instances

to take part in the

reaction.

several excited states are known

turn out

to

' " p h e o c c u r r e n c e of d i a t o m i c ions of the r a r e gases has b e e n n o t e d b y s e v e r a l experimenters (29, 30,43),

b u t f o r a n u m b e r of years t h e exact

n a t u r e o f the processes l e a d i n g to t h e i r f o r m a t i o n w a s not u n d e r s t o o d . I n 1951, H o r n b e c k a n d M o l n a r (18)

m e a s u r e d the a p p e a r a n c e p o t e n t i a l s

of t h e h o m o n u c l e a r d i a t o m i c ions for a l l of the rare gases.

T h e i r mass

spectrometer w a s not i d e a l l y s u i t e d to studies of mass spectra of gases at e l e v a t e d pressures. T h e i n s t r u m e n t h a d r e l a t i v e l y p o o r r e s o l v i n g p o w e r w i t h the result t h a t t h e i r a p p e a r a n c e p o t e n t i a l s w e r e not precise. N e v e r theless, t h e y d e m o n s t r a t e d t h a t the a p p e a r a n c e p o t e n t i a l s of t h e d i a t o m i c ions w e r e i n a l l cases 0.7 to 1.5 e. v. b e l o w the i o n i z a t i o n potentials of the c o r r e s p o n d i n g r a r e gas atoms.

T h e pressure d e p e n d e n c e

i n e a c h case

w a s f o u n d to b e s e c o n d order. B o t h of these results s h o w t h a t the r e a c t i o n d i d not i n v o l v e a t o m i c ions, a n d H o r n b e c k a n d M o l n a r c o n c l u d e d t h a t t h e r e a c t i o n i n v o l v e d a t t a c k of a n e x c i t e d a t o m u p o n a n a t o m i n t h e g r o u n d state—i.e. X* + X - > X

2

+

+ e

(1)

w h e r e the asterisk signifies electronic e x c i t a t i o n . R e a c t i o n s of t h i s k i n d , i n w h i c h a n i o n is f o r m e d b y the r e a c t i o n of t w o or m o r e n e u t r a l entities, 1 Margrave; Mass Spectrometry in Inorganic Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

2

MASS SPECTROMETRY I N INORGANIC

Table I.


Excited Atom, X*

2

CHEMISTRY

Appearance Potentials of Appearance

Lowest Excitation Energy

Ionization Potential

He

19.8

24.6

Ne

16.8

21.6

Ar

11.5

15.8

Kr

9.9

Xe

8.3

He 23.2 (18) 23.3 (35) 23.4 (22)

14.7 14.0" b

13.4 12.1 h

e

P / 2 state. 3

is c a l l e d c h e m i o n i z a t i o n . F o l l o w i n g the studies of H o r n b e c k a n d M o l n a r (18),

v a r i o u s w o r k e r s h a v e s t u d i e d m i x t u r e s of c e r t a i n of the r a r e gases

i n a n effort to a s c e r t a i n w h e t h e r h e t e r o n u c l e a r d i a t o m i c ions c o u l d formed.

T h u s , P a h l a n d W e i m e r (36,

d i s c h a r g e b u t f a i l e d to find H e A r F u c h s a n d K a u l (13)

or N e A r

+

studied N e A r

observed H e N e

37)

+

+

reported A r X e

+

in a

be

glow

i n m i x t u r e s of those gases.

a n d A r K r , a n d t h e latter w a s e x a m +

i n e d i n greater d e t a i l b y K a u l , L a u t e r b a c h , a n d F u c h s (21). T a u b e r t (22)

+

and KrXe . +

Kaul and

Munson, Franklin, and Field

m a d e a r a t h e r c o m p l e t e s t u d y of t h e d i a t o m i c ions of t h e r a r e gases t a k e n a l o n e a n d i n c o m b i n a t i o n (35).

T h e y f o u n d that a l l of the possible r a r e

gas d i a t o m i c gas ions are f o r m e d , a n d t h e y m e a s u r e d a p p e a r a n c e p o t e n tials of a l l except H e X e , w h i c h w a s f o r m e d i n s u c h s m a l l a m o u n t s t h a t +

a n a p p e a r a n c e p o t e n t i a l c o u l d not b e d e t e r m i n e d satisfactorily. T a b l e I gives a l l of the results of these studies as w e l l as of the a p p e a r a n c e p o t e n tials m e a s u r e d f o r t h e r a r e gas d i a t o m i c ions f r o m t h e l i t e r a t u r e . I t is s u r p r i s i n g that, c o n s i d e r i n g t h e r e l a t i v e l y s m a l l intensities of these ions, t h e r e has b e e n s u c h g o o d agreement a m o n g the m e a s u r e d

appearance

potentials. B e f o r e d i s c u s s i n g these measurements i t w o u l d b e of interest to n o t e t h e shape of a t y p i c a l i o n i z a t i o n efficiency c u r v e for a r a r e gas d i a t o m i c ion (Figure 1).

