DISSOCIATIVE MULTIIONIZATION IN MOLECULES
J.H.D. E l a n d a n d B.J. T r e v e s - B r o w n
P h y s i c a l C h e m i s t r y L a b o r a t o r y , O x f o r d , OX1 3QZ, U.K.
ABSTRACT
T h e m a n i f e s t a t i o n s of d i s s o c i a t i v e m u l t i i o n i z a t i o n i n t w o - d i m e n s i o n a l
time-of-flight mass spectra are described and are illustrated by results
o n ICN a n d SF 6. C o n c e r t e d r e a c t i o n s a n d t w o s e q u e n t i a l m e c h a n i s m s c a n
b e d i s t i n g u i s h e d o n t h e b a s i s of v e c t o r c o r r e l a t i o n s b e t w e e n i n i t i a l
momenta, which determine the peak shapes.
Slower sequential reactions
produce metastable signatures that give information on ion lifetimes,
complementing deductions from the peak shapes.
Developments which
will
enhance this technique
f o r s t u d y of r e a c t i o n d y n a m i c s a r e
discussed.
INTRODUCTION
A l m o s t all m u l t i i o n i z a t i o n of s m a l l m o l e c u l e s i s d i s s o c i a t i v e , e v e n
double ionization. Although many molecules possess low-lying states in
w h i c h t h e y a r e e f f e c t i v e l y s t a b l e , i n t h e g r e a t m a j o r i t y of s t a t e s f o r m e d
b y i m p a c t p r o c e s s e s all d o u b l y c h a r g e d s m a l l m o l e c u l e s d i s s o c i a t e . By
c o n t r a s t to s i n g l y c h a r g e d i o n s w h i c h c a n o n l y e j e c t a n e u t r a l f r a g m e n t ,
t h e f i r s t s t e p of d o u b l y c h a r g e d i o n d e c a y c a n b e o n e of t w o p r o c e s s e s ,
covalent dissociation
m2§ . . . .
> m t z* + m!
or coulombic dissociation, also called charge
m 2. . . . .
> ml §
separation
+ m2 §
A f t e r c o v a l e n t d i s s o c i a t i o n a s a f i r s t s t e p , c h a r g e s e p a r a t i o n is
l i k e l y t o follow a s a s e c o n d , a n d a f t e r e i t h e r p r o c e s s s e c o n d a r y d e c a y
of t h e p r i m a r y f r a g m e n t s may o c c u r .
The time scales may r a n g e from
f e m t o s e c o n d s to m i c r o s e c o n d s .
T h e s e r i c h m e c h a n i s t i c p o s s i b i l i t i e s of
dissociative double ionization (and the easily extrapolated
greater
r i c h n e s s of h i g h e r m u l t i p l e i o n i z a t i o n ) a r e e x p l o r e d e f f e c t i v e l y b y
t e c h n i q u e s i n w h i c h all i o n i c p r o d u c t s a r e d e t e c t e d i n c o i n c i d e n c e w i t h
electrons from the initial ionization event. For double ionization a new
t h r e e - d i m e n s i o n a l m a s s s p e c t r o m e t r y a r i s e s l, p r o v i d i n g n o v e l t o o l s t o
investigate
ion reaction
mechanisms; there
are
n e w v a r i e t i e s of
m e t a s t a b l e p e a k s a n d a b o v e all t h e m a i n i o n - p a i r p e a k s h a p e s r e p r e s e n t
vector correlations between initial ion momenta which give intimate clues
t o t h e r e a c t i o n d y n a m i c s ] . I n t h i s p a p e r we t r y t o i l l u s t r a t e t h e p o w e r
100
9 1992 American Institute of Physics
J . H . D. E l a n d a n d B. J . T r e e v e s - B r o w n
101
of t h e P E P I P I C O ( p h o t o e l e c t r o n - p h o t o i o n - p h o t o i o n c o i n c i d e n c e ) t e c h n i q u e ,
a s f a r a s i t is y e t u n d e r s t o o d , c o n c e n t r a t i n g
o n s p e c t r a of s m a l l
m o l e c u l e s , p a r t i c u l a r l y ICN a n d SF 6 a s e x a m p l e s .
EXPERIMENTAL TECHNIQUE
T h e PEPIPICO t e c h n i q u e i s n o w u s e d b y s e v e r a l g r o u p s i n t h e
w o r l d , a n d d e t a i l s of t h e d i f f e r e n t e x p e r i m e n t s a r e g i v e n i n s p e c i a l i s t
p a p e r s 3"6. Common t o all i s t h a t p h o t o i o n i z a t i o n b y e x t r e m e u l t r a v i o l e t
r a d i a t i o n o r s o f t X - r a y s t a k e s p l a c e i n a r e g i o n of u n i f o r m e l e c t r i c f i e l d ,
w h i c h a c c e l e r a t e s all t h e i o n s i n t o t h e f l i g h t p a t h of a t i m e - o f - f l i g h t
(TOF) m a s s s p e c t r o m e t e r a n d a c c e l e r a t e s t h e e l e c t r o n s t o w a r d s
a
multiplier detector.
D e t e c t i o n of a n e l e c t r o n p r o v i d e s a s i g n a l f r o m
w h i c h t h e a r r i v a l s of t w o o r m o r e i o n s a r e t i m e d . T h e s p e c t r a a r e m a p s
of i n t e n s i t y a s a f u n c t i o n of t h e a r r i v a l t i m e s of b o t h t h e f i r s t a n d t h e
second detected ions as parameters.
To r e d u c e t h e b a c k g r o u n d " n o i s e "
f r o m f a l s e c o i n c i d e n c e s , a n d to s i m p l i f y i n t e r p r e t a t i o n of p e a k s h a p e s
i n t h e s p e c t r a i t is d e s i r a b l e t h a t i o n i z a t i o n s h o u l d b e c o n f i n e d t o a
small d e f i n e d zone; for t h i s r e a s o n t a r g e t molecules a r e s u p p l i e d in the
f o r m of a g a s j e t w h i c h c r o s s e s t h e b e a m of i o n i z i n g r a d i a t i o n .
For
s t u d i e s of r e a c t i o n d y n a m i c s t h e u n i f o r m i t y of t h e e l e c t r i c f i e l d i n t h e
i o n i z a t i o n r e g i o n is c r u c i a l l y i m p o r t a n t , a s i s c o l l i m a t i o n of t h e t a r g e t
g a s beam.
C u r r e n t r e s u l t s p o i n t , a s we s h a l l s e e , t o t h e n e e d f o r a
true molecular beam, preferably a beam cooled by supersonic expansion.
As i n all c o i n c i d e n c e e x p e r i m e n t s h i g h c o l l e c t i o n e f f i c i e n c y f o r t h e
p a r t i c l e s is n e e d e d ; t h i s militates a g a i n s t
e n e r g y a n a l y s i s of t h e
electrons.
N e v e r t h e l e s s h e r o i c e x p e r i m e n t s with e n e r g y a n a l y s i s to
p r o v i d e i n i t i a l s t a t e s e l e c t i v i t y h a v e a l r e a d y b e e n p e r f o r m e d 6'7, t h o u g h
not yet under conditions yielding detailed dynamical information.
