New routes
to dissociation:
the fourth way
A new dissociation mechanism was
elaborated from the analysis of the ultrafast
nuclear dynamics in highly excited molecules
produced by resonant X-ray absorption.
Observation of ultrafast dissociation
occurring on a femtosecond timescale
was explained with the help of theoretical
modeling, which show that, in the very early
stages of the photodissociation, the
internal
rotation
of the light carbon atoms in
1-bromo-2-chloroethane (Br-CH
2
-CH
2
-Cl,
BCE) has a significant impact on the whole
dissociation process while the halogen
atoms remain nearly still.
Dissociation is one of the simplest
chemical reactions, where a molecule
breaks apart into two or more fragments,
i.e., other molecules, atoms, ions,
or radicals. Understanding the
mechanisms of dissociation reactions
is essential as this knowledge can be
the guide to the complex world
of biomolecular processes. Yet dissociation
pathways may remain elusive in larger
systems, constituted of more than three
atoms.
Until recently only two dissociation
mechanisms were known: (1) stretching
a bond until it breaks; (2) dissociation
over a potential energy barrier through
a transition state, where electrons
are rearranged so that the old bonds
are broken and new ones are formed.
Recently a third dissociation modality
was discovered – (3) the so-called
‘roaming’ reactions, which don’t follow
the conventional mechanics of transition
state theory. In this last type of chemical
reactions, one atom wanders, or “roams”,
around the molecular fragment until
it finds another atomic partner to make
a bond with, and leave the molecule [1].
The resonant electron spectroscopy
and electron-ion coincidence experiments,
performed at the beamline PLEIADES
and combined with theoretical modeling,
revealed a new dissociation mechanism,
which doesn’t fall into any of the three
categories of dissociation reactions
described above. In this work it is shown
that in some cases stretching of the bond
can be not that simple, as anticipated.
The breakage of the bond can be mediated
by the internal motions of other lighter
atom groups of the molecule, the bond
thus being broken not strictly along the
bond axis. This allows yielding heavy
fragments on very short timescales,
by far faster than the classical two-
body dissociation would predict. It was
demonstrated that in the prototypical
molecular system – 1-bromo-2-
chloroethane – just the rotation
of the light-weight group of carbon
and hydrogen atoms (-CH
2
-CH
2
-) around
one of the halogen atoms (Br/Cl) may
result in the breakage of the carbon-
halogen (C-Cl/C-Br) bond (Figure
➊
).
In reality, both the -CH
2
-CH
2
-rotation
and the C-Cl (or C-Br) bond stretching
occur simultaneously. However, due
to the considerably lower masses
of the carbon atoms relative to that
of the halogen (Cl/Br) atom, the rotation
is very fast and, therefore, extremely
important in the total dissociation
process. As shown in the Figure
➋
, which
represents the potential energy surface
of the neutral Cl 2p core-excited BCE
molecule (lifetime
τ
~7 fs), produced
by absorption of X-rays, the blue path
would be followed if the molecule were
to dissociate in a ‘classical way’ –
the halogen atom were to separate from
the residual molecular fragment by moving
apart along the C-Br/C-Cl bond. However,
there exists a faster (red) slope from
the top of the mountain on this potential
energy surface, if a third dimension, i.e.
-CH
2
-CH
2
-rotation, is taken into account.
This new dissociation mechanism
is expected to be rather general
and applicable to other complex
molecular systems, especially to the ones
with light (-CH
2
-)
x
-linkages. Therefore,
consideration of other reaction coordinate
dimensions could be critical in modeling
of dissociation reactions for prediction
of proper reaction kinetics and product
outcomes.
ATOMIC AND MOLECULAR PHYSICS, DILUTE MATTER, UNIVERSE SCIENCE
80
SOLEIL
HIGHLIGHTS
2013