T h i s c u r v e is r e m i n i s c e n t of the e x c i t a t i o n c u r v e

for

o p t i c a l l y f o r b i d d e n transitions b y e l e c t r o n i m p a c t a n d is q u i t e different

Margrave; Mass Spectrometry in Inorganic Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

1.

FRANKLIN

Studies of

3

Chemionization

the Rare Gas Diatomic Ions in e.v. Potential

where Y is

of XY

+

23.4 (35) 22.6 (22)

17.9 19.9

(35) (35)

19.9 (35)

20.9 (18, 35)

16.8 16.5

(35) (13)

16.6

14.7 (35) 15.1 (18, 20) Downloaded by 80.82.77.83 on May 17, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0072.ch001

Xe

Kr

Ar

Ne

16.0 (35)

(35)

14.0 (35)

13.5 (22,

13.0 (35) 13.2 (18,22)

12.2 (22) 12.3 (35)

35)

11.2 (35) 11.6 (22) » Pi/ 2

2

state.

f r o m the t y p i c a l i o n i z a t i o n efficiency c u r v e for the f o r m a t i o n of m o n a t o m i c ions b y electrons, also s h o w n i n F i g u r e 1 for c o m p a r i s o n . A s w i l l b e d i s c u s s e d i n m o r e d e t a i l later, the d i a t o m i c ions i n a l l cases w e r e s e c o n d o r d e r i n pressure.

F u r t h e r , the m o l e c u l a r - i o n f o r m a t i o n

was

i n d e p e n d e n t of field strength, a n d thus the p o s s i b i l i t y of t h e i r f o r m a t i o n b y i o n - m o l e c u l e reactions is e l i m i n a t e d . T h u s , w e c o n c l u d e that H o r n b e c k a n d M o l n a r ' s (18)

i n t e r p r e t a t i o n of

t h e i r observations

applies

b r o a d l y a n d t h a t the h e t e r o n u c l e a r d i a t o m i c ions are f o r m e d b y t h e f o l lowing generalized reaction: X* + Y - » X Y + e

(2)

+

It w i l l be of interest to note the a p p e a r a n c e potentials of the h e t e r o n u c l e a r ions i n T a b l e I. I n e a c h case the a p p e a r a n c e p o t e n t i a l is greater t h a n t h e i o n i z a t i o n p o t e n t i a l of the constituent a t o m of lowest i o n i z a t i o n p o t e n t i a l so that the r e a c t i o n m u s t a l w a y s i n v o l v e a n e x c i t e d state of the a t o m of highest i o n i z a t i o n p o t e n t i a l . P o s s i b l e exceptions to t h i s w o u l d b e A r K r , A r X e , a n d K r X e . I n the case of K r X e +

+

+

ance p o t e n t i a l is close to the l o w e r ( P^/ ) 2

respectively.

For ArXe

+

2

and A r K r

+

the a p p e a r -

state of x e n o n a n d k r y p t o n ,

the a p p e a r a n c e p o t e n t i a l is q u i t e close to the

h i g h e r ( P i / ) state of xenon. 2

2

+

If i n d e e d , a state of the a t o m of l o w e r

i o n i z a t i o n p o t e n t i a l is i n v o l v e d , it m u s t b e a n i o n i c state if the a p p e a r a n c e potentials are correct.

S i n c e the r e a c t i o n of a n i o n w i t h a n e u t r a l a t o m

w o u l d b e t h i r d o r d e r i n pressure a n d these reactions are s e c o n d order, it



4

MASS SPECTROMETRY

E L E C T R O N

Figure 1.

I N INORGANIC

CHEMISTRY

VOLTAGE

Ionization efficiency curves

seems c e r t a i n that the reactions l e a d i n g to the f o r m a t i o n of A r K r

+

and

ArXe

+

are reactions of the e x c i t e d a r g o n , a n d that for the f o r m a t i o n of

KrXe

+

involves excited krypton.

I n a l l of the h e t e r o n u c l e a r m o l e c u l a r rare gas ions the e n e r g y at w h i c h the i o n appears is greater t h a n one of its possible asymptotes.

decomposition

T h e r e is thus a p r o b l e m of u n d e r s t a n d i n g w h y s u c h a

m o l e c u l a r i o n s h o u l d be stable. T h e r e seems no d o u b t that t h e y are, a n d w e t h i n k t h a t F i g u r e 2 w i l l illustrate the reason. HeNe . +

P r e s u m a b l y , the i n t e r a c t i o n of N e

whereas that of H e

+

+

C o n s i d e r the case of

w i t h h e l i u m is r e p u l s i v e ,

w i t h N e is attractive. H o w e v e r , the n o n c r o s s i n g

r u l e , as i l l u s t r a t e d i n F i g u r e 2, results i n a m i n i m u m i n the p o t e n t i a l energy c u r v e w h i c h enables the i o n to exist i n spite of its energy. I n T a b l e I the H e A r