The
spectra discussed here have been taken using wavelength-selected
vacuum ultraviolet light from atomic discharges, with only weak and
unintentional electron energy selectivity.
RESULTS AND DISCUSSION
T h e PEPIPICO s p e c t r a of SF 6 a n d ICN t a k e n a t a w a v e l e n g t h of
25.6 nm a r e s h o w n i n F i g u r e s 1 a n d 2, u s i n g two d i f f e r e n t f o r m s of
presentation.
T h e d e n s i t y of p r i n t e d p o i n t s , a s i n t h e SF 6 s p e c t r u m ,
c a n r e p r e s e n t o n l y a v e r y l i m i t e d d y n a m i c r a n g e of i n t e n s i t y v a r i a t i o n s ,
so some c o n t r a s t e n h a n c e m e n t h a s b e e n u s e d t o e x h i b i t w e a k f e a t u r e s .
Each ion pair produces a peak in the spectra at coordinates (times)
p r o p o r t i o n a l to t h e s q u a r e r o o t s of t h e m a s s e s of t h e two i o n s , s o t h e
i d e n t i t i e s o f t h e p a i r s c a n b e r e a d off. I n d e t a i l m o s t p e a k s c o n s i s t of
a more o r l e s s l o n g a n d t h i n l i n e a t a d e f i n i t e s l o p e to t h e a x e s ; i n
a d d i t i o n c e r t a i n p e a k s h a v e w e a k t a i l s e x t e n d i n g to t h e p r i n c i p a l
d i a g o n a l of t h e F i g u r e . T h e s e t a i l s r e p r e s e n t
a f o r m of " m e t a s t a b l e
p e a k " p e c u l i a r to PEPIPICO s p e c t r o s c o p y , f o r
r e l a t i v e l y slow c h a r g e
102
Dissociative M u l t i i o n i z a t i o n in M o l e c u l e s
5.6
SF5+ - ~,
SF~ 2 5 . 6 n m
Fig.
i.
PEPIPICO
spectrum
of SF 6 at
25.6 n m as a dot plot,
showing
metastable
tails for slow decays
o f SF42+ a n d SF32+.
5.0
SF4+ - ~
09
=L
-SF3+ - - ~1~
4.4
"%
I:
SF2+-- lk~.
3.8
3.2
:.....
..%
..
I
I
F+
F2+
9... ,.~.,~'_ SF42+
I
I
I
2.1
2.7
3.3
3.9
Time t 1/gs
separation
reactions.
The
of SF 6 corresponds
to the
SF42§ .... > SF3 §
tail seen
fragment
on the main
ion reaction
peak
in the
spectrum
+ F+
occurring in times from a few tens of nanoseconds
to several
microseconds.
The ion pairs contributing to the main peak represent
SF42§ ions which dissociate within less than i0 ns of their formation;
those that fall in the tail are longer-lived ions that form a small
minority of the population. The distance along the tail a w a y from the
peak towards the diagonal is a measure of the actual lifetime; any ion
that lives long e n o u g h to be fully accelerated before decay contributes
to the "V" shape abutting the diagonal at the normal time of flight for
SF42+. The distance from the main peak to the "V" thus represents a
lifetime range equal to the acceleration time of the doubly charged
progenitor. The intensity distribution along the tail a n d in the "V" can
thus be used to characterize the true lifetime distribution of the ions;
simulations for different mixtures of m e a n lifetimes are used to match
the experimental data. It turns out in this case that to fit the intensity
variation along the tail two m e a n lifetimes of SF42+ of 2 Ns and 40 ns are
needed.
In the spectrum of ICN there is also a metastable tail on the
main peak for I+ + C N § pairs, not visible in the contour diagram of
Figure 2, showing that some of the ICN 2§ parent ions are a/so metastable
with
a similar lifetime.
J. H. D. Eland and B. J. Treeves-Brown
CI +
E i+
F-
-
ICN 2 5 . 6 n m
\
-%\ \
103
Fig. 2.
PEPIPICO s p e c t r u m
of ICN a t 25.6 n m a s a
contour
diagram.
Successive contours are at
i n t e r v a l s o f a f a c t o r of t w o
in intensity.
Note t h e s c a l e
breaks.
N~
C+
Time t 1
N+
I
CN +
1l ~ s
Two-body ion pair processes
The s i m p l e s t p e a k s in PEPIPICO s p e c t r a r e p r e s e n t ion p a i r s from
c o u l o m b d i s s o c i a t i o n of p a r e n t d i c a t i o n s p r o d u c i n g t w o b o d i e s o n l y :
examples are
ICN 2~ --> i§
+ C N § ICN2§
IC § + N § SF6 2§ --> SF5 + + F ~
Two i o n s f r o m d i s s o c i a t i o n of a d i c a t i o n i n i t i a l l y a t r e s t h a v e
anticorrelated momenta from their mutual coulomb repulsion.
Because
t h e time of f l i g h t i n p r o p e r l y t u n e d a p p a r a t u s d e v i a t e s f r o m i t s n o m i n a l
v a l u e b y an a m o u n t p r o p o r t i o n a l to t h e i o n ' s initial m o m e n t u m a l o n g t h e
s p e c t r o m e t e r a x i s , a n d t h e d i r e c t i o n s a r e r a n d o m i n s p a c e , all t w o - b o d y
p a i r s s h o u l d f o r m l i n e s i n t h e s p e c t r a w i t h s l o p e s of p r e c i s e l y - 1 . T h e
r e a l t w o - b o d y i o n p a i r s do f o r m n a r r o w d i a g o n a l p e a k s , a s s h o w n i n
F i g u r e s I a n d 2, w i t h v a r y i n g l e n g t h s a n d f i n i t e w i d t h s .
To i n t e r p r e t
t h e l e n g t h s a n d w i d t h s we m u s t b r i e f l y l o o k a t t h e t h e o r y i n m o r e
detail.
Under space-focussing conditions the time-of-flight t for any ion
is g i v e n to a g o o d a p p r o x i m a t i o n b y :
t = t o - pz/{eE)
(1)
n
w h e r e t ~ i s t h e f l i g h t time f o r a n i o n s t a r t i n g a t r e s t , Pz i s t h e i n i t i a l
ion momentum p r o j e c t i o n on t h e line t o w a r d s t h e d e t e c t o r , E is t h e
d r a w o u t f i e l d a n d e t h e e l e m e n t a l c h a r g e . T h e a c t u a l i n i t i a l m o m e n t a of
ions from charge
separation
can be considered
to
have
three
components.
F i r s t , t h e r e is t h e m o m e n t u m p a l o n g the d i s s o c i a t i o n
c o o r d i n a t e f r o m r e l e a s e of t h e c o u l o m b r e p u l s i o n e n e r g y .
