Abstract
Time-of-flight mass analysis with multi-stop coincidence detection has
been used to study the multi-cation ionic fragmentation
via
fission of three isomeric carborane icosahedral cage compounds
closo
-1,2-orthocarborane,
closo
-1,7-metacarborane,
closo
-1,12-paracarborane (C
2
B
10
H
12
) following
inner-shell excitation at or above the B 1s
regime. Photoelectron-photoion-photoion coincidence (PEPIPICO)
spectroscopy was used to study the dominant fission routes in the core
level excitation regime. Series of ion pairs are identified, where
asymmetric fission dominates, leading to ion pairs of different
mass. The fragmentation yields and charge separation mass spectra of
all three isomers are generally quite similar in that the ion pairs
(H
+
, Y
+
), (Y
+
, Y
11
+
), and (Y
3
+
,
Y
9
+
) dominate, where Y
+
denotes the mass region from
B
+
to CH
+
. Slight differences are observed at the B
1s-threshold, where the H
+
and BH
2
+
/CH
+
ion pairs
dominate for ortho- and meta-carborane, while (Y
+
, Y
11
+
)
ion pairs dominate the multi-photofragment ion yield of
paracarborane. These similarities and distinct differences in charge
separation are discussed by considering the energetics of these three
major species of ion pairs, as well as charge distributions in
closo
-carboranes and charge distributions in the
carborane cage. It is shown that product formation
via
charge
separation is driven by electronic relaxation, so that the lowest energy products are not
necessarily formed.