+

i o n has t w o a p p e a r a n c e potentials. T h e u p p e r

one at 19.9 e. v. corresponds closely to the energy of the h e l i u m metastable a t o m . T h e i o n i z a t i o n efficiency c u r v e shows a s h a r p c h a n g e i n slope at this p o i n t . T h e r e is, h o w e v e r , a r a t h e r l o n g t a i l to the c u r v e w h i c h has a l o w e r a p p e a r a n c e p o t e n t i a l at a r o u n d 17.9 e. v. T h i s occurs i n a n energy r a n g e for w h i c h there are no k n o w n states for either a r g o n or h e l i u m , a n d it is difficult to u n d e r s t a n d h o w a n i o n w i t h this a p p e a r a n c e p o t e n t i a l


1.

FRANKLIN

is f o r m e d .

Studies

of

5

Chemionization

T h i s was, however,

represented a real phenomenon.

a reproducible value and

presumably

T h e r e has b e e n some e v i d e n c e b y mass

spectrometrists e m p l o y i n g e l e c t r o n beams of n a r r o w energy s p r e a d w h i c h suggest that there are i n d e e d states of a r g o n i n the i o n i z a t i o n c o n t i n u u m , a n d one of these has b e e n i d e n t i f i e d i n the n e i g h b o r h o o d of 18 e. v .

(8).

P e r h a p s it is this state that is responsible for the l o w e r a p p e a r a n c e p o t e n t i a l of H e A r . +

A s i m i l a r result is f o u n d w i t h N e X e , w h o s e

appearance

+

p o t e n t i a l at 16.0 volts is w e l l a b o v e the i o n i z a t i o n p o t e n t i a l of x e n o n a n d d e f i n i t e l y b e l o w the lowest state of neon. P r e s u m a b l y , s i m i l a r c o n s i d e r a tions w o u l d a p p l y , b u t there h a v e b e e n n o e x c i t e d states of x e n o n r e p o r t e d to o u r k n o w l e d g e . Downloaded by 80.82.77.83 on May 17, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0072.ch001

T h e f o l l o w i n g reactions thus a p p e a r to a c c o u n t for the v a r i o u s r a r e gas d i a t o m i c i o n s : He* + He - » H e

2

+ e

+

(3)

He* + Ne - » HeNe* + e

(4)

H e (2s»S) + A r - » H e A r + e

(5)

+

A r * + H e -> H e A r * + e

(6)

(?)

He(2s3S) + K r -> H e K r * + e

(7)

Ne* + Ne -> Ne * + e

(8)

2

Ne(3s»P) + A r -> N e A r + e

(9)

Νβ(3ί»Ρ) + K r - » N e K r * + e

(10)

+

X e * + N e - » NeXe* + e

(?)

(H)

A r * + A r -*• A r * + e

(12)

A r * + K r -> A r K r * + e

(13)

A r * + X e ->· A r X e + e

(14)

Kr* + Kr - » Kr * + e

(15)

Kr* + Xe - » KrXe + e +

(16)

Xe* + Xe - » X e

+ β

(17)

2

+

2

T h e strength of the b o n d i n H e

2

+

2

+

has b e e n e s t i m a t e d i n v a r i o u s w a y s

w i t h results v a r y i n g f r o m 0.05 e. v. to 2.2 e. v. R e c e n t l y , R e g a n , B r o w n e , a n d M a t s e n (38)

made a careful a n d apparently definitive calculation

w h i c h shows D ( H e

2

+

)

to b e at least 2.2 e. v.

T h i s b o n d strength, i f

correct, w o u l d suggest that the a p p e a r a n c e p o t e n t i a l of H e

2

+

should be

a b o u t 22.4 e. v. I n fact, as is seen i n T a b l e I , the a p p e a r a n c e p o t e n t i a l is a b o u t 23.2 e. v., a n d the q u e s t i o n arises as to w h y the p o t e n t i a l is not


6


CHEMISTRY

l o w e r a n d nearer to 22.4 e. v. T h e f o l l o w i n g relevant states of h e l i u m a t o m a n d t h e i r energies ( i n e. v . ) are k n o w n

(33):

2*P, 21.22; 3»S, 22.72; 3 ^ , 22.92; 3*P, 23.01; 3*P, 23.09. A t least f o u r of these h a v e , sufficient energy to c o n t r i b u t e to the f o r m a t i o n of the d i a t o m i c i o n . R e c e n t l y , St. J o h n et al. ( 3 9 ) h a v e s t u d i e d the cross-section for e x c i t a t i o n of v a r i o u s electronic states of h e l i u m b y e l e c t r o n i m p a c t . T h e y find t h a t a l l of the relevant states except P states 1

d r o p p e d to v a n i s h i n g l y s m a l l values at a b o u t the i o n i z a t i o n p o t e n t i a l of helium.