Its projection
along the s p e c t r o m e t e r axis is pcos8 , 8 b e i n g t h e a n g l e b e t w e e n t h e
104
Dissociative Multiionization in Molecules
t w o d i r e c t i o n s . I f t h e d i s s o c i a t i o n c o o r d i n a t e is a s s u m e d t o h a v e a
random distribution
i n s p a c e i% h a s a s i n 8 d i s t r i b u t i o n
to t h e
s p e c t r o m e t e r a x i s . S e c o n d l y , t h e t h e r m a l v e l o c i t y vth of t h e m o l e c u l e s
before ionization has a Gaussian distribution along the spectrometer
axis: e a c h p r o d u c t i o n i n h e r i t s a m o m e n t u m mvth p r o p o r t i o n a l t o i t s
mass. T h i r d l y , r o t a t i o n of t h e molecule is c o n v e r t e d i n t o a n t i - c o r r e l a t e d
e q u a l m o m e n t a of t h e t w o f r a g m e n t s p e r p e n d i c u l a r t o t h e b o n d axis. To
i n t e r p r e t t h e s h a p e s of t w o - b o d y p e a k s i t i s c o n v e n i e n t t o t a k e o n e
d i m e n s i o n a l d i s t r i b u t i o n s of t h e s u m s a n d d i f f e r e n c e s of t h e f l i g h t t i m e s
for the two ions in each pair.
I n t h e d i s t r i b u t i o n of t h e s u m , t I + t~, t h e e f f e c t s of t h e
a n t i c o r r e l a t e d c o n t r i b u t i o n s to t h e i n i t i a l momenta c a n c e l , l e a v i n g o n l y
the thermal contribution. The shapes have exactly the same origin and
m e a n i n g a s TOF p e a k s h a p e s f o r ] ~ a r e n t i o n s , w h i c h w e r e d e s c r i b e d l o n g
a g o b y F r a n k l i n , H i e r l a n d Whan.~ T h e p e a k s s h o u l d h a v e a G a u s s i a n
shape
corresponding
to
the
one-dimensional
Maxwell-Boltzmann
distribution of molecular velocities along the spectrometer axis in the
target gas before ionization. O u r peak shapes for parent ions and for
tI + t2 distributions from two-body reactions are approximately Gaussian,
but their widths imply kinetic temperatures lower than the actual gas
temperature. The peak width a n d shape d e p e n d on the distance of the
gas nozzle from the light beam. This is because at the pressure in the
source there are no more molecular collisions, and the narrowness of the
light b e a m selects the central collimated part of the gas jet for
ionization.
The m e a s u r e d widths define effective transverse kinetic
temperatures of about 70 K for singly charged ions a n d 120 K for pairs.
W e do not fully understand the difference, but part is due to the
double contribution of instrumental broadening to the ion pairs. The
narrowing (by comparison with room-temperature widths) is crucial for
good mass resolution, and for detailed study of peak shapes, where
thermal velocities provide u n w a n t e d broadening.
The distribution of the difference t2 - tI for a single
PEPIPICO peak is equivalent to the shape of a 9peak in the simpler
PIPICO (photoion-photoion coincidence) technique ,10. The distribution
contains contributions from all three sources of initial m o m e n t u m , except
in the case of two equal mass fragments where the thermal velocity
contribution vanishes.
Because the coulomb repulsion energy is verb"
large by comparison with the other contributions, the m o m e n t u m given
to the ions can be considered
almost single-valued.
A pure
single-valued energy release would produce square flat-topped peaks,
exactly as for large kinetic energy releases in T O F mass spectrometryS;
the distributions over t2 - tI are indeed broad and flat-topped.
Initial
intercharge distances in dissociating dications are not single-valued,
however, but cover a range corresponding
to the width of the
F r a n c k - C o n d o n zone in the ions' formation, so the m o m e n t u m release p
is not single-valued either and the t2 - tI distributions have rounded
ends. The intrinsic coulomb energy release distributions cannot be
J . H . D. E l a n d a n d B. J . T r e e v e s - B r o w n
105
deduced directly from the observed peak shapes, because thermal
velocities and
the
rotational momentum release
smear the
TOF
d i s t r i b u t i o n s . T h e s m e a r i n g e f f e c t of t h e r m a l v e l o c i t i e s c a n b e e l i m i n a t e d
if n e c e s s a r y , s i n c e t h e d e v i a t i o n of t 1 + t 2 f o r e a c h i n d i v i d u a l p a i r f r o m
the mean value gives the thermal velocity on axis, which can be used
to c o r r e c t t h e v a l u e s of t I a n d t 2 i n d i v i d u a l l y b e f o r e s u b t r a c t i o n .
The
s m e a r i n g e f f e c t s of m o l e c u l a r r o t a t i o n , w h i c h c a n n o t b e r e m o v e d i n t h i s
w a y h a v e n o t b e e n c a l c u l a t e d , b u t a r e c e r t a i n l y s i g n i f i c a n t . T h e u s e of
a cooled molecular beam would e f f e c t i v e l y eliminate them.
Kinetic energy releases in two-body
charge
separations
are
deduced in practice
from the half-height
w i d t h s o f t h e LZ - t 1
d i s t r i b u t i o n s u s i n g f o r m u l a e g i v e n b e f o r e z. R e s u l t s f o r t h e ICN a n d S F 6
spectra are listed in the Table.
Peak s l o p e s
In a two-body
d i s s o c i a t i o n t h e i n i t i a l l i n e a r m o m e n t a of t h e
f r a g m e n t s a r e e x a c t l y a n t i c o r r e l a t e d , so t h e t r u e s l o p e of t h e p e a k i n
PEPIPICO is u n a m b i g u o u s l y -1. The m e a s u r e d slope o f t e n d i f f e r s from
t h i s v a l u e b y more t h a n the a p p a r e n t s t a n d a r d d e v i a t i o n , h o w e v e r , w h e n
i t is d e t e r m i n e d i n a l e a s t - s q u a r e s c a l c u l a t i o n t r e a t i n g t h e t w o t i m e s i n
e a c h p a i r a s of e q u a l s t a t i s t i c a l w e i g h t . A m a j o r r e a s o n is t h a t t h e
t h e r m a l v e l o c i t y of t h e n e u t r a l m o l e c u l e b e f o r e i o n i z a t i o n , w h i c h b o t h
Table.
D e t a i l s of c a t i o n p a i r s f r o m ICN a n d S F 6 a t 25.6 n m
J
Pair
Intensity ~
CN+ + I t
N+
+ CI +
Ct
64.0
Ucs/eV
5.0+/-0.4
U~eut/eV
-
2.7
5.5+/-I
-
+ I+
34.0
5.5+/-I
Ib
Nt
+ It
18.0
5 +/-i
0.8 c
Ct
+ Nt
2.2
5 +/-i
?
Ft
+ SF5+
1.5
4.8+/-0.5
-
F§
+ SF~+
0.4
4.9+/-1
?
FF~++ SF 3.