The

1

P states s h o w a p p r e c i a b l e cross sections at voltages

con-

s i d e r a b l y b e l o w the i o n i z a t i o n p o t e n t i a l ; hence, one c o n c l u d e s t h a t i n a l l p r o b a b i l i t y it is the large cross section for e x c i t a t i o n of the P states t h a t Downloaded by 80.82.77.83 on May 17, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0072.ch001

X

d e t e r m i n e the a p p e a r a n c e p o t e n t i a l of the d i a t o m i c i o n . T h u s , the 3 P , 3

the 3*S a n d 3 S , w h i l e b e i n g e n e r g e t i c a l l y c a p a b l e of r e a c t i o n , are present 3

at l o w energies i n s u c h s m a l l amounts as to be u n d e t e c t a b l e .

The 2 P 1

state is of too l o w a n e n e r g y to f o r m the d i a t o m i c i o n a n d t h u s , the a p p e a r a n c e p o t e n t i a l is l i m i t e d to the 3*P state w h o s e e n e r g y of 23.09 e. v. a b o v e the g r o u n d state corresponds

closely to the average

appearance

p o t e n t i a l d e t e r m i n e d b y various mass spectrometrists for the d i a t o m i c i o n . States of h i g h e r e n e r g y m a y c o n t r i b u t e b u t w o u l d not b e d e t e c t e d

at

onset. U n t i l n o w the d i s c u s s i o n has b e e n l i m i t e d to rare gases.

Although

t h e y h a v e i n d e e d p l a y e d a l a r g e p a r t i n the d e v e l o p m e n t of this

field,

several other reactions h a v e b e e n r e p o r t e d i n v o l v i n g the r a r e gases w i t h other m a t e r i a l s a n d i n d e e d , i n v o l v i n g atoms a n d m o l e c u l e s , exclusive of the r a r e gases. C e r m a k a n d H e r m a n ( 6 ) a n d C e r m a k (4)

have reported

several reactions of the rare gases w i t h m e r c u r y a n d of e x c i t e d m e r c u r y atoms w i t h a m m o n i a , m e t h a n e , acetylene, a n d m e t h a n o l , as w e l l as a r e a c t i o n w i t h n i t r o g e n i n w h i c h i t c o u l d not b e d e t e r m i n e d w h i c h r e actant was the e x c i t e d entity. T y p i c a l reactions i n v o l v i n g m e r c u r y are listed below. Xe* + H g - » X e H g + e

(18)

Kr* + H g

KrHg + e

(19)

A r * + H g -> A r H g + e

(20)

+

+

+

Hg* + H 0 - » H g H 0

+

+ e

(21)

H g * + N H * -> H g N H

+

+ e

(22)

+ e

(23)

2

2

H g * + C H -> H g C H 4

Hg* + C H 2

2

3

4

+

-> H g C H 2

2

Hg* + C H O H - » H g C H 0 3

(24)

+ e 4

+

+ e


(25)

1.

FRANKLIN

Studies of

7

Chemionization

M o r e r e c e n t l y H e r m a n a n d C e r m a k (17)

h a v e f o u n d that t h e r e a c t i o n o f

a r g o n a n d m e r c u r y ( R e a c t i o n 20) i n v o l v e s the metastable state of a r g o n . M u n s o n , F i e l d , a n d F r a n k l i n (34)

h a v e s t u d i e d reactions of c e r t a i n

r a r e gases w i t h n i t r o g e n a n d c a r b o n m o n o x i d e , a n d F i e l d a n d F r a n k l i n (11)

h a v e s t u d i e d reactions of the rare gases w i t h m e t h a n e , acetylene,

a n d other s i m p l e gases. T h e latter o b s e r v e d the f o r m a t i o n , b y c h e m i o n i z a t i o n reactions, of c o m p o u n d s of x e n o n w i t h m e t h a n e , acetylene, o x y g e n , a n d w a t e r . T h e f o r m e r r e p o r t e d reactions of a r g o n , k r y p t o n , a n d x e n o n w i t h nitrogen and carbon monoxide.

I n e a c h case i t a p p e a r e d to b e the

e x c i t e d r a r e gas a t o m t h a t a t t a c k e d , a l t h o u g h this c o n c l u s i o n is s o m e w h a t


speculative.

EÎ

Figure 2.