+ SF~+
0.8
27.3
4.6+/-0.8
4.3+/-0.5
~
0.5d
F §
SF 2'
F~ ++ SF2t
0.i
1.9
"~
3.. 5 + / - 1
"
?.
aIntensi~ies are on a scale where the total singly
i s 250. E x p o n e n t i a l d i s t r i b u t i o n w i t h t h i s m e a n ,
i n t e r m e d i a t e CNt. C G a u s s i a n d d i s t r i b u t i o n w i t h t h i s
width, retaining linearity. Gaussian distribution
this mean, truncated at zero.
charged ion intensity
w i t h f r e e r o t a t i o n of
m e a n a n d 0.3 eV h a l f w i t h 2 eY w i d t h a n d
106
Dissociative Multiionization
in Molecules
fragments
inherit, gives each one a random momentum component
proportional to its mass.
The spread or "error" in the time for the
heavier ion is thus proportionally larger than the spread in that for the
lighter one: the correct
unit slope can be determined
if t h e t w o
variables are properly weighted in the least-squares
treatment.
When
pairs of ions are produced by more complex reactions, perhaps involving
several steps, the spreads
of momentum carried
by each ion are
unknown and may be considerably
larger than those produced
by
t h e r m a l e n e r g i e s . To c i r c u m v e n t t h i s d i f f i c u l t y , a n d c o n t r a r y t o e a r l i e r
p r a c t i c e 2, w e n o w c o n s i d e r t h a t t h e o n l y r e l i a b l e m e t h o d o f d e t e r m i n i n g
peak slopes is to use a contour diagram, after smoothiDg if necessary.
If the long axis of a peak is marked by straight, parallel contours their
slope should be used.
For peaks with curved or non-parallel contours
the concept of slope is of less value.
This is unfortunately
also the
case when a peak is "hollow" because of the loss of ions with large
sideways velocity components.
Three-body
peaks with a single slope of -1
Peaks in PEPIPICO spectra
for three-body
or more complex
reactions come with a great variety of shapes, some of which are
e x h i b i t e d i n F i g u r e 2. W h e r e s i n g l e p r o c e s s e s a r e i n v o l v e d t h e s h a p e
is governed mainly by the momentum given to the unseen
neutral
fragment(s).
Peaks with a true slope of -1 can arise either from
concerted simultaneous reactions in which neutral fragments receive no
correlated impulse, or from deferred charge separation.
The deferred
c h a r g e s e p a r a t i o n m e c h a n i s m i s o n e in w h i c h a d o u b l y c h a r g e d p a r e n t
ion first ejects a neutral fragment, after which the remaining doubly
charged fragment
splits into two singly charged ions.
m 2~ __> ml 2t
+ m2:
ml 2§ - - >
m3§ + m4~
E v i d e n c e f o r t h e t e m p o r a l s e p a r a t i o n o f t h e s e t w o s t e p s m a y come
from the existence of the intermediate doubly charged fragment as a
distinct ion in the mass spectrum, or from inference that this same
d i c a t i o n i s t h e i m m e d i a t e p a r e n t o f s e v e r a l d i f f e r e n t i o n p a i r s . More
d i r e c t e v i d e n c e i s t h e p r e s e n c e o f a " d i a g o n a l " m e t a s t a b l e t a i l in t h e
P E P I P I C O s p e c t r u m , a s i l l u s t r a t e d in F i g u r e 1, w h i c h s h o w s t h a t t h e
c h a r g e s e p a r a t i o n s t e p i n f o r m a t i o n o f SF3 * + F ~ f r o m S F 6 v i a SF42~ c a n
t a k e 1Ons o r l o n g e r . A s i m i l a r b u t d i s t i n g u i s h a b l e m e t a s t a b l e t a i l c o u l d
appear for simultaneous formation of three fragments, two ions and one
neutral, the whole process being delayed after ionization; it would
connect to a point on the diagonal corresponding
t o t h e t o t a l m a s s of
the doubly charged precursor.
I n t h i s c a s e t h e SF42§ i o n , w h i c h i s
definitely the precursor, is a prominent feature of the mass spectrum,
making this reaction one of the clearest examples of deferred charge
separation. Other examples of neutral ejection before charge separation
include:
J. H. D. Eland and B. J. Treeves-Brown
CH3CN 2~ - - > CH2CN 2§ + H
C2H42;
- - > C2H32;
(Acetonitrile)
+ H
(Ethylene}
CH3OH 2; - - > CHOH 2+ + H2
CF42;
C6F62;
-->
CF32+
+
107
F
(Methanol)
(Tetrafluorethylene)
- - > C6F52+ + F
(Hexafluorobenzene)
2;
C4H4N22; - - > C4H4 + N2
(Pyridazine)
Reactions of these types are very common, in fact loss of a neutral
F atom as a first step has been observed in the PEPIPICO spectrum of
every single perfluorinated
compound so far examined.
In deferred charge separation the peak is expected to have a slope
of -1; the peak shape is also characteristic
of the reaction mechanism.
The energy release in the first step, although usually small, is not zero.
If the time between the two steps is sufficient for free rotation of the
dication fragment before charge separation, the directions of the two
m o m e n t u m r e l e a s e s will b e a l m o s t c o m p l e t e l y u n c o r r e l a t e d ; s i m u l a t i o n s b y
a Monte-Carlo method confirm that the result is a peak of slope -1 with
more than thermal width and a lozenge-like shape.
The shape for the
SF3 + + F + p a i r i s s h o w n in F i g u r e 3, a n d i s e c h o e d i n a s i m u l a t i o n . T h e
._
~
,
0
ns
hrne t l ~
F i g . 3. C o n t o u r d i a g r a m s (a) f o r t h e o b s e r v e d F § + SF3 + p e a k a n d
(b) f r o m a s i m u l a t i o n a s s u m i n g a n F 2 n e u t r a l f r a g m e n t e j e c t e d
w i t h a t o t a l e n e r g y r e l e a s e o f 0.5 eV w i t h a b r o a d d i s t r i b u t i o n
given in the Table.
108
Dissociative Multiionization in Molecules
p e a k s h a p e is a l o z e n g e of u n i t slope, r a t h e r t h a n a b a r , b e c a u s e t h e
momentum spreads
of the ions from the first step of the reaction,
proportional to the masses, are unequal.
The e n d s of t h e l o z e n g e
s h o u l d h a v e s l o p e s o f m4/m 3, t h e r a t i o o f t h e i o n s ' m a s s e s , h e r e 8 9 / 1 9 .
In c a s e s w h e r e t h e e n e r g y r e l e a s e in t h e f i r s t s t e p is small or w h e r e
t h e e n e r g y r e l e a s e d i n t h e s e c o n d s t e p h a s a b r o a d s p r e a d , a p e a k of
u n i t s l o p e w i t h r o u n d e d e n d s i s s e e n , a n d t h e w e a k p e a k s f o r SF4 § + F §
a n d SF3 § + F2§ i n F i g u r e 1 a r e e x a m p l e s . I n f o r m a t i o n o n t h e e n e r g y
r e l e a s e in n e u t r a l f r a g m e n t e j e c t i o n a n d on its d i s t r i b u t i o n
can be
o b t a i n e d f r o m t h e d i s t r i b u t i o n o f t 1 + t 2, a s f o r t w o - b o d y r e a c t i o n s . T h e
t 1 + t2 d i s t r i b u t i o n c o r r e s p o n d i n g t o F i g u r e 3 i s s h o w n i n F i g u r e 4 a n d
i n d i c a t e s a b r o a d e n e r g y r e l e a s e d i s t r i b u t i o n in t h e n e u t r a l e j e c t i o n
step; unfortunately
it is n o t k n o w n w h e t h e r two F a t o m s o r a n F2
m o l e c u l e a r e e j e c t e d , t h o u g h w e s u s p e c t t h a t i t is m o l e c u l a r f l u o r i n e .