Schematic representation for non-crossing for states of R ^ R /

rule

P e r h a p s the earliest studies of c h e m i o n i z a t i o n w e r e those of M o h l e r et al. (31, 32) of C s

2

+

a n d R b . I n these studies m o n o c h r o m a t i c r a d i a t i o n 2

+

f r o m l i n e spectra w e r e passed i n t o the v a p o r of C s or R b , a n d i o n i z a t i o n w a s d e t e c t e d at energies d e f i n i t e l y b e l o w the i o n i z a t i o n p o t e n t i a l of the a l k a l i a t o m i n q u e s t i o n . S i m i l a r studies e m p l o y i n g r a d i a t i o n h a v e r e c e n t l y b e e n m a d e b y L e e a n d M a h a n ( 2 7 ) , w h o c o n f i r m e d the results of M o h l e r et al. a n d also o b s e r v e d the f o r m a t i o n of K

2

+

by chemionization.

I n f u r t h e r studies M u n s o n , F i e l d , a n d F r a n k l i n (34) and C 0 2

2

+

respectively i n nitrogen and carbon monoxide.

observed N

4

+

Both exhibited

i o n i z a t i o n efficiency curves h a v i n g the c h a r a c t e r i s t i c s h a r p m a x i m u m a f e w volts a b o v e the onset for f o r m i n g the p r o d u c t i o n . F r o m the shape of the c u r v e i t w a s suspected that a n e x c i t a t i o n process w a s i n v o l v e d a n d


8


CHEMISTRY

thus that the ions r e s u l t e d f r o m a c h e m i o n i z a t i o n r e a c t i o n . T h e s e ions e x h i b i t e d second-order d e p e n d e n c e o n pressure, a n d t h e i r intensities w e r e independent

of

field

s t r e n g t h ; hence,

chemionization reaction. T h e N

4

t h e y are c l e a r l y f o r m e d

and C 0

+

2

2

by

a

ions w e r e present i n r a t h e r

+

s m a l l intensities e v e n at 300 m i c r o n s pressure. W h e n the pressure was r e d u c e d to 7 0 - 8 0 m i c r o n s , they c o u l d no l o n g e r b e detected. If, at this p r e s sure of n i t r o g e n , a r g o n was t h e n a d d e d i n a p p r o x i m a t e l y e q u a l a m o u n t s , N

+

4

reappeared a n d along w i t h it A r N

s h o w e d that A r N

2

w a s observed.

+

2

A pressure s t u d y

d e p e n d e d o n the first p o w e r of the pressure of e a c h

+

reactant a n d t h a t N

4

+

depended

o n b o t h the first p o w e r of t h e a r g o n

pressure a n d s e c o n d p o w e r of n i t r o g e n pressure. F u r t h e r , N

+

4

formation


d e p e n d e d u p o n field strength, suggesting t h a t a n i o n - m o l e c u l e r e a c t i o n w a s i n v o l v e d . C l e a r l y t h e n , the N the A r N

2

+

4

+

appears to h a v e b e e n f o r m e d f r o m

i o n . T h e a p p e a r a n c e potentials a l l c o r r e s p o n d to that of A r , 2

a n d i n d e e d the A r

2

+

T h u s , i t is e v i d e n t t h a t t h e same states w h i c h l e a d to A r ArN

2

+

.

+

i o n is r e d u c e d as the n i t r o g e n pressure increases. 2

l e a d also to

+

T h e sequence of reactions w o u l d t h e n a p p e a r to b e as f o l l o w s : A r * + A r -> A r

2

+ e

+

A r * + N -> A r N 2

ArN

2

2

(26)

+ e

+

+ N -> A r + N

+

2

(27) 4

(28)

+

S i m i l a r considerations a p p l y to reactions of k r y p t o n a n d C O . A l o n g s i m i l a r lines C e r m a k ( 5 ) has r e p o r t e d the f o r m a t i o n of i o n b y a c h e m i o n i z a t i o n process i n v o l v i n g r e a c t i o n of e x c i t e d N

N 0 3

+

with

2

N O a c c o r d i n g to R e a c t i o n 29. N * + NO -» N 0 2

8

+ e

+

(29)

T h e i o n i z a t i o n efficiency c u r v e s h o w e d the same g e n e r a l shape as t h a t for N

4

+

, r e p o r t e d b y M u n s o n , F i e l d a n d F r a n k l i n (34).

C e r m a k (17)

Herman and

h a v e also r e p o r t e d the r e a c t i o n of e x c i t e d C O w i t h s o d i u m

a n d p o t a s s i u m vapors to give the ions C O N a R e c e n t l y K e e n a n a n d C l a r k e (23) H * + H 2

+

or C O K

+

by chemionization.

h a v e r e p o r t e d R e a c t i o n 30. 2

^ H / + H + e

(30)

T h i s w a s a q u i t e s u r p r i s i n g result since the f o r m a t i o n of H

8

+

had previ-

o u s l y b e e n c o n s i d e r e d to arise o n l y f r o m the r e a c t i o n : H

2

+

+ H -» H 2

3

+

+ H

S i m i l a r l y , K o y a n o et al. ( 2 5 ) , u s i n g p h o t o e x c i t a t i o n h a v e o b s e r v e d f o r m a t i o n i n acetylene of C H ^ 4

+

and C H 4

2

+

w e l l b e l o w the i o n i z a t i o n p o t e n t i a l of acetylene.

T h e s e ions h a d p r e v i -

o u s l y b e e n o b s e r v e d o n l y i n i o n - m o l e c u l e reactions b e t w e e n C H 2

C H . 2

the

b y c h e m i o n i z a t i o n at energies

2


2

+

and

1.

FRANKLIN

Studies of

9

Chemionization

Rates of Chemionization

Reactions

I n studies o f i o n - m o l e c u l e reactions it is r e l a t i v e l y s i m p l e to o b t a i n e x p l i c i t d e t e r m i n a t i o n s of r e a c t i o n rate because the mass

spectrometer

c l e a r l y identifies a n d p r o v i d e s q u a n t i t a t i v e analysis for b o t h reactant a n d p r o d u c t ions. U n f o r t u n a t e l y , i n c h e m i o n i z a t i o n processes the reactant is not c h a r g e d a n d , h e n c e , cannot b e o b s e r v e d d i r e c t l y . T h e reactant m u s t b e i n f e r r e d f r o m other observations, a n d its q u a n t i t a t i v e m e a s u r e m e n t m u s t b e o b t a i n e d i n d i r e c t l y . T h e most o b v i o u s a p p r o a c h is to relate t h e i n t e n s i t y of the d i a t o m i c i o n to that of the m o n a t o m i c i o n w h i c h c a n n o t u n d e r g o r e a c t i o n at m o d e r a t e pressures.

D a h l e r et al. (10)

employed


this d e v i c e i n s t u d y i n g the rates of c h e m i o n i z a t i o n reactions of h e l i u m , n e o n , a n d a r g o n to f o r m the d i a t o m i c ions. T h e y c o n s i d e r e d the r e a c t i o n m e c h a n i s m to b e i s f o l l o w s . κ Ar + e->Ar* + e

(31)

A r + e - » A r + 2e

(32)

+

(33) Ar*

->

U

Ar + h

Κ Ar* + Ar - » A r

2

+

v

+ e

A s s u m i n g the steady state to a p p l y to A r * w e

(34) find

= ί £ τ $ £ )

( 3 5 )

W h e r e L refers to electron c u r r e n t

«£2

_

W

A , ) d(Ar') dt

(36, = *&(Ar)

(37)

D i v i d i n g ( 3 6 ) b y ( 3 5 ) a n d r e c a l l i n g that jr = — w h e r e σ is cross-section, we

find d(Ar

+

d(Av

+

2

S i n c e ( A r ) a n d ( A r ) are i n d e p e n d e n t of e a c h other, w e c a n w r i t e +

2

+

(Ar ) 2

+

σ.|_

WAr)J

'Ar * 2


v

10


CHEMISTRY

w h e r e w e assume the o b s e r v e d i o n intensities are p r o p o r t i o n a l to c o n c e n t r a t i o n a n d w i t h the same constant of p r o p o r t i o n a l i t y . 7

Ar

+

F r o m E q u a t i o n 39 it w i l l b e o b s e r v e d that a p l o t of - r — - against the 7

r e c i p r o c a l of pressure s h o u l d give a straight l i n e w h o s e i n t e r c e p t is — a n d w h o s e slope is — .

P l o t s of e x p e r i m e n t a l d a t a d i d i n d e e d g i v e

straight lines, a n d i n T a b l e I I the results of D a h l e r et al. (10) the r a t i o of cross sections a n d . the p r o d u c t of k r r

n

these results, h o w e v e r , there is no w a y to d e t e r m i n e r

and k

n

R e a s o n a b l e values of t

r

From

separately.

w o u l d b e i n the r a n g e of 10" to 10" seconds so 6

u


for b o t h

are r e p o r t e d .

that k w o u l d necessarily b e i n the r a n g e of 1 0 " r

10

8

to 10" c c . / m o l e c u l e 8

sec.

S u c h rate constants w o u l d b e c o m p a r a b l e to those for i o n - m o l e c u l e r e a c tions or at the u p p e r l i m i t w o u l d exceed b y a factor of 10 the rates of the fastest i o n - m o l e c u l e reactions. Table II.

Steady State Values for Rates of Diatomic Ion Formation

Rare Gas

k „ Χ

^/

σ

He Ne Ar

σι

r T

0.06 0.01 0.055

10

16

0.58 11. 3.6

I n the studies of D a h l e r et al. (10)

there w a s n o w a y to v a r y the

t i m e of r e a c t i o n , a n d this necessarily l i m i t e d the i n f o r m a t i o n that c o u l d b e o b t a i n e d c o n c e r n i n g rates. a n d L a m p e ( J , 2)

L a m p e a n d H e s s (26)

a n d later Becker

e x t e n d e d the s t u d y of a r g o n b y u s i n g a time-of-flight

mass spectrometer c a p a b l e of p u l s i n g the electron b e a m a n d i o n d r a w - o u t p o t e n t i a l . I n this s t u d y t h e y c o u l d , at v a r i o u s pressures, d e l a y t h e d r a w out p u l s e a n d h e n c e v a r y r e a c t i o n t i m e i n the i o n i z a t i o n c h a m b e r .