In deferred charge separation both ions receive the same velocity
from r e a c t i o n to t h e d e p a r t u r e of t h e n e u t r a l f r a g m e n t a n d t h i s v e l o c i t y
is k n o w n f o r e a c h i o n p a i r f r o m t h e d e v i a t i o n o f t 1 + t 2 f r o m i t s m e a n
v a l u e . The e f f e c t of e n e r g y
r e l e a s e in t h e f i r s t s t e p c a n t h u s be
subtracted
in calculating
the distribution
o f t~ - t I i n o r d e r
to
characterize
t h e e n e r g y r e l e a s e in t h e s e c o n d s t e p of t h e r e a c t i o n .
T h e r e is a d e f i n i t e i m p r o v e m e n t in s h a r p n e s s of t h e t z - t I d i s t r i b u t i o n
a s d e m o n s t r a t e d i n F i g u r e 4, w h e n t h e t w o i o n s a r e o f v e r y u n e q u a l
m a s s .
Ii I (a}
,.,..,..,..,.~..'t "l..t '~
IIIII llllt I'h..,o I
(c)
.... ~ ~ll
IIlIplil~,fh
,J]tk|~t (b)
~ ~i
IIIIllllllllllllllll~lllllllllll
(d)
tI
tl
I
11l
,I II
|11111Iiiii, ill ~ I
6
+sb
illJ
- 6o
Time deviation
|II
b
+lbo
(ns)
F i g . 4. One d i m e n s i o n a l d i s t r i b u t i o n s f r o m t h e s p e c t r u m of SF 6 a s
i n F i g . 1. (a): t 1 + t 2 d i s t r i b u t i o n f o r F* + SF5 § ( p s e u d o - t h e r m a l )
9
.
.
§
§
.
(b): t~ + t~ d i s t r i b u t i o n f o r F + SF~ s h o w i n g t h e e f f e c t o f n e u t r a l
e j e c t i o n . (c): t 2 - t l d i s t r i b u t i o n f o r F
SF 3 as m e a s u r e d and
(d): t h e s a m e a s (c) a f t e r r e m o v a l o f t h e e f f e c t o f m o m e n t u m
r e l e a s e in e j e c t i o n o f t h e n e u t r a l , a s e x p l a i n e d i n t h e t e x t .
J . H . D. E l a n d a n d B. J . T r e e v e s - B r o w n
109
C o n c e r t e d b r e a k i n g of t w o b o n d s i n a d i c a t i o n , t h e m e c h a n i s m
sometimes called simultaneous coulomb explosion, can also result in
singly ionized fragments with anticorrelated momenta and one neutral
f r a g m e n t , all l i b e r a t e d s i m u l t a n e o u s l y . A n e x a m p l e i s t h e d i s s o c i a t i o n of
doubly ionized carbon disulphidell:
32SC34S2§ --> 32S+ + C § 34S+
A simple peak with p a r a l l e l s t r a i g h t c o n t o u r s a n d u n i t slope (as
s e e n i n t h i s e x a m p l e ) will a p p e a r f o r s u c h a r e a c t i o n if t h e m o m e n t u m
g i v e n to t h e n e u t r a l is z e r o , o r if i t is r a n d o m i n d i r e c t i o n r e l a t i v e t o
t h e d i r e c t i o n of c h a r g e s e p a r a t i o n .
More c o m p l e x p e a k s h a p e s a r e
produced by concerted dissociations in which the neutral fragment
receives a significant correlated impulse, however, and these are
discussed later.
T h r e e - b o d y p e a k s w i t h n o n - u n i t single slopes
P E P I P I C O p e a k s of n o n - u n i t s i n g l e - v a l u e d s l o p e c a n a r i s e f r o m
simultaneous explosions, from deferred
charge separation
with the
n e u t r a l e j e c t e d on the same line as t h e ions, or from s e c o n d a r y d e c a y
of t h e p r o d u c t s
of c h a r g e s e p a r a t i o n 2.
We b e l i e v e t h a t t h e l a s t
m e c h a n i s m is t h e m o s t c o m m o n .
The secondary decay mechanism can be written
m 2§ __> ml * + m2 +
ml § --> m3 ~ + m 4
though the starting point m a y already be a fragment, a n d both ions
from charge separation m a y subsequently dissociate.
T h e critical
parameter determining both the description of the m e c h a n i s m a n d the
appearance of the peaks is time, the time between charge separation a n d
loss of the neutral fragment. The deferred charge separation m e c h a n i s m
discussed above corresponds to negative times in this sense, concerted
immediate explosions to zero time (within a few femtoseconds) and
secondary decays to positive time. Details of the peak shapes respond
to magnitudes of the time from microseconds, in metastable peak
phenomena, to femtoseeonds in secondary decay.
Secondary decay of a primary ion about a picosecond or later after
charge separation leads to a PEPIPICO peak w h o s e slope is equal to the
mass ratio m3/m I (or the inverse if m 2 is heavier than m3). For times in
this range a n d for normal rotational temperatures the intermediate ion
m I can rotate freely before the secondary decay occurs, so the
m o m e n t u m release directions in the two steps are uncorrelated.
Since
the time is long e n o u g h for escape, secondary decay occurs outside the
range of coulomb repulsion.
If an e n e r g y release occurs in the second
step it affects only one of the two ions detected, m3 ~, so the peak shape
will be a lozenge with vertical (or horizontal) ends, showing that one of
the ions has a m u c h wider spread of m o m e n t u m along the axis than the
110
Dissociative Multiionization in Molecules
o t h e r . S e c o n d a r y d e c a y of p r i m a r y i o n s i s j u s t t h e s a m e p r o c e s s a s
m o n o c a t i o n d i s s o c i a t i o n s t u d i e d i n n o r m a l m a s s s p e c t r a , a n d i t is n o
s u r p r i s e t h a t the e n e r g y r e l e a s e is n o t u s u a l l y l a r g e r t h a n a few
h u n d r e d m i l l i e l e c t r o n v o l t s . As a r e s u l t t h e m a j o r i t y of p e a k s of t h i s
t y p e h a v e r o u n d e d e n d s a n d f a i r l y small p e r p e n d i c u l a r widths.
Some
c l e a r e x a m p l e s of s e c o n d a r y d e c a y arel2'13:
CH3R2~
- - > Rt + CH~+: CH3t - - > CH2t + H
(R = OH, I, SCN, e t c . )
(CH3)2SO2§ --> CH3* + CH3SO+: CH3SOt --> SOt + CH3
C4H402+ --> C2H2t + C2H2Ot: C2H20t --> CH2t + CO
(Furan)
Close agreement between the measured slope and a mass ratio is in
itself good evidence for the secondary decay mechanism; the primary ion
pair is almost always p r e s e n t in the spectrum, and often the same ions
break up in two or more secondary decays. If the range of lifetimes in
secondary decay extends into the metastable range (10 ns to 10 lls) a
broad ridge is seen in the PEPIPICO spectrum extending along the line
of m2t from ml+ to m3t. I n such a case the decay of the singly charged
ion involved is likely to be known as a metastable process in normal
mass spectrometry, and perhaps to be visible in the photoionization
mass spectrum itself.
In the PEPIPICO spectrum of ICN the peak for It + C+ offers itself
as a probable case of secondary decay, as the slope is almost exactly
26/12 and the CN§ ion is certainly formed by charge separation. This
case involves an ambiguity of interpretation, however, because C is the
central atom. Even if the C-N bond were broken instantaneouslyat the
moment charge separation began, the C+ ion would have to
push the N
9
atom out of its way before escaping. Such a mechanism~, "obstructed
instantaneous explosion", cannot easily be distinguished from secondary
decay.
The means of distinction will be to v a r y the rotational
temperature of the molecules; at high rotational temperature the N atom
from an obstructed explosion should escape sideways at an earlier time,
and the slope of the peak should approach - I .
Some peaks are found to have single-valued slopes t h a t are neither
-1 nor close to a recognisable mass ratio. In polyatomic molecules the
p o s s i b i l i t y t h a t b o t h m e m b e r s of a p r i m a r y p a i r h a v e u n d e r g o n e
s e c o n d a r y d e c a y m u s t b e c o n s i d e r e d ; b o t h of t h e i n d i v i d u a l s e c o n d a r y
r e a c t i o n s s h o u l d be o b s e r v a b l e s e p a r a t e l y for t h i s to be p l a u s i b l e .
Experimental artefacts may sometimes
d i s g u i s e the c o r r e c t slope, b u t
t h e r e r e m a i n a g r o u p of v e r y i n t e r e s t i n g p e a k s w h o s e s l o p e s c a n n o t b e
e x p l a i n e d i n t h i s w a y ; a p r i m e e x a m p l e is t h e p a i r I t + N+ i n t h e
s p e c t r u m of ICN. T h e s l o p e of t h e p e a k is -1.36+0.04 w h e r e a s s e c o n d a r y
d e c a y of CN§
g i v e - 1 . 8 6 (26/140) a n d s e c o n d a r y d e c a y of CI § w o u l d
g i v e - 0 . 9 2 (127/139). One a r b i t r a r y
b u t p o s s i b l e e x p l a n a t i o n is t h e
r e l e a s e of a s i n g l e d e f i n i t e k i n e t i c e n e r g y i n s e c o n d a r y d e c a y a l o n g t h e
J . H . D. E l a n d a n d B. J . T r e e v e s - B r o w n
111
d i r e c t i o n of c h a r g e s e p a r a t i o n , b e f o r e t h e d i a t o m i c f r a g m e n t h a s t i m e t o
rotate.
A f o r m u l a f o r t h e r e s u l t i n g s l o p e i n t e r m w of t h e m a s s e s a n d
k i n e t i c e n e r g i e s w a s g i v e n i n t h e e a r l i e r p a p e r ,~ a n d p r o v i d e s a n
excellent starting point for simulations; the best match on this model is
given in the Table. Another possibility is that secondary decay takes
place at a short distance from the other (undissociated} primary ion,
within the coulomb repulsion zone. If the secondary decay involves no
energy release the slope for a single-valued critical distance is easily
calculable, though simulations using exponential lifetime distributions are
probably more realistic.
F o r I t + N§ f r o m ICN a t 25.6 n m t h i s m o d e l
w o u l d r e q u i r e a m e a n CN§ l i f e t i m e o f a b o u t 30 fs. U p t o n o w i t h a s n o t
b e e n p o s s i b l e to d e c i d e b e t w e e n t h e s e a n d o t h e r model m e c h a n i s m s ; t h e
finer resolution made possible by a rotationally cold molecular beam may
allow a d e c i s i o n t o b e m a d e .
E n e r g y r e l e a s e s i n s e c o n d a r y d e c a y s w h i c h g i v e p e a k s of t h e
expected slope can be determined
f r o m t h e w i d t h s of t h e p e a k s
p e r p e n d i c u l a r to t h e i r l e n g t h s , or b y c o m p a r i s o n s of t h e p e a k s h a p e s
with s i m u l a t i o n s . T h e r e is r e a s o n to e x p e c t t h e e n e r g y r e l e a s e s a n d
t h e i r d i s t r i b u t i o n s to be v e r y similar to t h o s e e x h i b i t e d b y the same
m o n o c a t i o n s i n n o r m a l m a s s s p e c t r a , b u t n o s y s t e m a t i c i n v e s t i g a t i o n of
t h i s a s p e c t of t h e s p e c t r a h a s y e t b e e n m a d e .
Peaks with curved contours
I n t h r e e - b o d y d e c a y s w h e r e t h e n e u t r a l f r a g m e n t is e j e c t e d w i t h i n
l e s s t h a n a few p i c o s e c o n d s
before or after
charge
separation,
d i r e c t i o n a l c o r r e l a t i o n of all t h r e e i n i t i a l m o m e n t a i s t o b e e x p e c t e d .
S i m u l a t i o n s ll'N a n d a l g e b r a i c a n a l y s i s 14 i n d i c a t e t h a t t h e c o r r e l a t i o n
s h o u l d s h o w u p i n t h e f o r m of p e a k s w i t h c u r v e d c o n t o u r s , a n d t w o
limiting cases can be delineated clearly.
If the neutral fragment is
e j e c t e d a t a s i g n i f i c a n t a n g l e t o t h e l i n e of c h a r g e s e p a r a t i o n , t h e
PEPIPICO p e a k s h o u l d b e b r o a d a t i t s c e n t r e b u t n a r r o w a t t h e e n d s .
For near-perpendicular
neutral
ejection the centre
of t h e
peak
corresponds
to
charge
separation
perpendicular
to
the
mass
s p e c t r o m e t e r axis; in t h i s o r i e n t a t i o n r e a c t i o n to t h e n e u t r a l m o m e n t u m
c o m p o n e n t c a n lie a l o n g t h e s p e c t r o m e t e r a x i s a n d so h a v e m a x i m u m
e f f e c t o n t h e f l i g h t t i m e s . T h e e n d s of t h e p e a k , b y c o n t r a s t , r e p r e s e n t
charge separation along the axis, where an additional perpendicular
momentum component, perpendicular
to the axis, has minimal effect.