As a

result t h e y c o u l d o b t a i n e x p l i c i t measurements of r e a c t i o n rates.

The

f o l l o w i n g equations d e v e l o p e d this r e l a t i o n s h i p , b a s e d u p o n the m e c h a n i s m of D a h l e r et al. (10)

( E q u a t i o n s 31-34).

S i n c e the e l e c t r o n p u l s e

is of short d u r a t i o n , it c a n r e a s o n a b l y b e a s s u m e d that e x c i t e d atoms are f o r m e d w i t h o u t r e a c t i o n a n d t h e n d e c a y b y R e a c t i o n s 33 a n d 34. W e c a n then write ~ ^ d

r > )

=[*«- -*r(Ar)](Ar*) |

(40)

w h i c h integrates to (Ar*) =

( A r * ) o exp [ - [ * „

+

MAr)]f]

d(Ar ) =fe (Ar*) 2

+

r


(41) (42)

1.

FRANKLIN

Studies of

11

Chemionization

s u b s t i t u t i n g E q u a t i o n 41 i n t o E q u a t i o n 42 a n d i n t e g r a t i n g (Ar +) = 2

[1 - exp [*. + * ( A r ) ] f ]

*jg£jt2^

(Ar*) = ^ ( A r - ) Kl 0

Ar

+

(43)

r

0

= ^(Ar*) σι

(44)

b e i n g i n c a p a b l e of r e a c t i o n at the l o w pressures e m p l o y e d .

Finally,

then

^ = i ^ 0 B ^ n ( Ar ) O b v i o u s l y , studies of ^ r ) ( A r )

l

^

l

l

k

"

+

l

A

M

m

( 4 5 )

+


+

p e r m i t s e p a r a t i o n of k

against t i m e at v a r i o u s pressures w i l l

^ and k.

u

r

the v a l u e of σ / σ * D a h l e r et al (10)

L a m p e a n d H e s s (26)

employed

o b t a i n e d b y the steady-state m e t h o d ,

Γ

together w i t h t h e i r o w n results to o b t a i n values of k

=

molecule"

sec.

T h e p r o d u c t is i n

(10).

Cermak and Her

r

1

cm."

1

and r

u

=

4.76 ±

0.45 Χ

10"

reasonable agreement w i t h that of D a h l e r et al

7

4.3 Χ

10"

10

cc.

m a n ( 7 ) , u s i n g a b e a m of e x c i t e d rare gas atoms f o u n d a p p r e c i a b l e n u m bers of l o n g l i v e d ( ^ 1 0 * sec.)

e x c i t e d atoms w i t h states a p p r o a c h i n g

i n e n e r g y to the i o n i z a t i o n p o t e n t i a l . Q u a l i t a t i v e l y this supports t h e o b servations of L a m p e (1, 2, Table III.

26).

Rate Constants for Various States Leading to A r k Χ 10 cc/molecule/sec.

Electron Energy, e.υ.

r

18 28 65

9

1.7 ± 0.2 2.0 ± 0.1 1.3 ± 0.2

τ„, micro sec. 0.77 ± 0.07 0.55 ± 0.03 0.33 ± 0.05

2

+

N 2

2Ν + N O * - » N

2

N * + NO* - » N 2

2

+ ΝΟ + e

+ NO 2

+

+ e

+

+ NO

+

+ e

(48) (49) (50)

G a t z et al. (14) h a v e also o b s e r v e d c e s i u m ions w h e n c e s i u m w a s present in the nitrogen afterglow a n d attribute i t to Reaction 51. 2N + Cs - » N

2

+ Cs + e +

(51)


I n this l a b o r a t o r y w e h a v e f o u n d that i o n i z a t i o n occurs w h e n b e n z e n e i s a d d e d to t h e n i t r o g e n a f t e r g l o w , b u t n e i t h e r t h e ions n o r t h e process of t h e i r f o r m a t i o n has b e e n i d e n t i f i e d . Ions also o c c u r i n shock w a v e s a n d p r o b a b l y are f o r m e d b y c h e m i o n i z a t i o n . H o w e v e r , t h e scope o f this p a p e r does n o t p e r m i t a d i s c u s s i o n of these processes. Literature