Several peaks with the resulting "egg" shape have been found and are
particularly common in cases where a triply charged ion dissociates into
t h r e e f r a g m e n t i o n s , o n l y two of w h i c h a r e o b s e r v e d . N e u t r a l f r a g m e n t s
n o r m a l l y c a r r y m u c h l e s s m o m e n t u m t h a n i o n s , e v e n if i t is c o r r e l a t e d ,
so o v o i d p e a k s a r e l e s s o f t e n o b s e r v e d i n d o u b l e i o n i z a t i o n , w h e r e t h e i r
t r u e s h a p e may be swamped b y t h e t h e r m a l v e l o c i t y d i s t r i b u t i o n . One
exceptionally clear and intense example 2 is the reaction
S022§ .... >
0§ +
S§ + 0
112
Dissociative Multiionization in Molecules
I n t h e s p e c t r u m of ICN a n e x a m p l e is s e e n i n t h e v e r y w e a k p e a k
f o r C~ + N~. I n t h i s c a s e t h e o v a l h a s i t s m a j o r a x i s a l m o s t v e r t i c a l ,
s h o w i n g t h a t t h e I a n d N§ f r a g m e n t s
travel
in nearly opposite
d i r e c t i o n s , l e a v i n g v e r y l i t t l e m o m e n t u m to b e c a r r i e d b y t h e C~ ion.
T h e c u r v a t u r e of t h e c o n t o u r s s h o w s t h a t t h e a n g l e s i n v o l v e d i n t h e
d i s s o c i a t i o n a r e n o t r a n d o m ( n o r z e r o ) , h o w e v e r . T h e w i d t h of t h e p e a k
s u g g e s t a l a r g e a n g l e b e t w e e n C+ a n d N§ d i r e c t i o n s , b u t b e c a u s e of t h e
low i n t e n s i t y of t h e p e a k a n e x a c t a n a l y s i s h a s n o t b e e n a t t e m p t e d . T h e
w i d t h of t h e d i s t r i b u t i o n of N§ t i m e s i n t h e p e a k i s a l m o s t e x a c t l y t h e
s a m e a s i t s w i d t h i n t h e N§ + CI § p e a k , s u g g e s t i n g t h a t t h e i n i t i a l
process resembles the simple charge separation.
A d i f f e r e n t p e a k s h a p e c a n a r i s e f r o m p r e f e r e n t i a l e j e c t i o n of a
n e u t r a l f r a g m e n t w i t h s i g n i f i c a n t m o m e n t u m a l o n g t h e l i n e of c h a r g e
s e p a r a t i o n . If t h e m o m e n t u m r e l e a s e is s i n g l e - v a l u e d (as a s s u m e d to
e x p l a i n t h e I § + N~ p e a k ) t h e p e a k s h o u l d b e a t h i n l i n e a t a n
u n e x p e c t e d slope, b u t for a m o m e n t u m r e l e a s e d i s t r i b u t i o n it s h o u l d
have an "hourglass" shape, fat at the ends but narrow in the middle
b e c a u s e of t h e o v e r l a p of m a n y l i n e s o f d i f f e r e n t s l o p e s .
No
satisfactory real example has yet been found.
Peak contours are often
d i s t o r t e d i n t o t h e s e m b l a n c e of a n h o u r g l a s s s h a p e b y l o s s of i o n s w i t h
l a r g e s i d e w a y s v e l o c i t y c o m p o n e n t s . I t is a l m o s t i m p o s s i b l e t o c o m p u t e
t h e e f f e c t s of d i s c r i m i n a t i o n w i t h c o m p l e t e f i d e l i t y , a n d s u c h p e a k s
unfortunately
convey little dynamical information beyond the bare
m a g n i t u d e of t h e c o u l o m b e n e r g y r e l e a s e . A n g u l a r a n i s o t r o p y i n t h e
initial ionization process, which has hitherto been neglected here, can
also produce similar appearances.
Peaks with non-parallel contours
PEPIPICO peaks for a n u m b e r of secondary decays are found on
close examination to have a "twist": the intense part of the peak has a
different slope from the less intense part, as illustrated in Figure 5 by
~
Fig. 5. PEPIPICO peak contours
for 35Clt + CF2 + from CF2CIBr at
30.4 nm, showing a strong "twist".
The diagonal line shows the slope
+
calculated for a
d e c a y of CF2Br §
CF2
I
3sCF+
tl"-"
slow
secondary
J . H . D. E l a n d a n d B. J . T r e e v e s - B r o w n
113
a n e x a m p l e f r o m t h e s p e c t r u m o f CF2C1Br. O n e o r b o t h o f t h e i n t e n s e
or less intense parts usually has a slope either close to -1 or close to
the mass ratio for a secondary decay, as in these examples. In some
cases the explanation is that a particular ion pair is formed by two
concurrent
mechanisms
with different
probabilities
and
different
characteristic slopes, and the two simply overlap.
This is certainly not
the explanation in all cases, however, and the twisted form of peak
generally implies a correlation between the probability of secondary
decay and one of the parameters which determine peak slope.
The
probability must be systematically related to either the momentum given
to the neutral along the charge separation direction, to the distance
between ions at which secondary fragmentation
takes place or to the
time of the event after charge separation.
If time is the critical variable, twisted peaks can be explained by
secondary decay occurring in a time
range of lOfs to lps, the range
in which the slopes vary markedly as functions of the position in the
coulomb field at which the neutral fragment is released.
The most
frequent
form of twisted peak has its greatest
intensity
near the
limiting slope for a secondary reaction, equal to a mass ratio, but is
surrounded
by an aureole with contours of slopes between the limiting
value and -1. The pile-up near the limiting mass ratio is well explained
on the time hypothesis,
where the lifetime distribution
should be
approximately exponential, by the
non- linear relationship
between
slope and time elapsed since charge separation.
After a time of the
order
o f 300 f s , w h i c h
depends
on the masses and
the initial
intercharge
distance, all later decays will contribute
to intensity at
slopes close to the limiting value.
Only extremely fast reactions,
essentially complete in one
or few vibration periods, will not have a
tail of lifetimes piling up near the limiting slope. Confirmatory evidence
for this interpretation
is that when the energy of the ionizing light is
reduced it is always the part of the peak at the secondary
reaction
slope that becomes relatively more intense, presumably because only the
slowest reactions persist.
Detailed tests to validate or contradict this
interpretation
will probably require the use of cold molecular beams,
and initial energy selection of the reacting ions. If they do validate it
we shall have a new route to femtosecond molecular reaction dynamics.
CONCLUSIONS
The PEPIPICO technique is shown to be a powerful tool for the
study of doubly charged ion dissociations; by obvious extension it can
be applied to reactions of more highly charged species.
To m a k e t h e
method really effective, two improvements are needed. First, target gas
must be provided in the form of a rotationally cold, tightly collimated
molecular beam. Secondly, initial state selectivity must be added to the
technique
instead
of the simple wavelength
variation
available at
present, or alternatively the statistics of existing energy-selective
forms
114
Dissociative Multiionization in Molecules
of the technique must be improved b e y o n d recognition.
on both these lines are currently under way.
Developments
REFERENCES
I,
2.
3.
4.
5.
J.H.D. Eland, Acc. Chem. Res., 22, 381 (1989).
J.H.D. Eland, Mol. Phys., 61, 725 (1987).
L.J. Frazinski, M. Stankiewicz, K.J. Randall, P.A. Hatherley
and
K. Codling, J. Phys. B. 19, L819 (1986).
P. Lablanquie, I. Nenner, P. Milli4, P. Morin, J.H.D. Eland, M.-J.
Hubin-Franskin and J. Delwiche, J. Chem. Phys., 82, 2951 (1985).
T. LeBrun, M. Lavoll4e and P. Morin, AIP Conf. Proc. 215, 846
(1990).
6.
7.
8.
9.
10.
ii.
12.
13.
14.
D.M. Hanson, C.I. Ma, K. Lee, D. Lapiano-Smith and D.Y. Kim, J.
Chem. Phys., 93, 9200 (1990).
R. M u r p h y and W. Eberhardt, J. Chem. Phys., 89, 4054 (1988).
J.L. Franklin, P.M. Hierl and D. A. Whan, J. Chem. Phys., 47, 3148
(1967).
G. Dujardin, S. Leach, O. Dutuit, P.-M. G u y o n and
M.
Richard-Viard, Chem. Phys., 88, 339 (1984).
P.M. Curtis and J.H.D. Eland, Int. J. Mass Spectrom. Ion
Processes, 63, 241 (1985}.
J.H.D. Eland, Laser Chem., 1991, in press.
E. Ruhl, S.D. Price, S. Leach and J.H.D. Eland, Int. J. Mass
Spectrom. Ion Processes, 97, 175 (1990).
Unpublished w o r k in this laboratory.
T. LeBrun, Doctoral Thesis, Universit4 Paris XI, 1991.
J. H. D. Eland and B. J. Treeves-Brown
115
QUESTION
B R U T S C H Y - 1. Have you studied these fragmentation patterns at different
energies, and did you possibly see a change of the pattern, telling you something
of the transition state ?
2. The structure of more complicated fragments is or may be still
open. Do you believe one may combine this elegant method with a collision
induced fragmentation (CID) section ?
E L A N D - In answer to the first question, we regularly take PEPIPICO spectra at
several wavelengths between 40.7nm and 25.6nm (300eV to 48eV). Where
reactions are simple two-step processes of deferred charge separation or
secondary decay, the peak shapes do not change. The only concerted reaction
studied over a wide range so far is SO 2+
2 - ) O + + S+ + O, where the fragments
separate at almost 120 ~. In this case the angles do not change much with
wavelength, but the magnitude of the energy release increases at high energy.
The shapes of "twisted" peaks do change strongly with wavelength.
On the second question, I fear that the technique you suggest would be too difficult
for technical reasons, unless detectors of true 100% detection efficiency become
available.
DE L A N G E - W h e n your doubly charged ion separates into two charged
fragments, you can only detect both fragments if their direction after formation
allows them to reach the detector. If the dissociation takes place far from the
detector this situation is not nearly as probable as when it occurs close to the
detector. Is it true that there is a large geometrical factor of this nature that one
should correct for? As a second point, I want to come back to what you said about
dissociation of ICN 2+. Of course neutral ICN is linear, but how about ICN2+? Will
non-linearity of the doubly charged ion have a significant effect on the line shape?
ELAND - On the first point, we normally operate our apparatus under conditions
where all fragments are detected, even if they fly apart in the source exactly
perpendicularly to the axis. If they dissociate later, they are certain to hit that active
area of the detector, so no convection is needed, the possibility of such ion losses
is however fully included in the Monte-Carlo modelling with which the
experimental data are compared. On the second point ICN 2+ may be non-linear in
excited states where electrons other than the outer ~-electrons have been lost.
This could certainly affect the peak shape, but will not explain it entirely:
particularly it does not explain the large momentum of the neutral I atom.
116
Dissociative Multiionization in Molecules
LEACH- One reason for the sideways ejection of carbon in the ICN dication case
could be the large change in the bending force constant expected when one
removes two electrons from ICN. Neutral ICN is a 16 valence electron linear
species, the outer 4 electrons being in a ~ orbital. In such species, the electron
occupation of this ~ orbital governs the ease of bending the linear molecule. This is
well known for the centrosymmetric series CO2, CO2 +, NCN .... C3 for which the
bending frequency diminishes strongly as successive ~g electrons are removed. A
greater flexibility is to be expected for ICN 2+ with respect to ICN. The existence of a
heavy Iodine atom may also help to tunnel energy into the bending vibration via
anharmonic coupling between stretch and bend vibrations. Thus a propensity for
sideways ejection of carbon, by strong excitation of the bending vibration, could
result from ejection of two valence electrons from the outer ~ orbital in ICN.
B E S W l C K - Could you comment on the possible contribution of the zero-point
bending energy of ICN in the suprisingly large contribution of the I + C + + N +
concerted reactions ?
E L A N D - The zero-point bending is probably very important , because it is
effectively amplified by this Coulomb repulsion, as we have seen in dissociation of
triply charged ions such as OCS 3+. The best model of the ICN 2+ --, C + + N + + I
reaction seems to be an "initial" separation into N + + CI +, followed by CI + ~ C + + I
before rotation of the diatomic fragment from its initial, small, zero-point angle. As
CI + is so heavy, it will rotate relatively slowly. As C + is light, by contrast, a small
energy release in CI + decay will propel it fast towards N +, whose repulsion will
deflect it to the observed large angle (--100 ~ from N +) outside the Coulomb zone.
DUJARDIN - When you analyse a particular peak shape, you assume that there
is a single process leading to the corresponding fragments ions. However it may
be that several processes are superimposed and give rise to more complicated
analysis. Could this explain the "unexplained" peak shapes?
E L A N D - You are right that because initial states are not selected, mixtures of
mechanism are observed. In many cases we see trivial superimposition of peak
shapes where two distinct pathways can produce the same ion pair. An interesting
example is the peak for H2+ + CH2 + from ethylene oxide C2H40, which arises at
30.4nm from both CH2+'+4 CO by deferred charge separation in CH 2+4 ' and by
secondary decay of CH2CO + from the primary pair H2+ + CH2CO +. At low photon
energy the first component vanishes.
In other cases, such as S + + O + from SO2, the peak shape remains the same at all
wavelengths, but the size changes. We take this to mean that the same general
mechanism (here concerted explosion) is operative in decay from a range of initial
states, with changing total energy release.
B A E R - Can you say anything about why some ions are metastable ? Is it
tunneling or a statistical process ? Are strong metastables related to large Eo's for
dissociation ?
J. H. D. Eland and B. J. Treeves-Brown
117
ELAND- We have good evidence, from detailed RRKM modelling, that in the
PEPIPICO spectra of large aliphatic hydrocarbons almost all the primary singlycharged ions from charge separation decay statistically including those that
produce metastable peaks. The charge separation step in these cases is fast, but
seems to allow a statistical energy distribution among the fragments.
By contrast, the metastable charge separations of such small ions as ICN2+, SF 2+
4 '
N202+, CO~ + or 02H22 seem likely to be due to tunneling processes undergone
by that small minority of ions in energy levels near the top of the barriers.
Metastable charge separations of aromatic species such as benzene or pyridazine
might be explained either way, and more detailed measurements are needed
before a distinction can be made.