Cited

(1) Becker, P. M., Lampe, F. W., J. Am. Chem. Soc. 86, 5347 (1964). (2) Becker, P. M., Lampe, F. W., J. Chem. Phys. 42, 3857 (1965). (3) Calcote, H. F., "Progress in Astronautics and Aeronautics," Vol. 12, pp. 107-144, Academic Press, New York, 1963. (4) Cermak, V., Proc. Intern. Conf. Phys. Electron At. Collisions, 3rd, Univ. Coll. London, 1963, 1080. (5) Cermak, V., J. Chem. Phys. 43, 4527 (1965). (6) Cermak, V., Herman, Z., Mass Spectrometry Conf., ASTM Committee E-14, New Orleans, La. (June 3-8, 1962). (7) Cermak, V., Herman, Z., Collection Czech. Chem. Commun. 29, 953 (1964). (8) Comes, F. J., Proc. Intern. Conf. Ionization Phenomena Gases, 6th, Paris, 1963, 159. (9) Comes, F. J., Lessman, W., Z. Naturforsch 16A, 1396 (1961). (10) Dahler, J. S., Franklin, J. L., Munson, M. S. B., Field, F. H.,J.Chem. Phys. 36, 3332 (1962). (11) Field, F. H., Franklin, J. L., J. Am. Chem. Soc. 83, 4509 (1961). (12) Fontijn, Α., Baughman, G. L., J. Chem. Phys. 38, 1784 (1963). (13) Fuchs, R., Kaul, W., Z. Naturforsch 15A, 108 (1960). (14) Gatz, C., Young, R. Α., Sharpless, R. L., J. Chem. Phys. 35, 1500 (1961). (15) Gatz, C., Smith, F. T., Wise, H., J. Chem. Phys. 40, 3743 (1964). (16) Gatz, C., Young, R. Α., Sharpless, R. L., J. Chem. Phys. 39, 1234 (1963). (17) Herman, Z., Cermak, V., Collection Czech. Chem. Commun. 31, 649 (1966). (18) Hornbeck, J. Α., Molnar, J. P., Phys. Rev. 84, 621 (1951). (19) Kaul, W., Proc. Intern. Conf. Ionization Phenomena Gases, 6th, Paris, 1963, 169. (20) Kaul, W., Fuchs, R., Z. Naturforsch 15A, 326 (1960). (21) Kaul, W., Lauterbach, U., Fuchs, R., Naturwiss 47, 353 (1960). (22) Kaul, W., Taubert, R., Z. Naturforsch 17A, 88 (1962). (23) Keenan, D. J., Clarke, Ε. M., Ann. Conf. Mass Spectrometry, 14th, Dal las, May 22-27, 1966, 42.


14

MASS SPECTROMETRY IN INORGANIC CHEMISTRY


(24) Knewstubb, P. F., "Mass Spectrometry of Organic Ions," F. W. McLafferty, ed., p. 255, Academic Press, New York, 1963. (25) Koyano, I., Tanaka, I., Omura, I.,J.Chem. Phys. 40, 2734 (1964). (26) Lampe, F. W., Hess, G. G.,J.Am. Chem. Soc. 86, 2952 (1964). (27) Lee, Y. T., Mahan, B. H., J. Chem. Phys. 42, 2893 (1965). (28) Melton, C. E., Hamill, W. H., J. Chem. Phys. 41, 1469 (1964). (29) Meyerott, R., Phys. Rev. 66, 242 (1944). (30) Ibid., 70, 671 (1946). (31) Mohler, F., Foote, P., Chenault, R., Phys. Rev. 27, 37 (1926). (32) Mohler, F., Boeckner,C.,J.Res. Nat. Bur. Stds. 5, 51, 399 (1930). (33) Moore, C. E., Natl. Bur. Std. Circ. 467, I (1949). (34) Munson, M. S. B., Field, F. H., Franklin, J. L.,J.Chem. Phys. 37, 1790 (1963). (35) Munson, M. S. B., Franklin, J. L., Field, F. H.,J.Phys. Chem. 67, 1541 (1963). (36) Pahl, M., Weimer, U., Naturwiss. 44, 487 ( 1957). (37) Pahl, M., Weimer, U., Z. Naturforsch 12A, 926 (1957). (38) Regan, P. N., Browne, J. C., Matsen, F.A.,Phys. Rev. 132, 1304 (1963). (39) St. John, R. M., Miller, F. L., Lin, Chun C., Phys. Rev. 134A, 888 (1964). (40) Shuler, R. E., Fenn, J. B., ed., "Progress in Astronautics and Aeronautics," Vol. 12, Academic Press, New York, 1963. (41) Sugden, T. M., "Progress in Astronautics and Aeronautics," p. 145, Vol. 12, R. E. Shuler and J. B. Fenn, eds., Academic Press, New York, 1963. (42) Symp. Combust., 10th, Combust. Inst., 1965. (43) Tüxen, Ο., Z. Physik 103,463 (1936). RECEIVED October 11, 1966.


Mass Spectrometry in Inorganic Chemistry

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