of the neutral species formed by near-resonant neutralization
were assumed to have the same difference in energies for both
of the isomeric cations. If these neutral species isomerize
before dissociation, then the branching ratios of the dissocia-
tion will depend only on the internal energy of acetylene and
vinylidene. As shown in Fig. 5, the energy level of excited acet-
ylene formed via near-resonant neutralization with a Cs target,
having the lowest ionization potential, is lower than that of the
excited vinylidene formed with a Na target having the largest
ionization potential in the alkali metals used in the present
work. Fig. 3 shows that the intensities of the C2ꢀ peak relative
to the C2Hꢀ peak for acetylene with a Cs target is much larger
than that of the vinylidene with a Na target. This difference in
relative intensity cannot be rationalized by the internal energy
of the excited neutrals, so is attributed to a difference between
the structures of the acetylene and vinylidene cations. The dif-
ferences in these relative intensities for acetylene and vinyli-
dene demonstrate that excited C2H2 neutrals formed via
neutralization dissociate prior to isomerization.
Fig. 5 Heats of formation of C2H2 , its fragments, and C2H2+ ions in
+
electron volts. The CH2
C
shown in this figure is the 2B1 ground
=
state. The values shown in parentheses are the energy values predicted
for near-resonant neutralization.
Since the dissociation process to form C2H + H from both
acetylene and vinylidene involves a simple cleavage of a C–H
bond, the barrier to the dissociation is expected to be smaller
than that for the process associated with the structural rearran-
gement to form C2 + H2 . While the barrier involved in the dis-
sociation process to form C2 + H2 from vinylidene is estimated
to be very small, the barrier to formation of C2 + H2 from acet-
ylene is expected to be larger due to the structural deformation
required to form the H–H bond.33 Therefore, the observed dif-
Interpretation of the results
+
The heats of formation for C2H2 , its fragments, and C2H2
ions are shown in Fig. 5, with most of the thermochemical data
shown in this figure being taken from ref. 32. The near-reso-
nant levels, which have energies lower than the energy level
of the corresponding cations by an amount equal to the ioniza-
tion potentials of the alkali metal targets, are also shown in
Fig. 5. The thermochemical data for vinylidene and its cation
have not been reported experimentally, so these levels were
estimated using the energy difference between acetylene and
vinylidene calculated by Jursic.2
ꢀ
ference in the intensities of the C2 peak relative to the C2Hꢀ
peak between acetylene and vinylidene is attributed to this dif-
ference in the barrier to dissociation into C2 + H2 . It is cur-
rently unclear whether the dissociation mechanism from
acetylene to form C2 + H2 proceeds via a transition state to dis-
sociation, or via a vinylidene structure prior to dissociation. It
should be possible to determine the dominant process by com-
parison with theoretical calculations.
Fig. 5 shows that two independent C–H bond cleavages to
form CC + 2H is endothermic by 0.79 eV compared with CC
triple bond cleavage, and H2 loss to form C2 is exothermic
by 3.73 eV compared with the CC triple bond cleavage. Two
endothermic C–H bond cleavages should result in a smaller
branching ratio than the CC bond cleavage because simple
bond cleavages do not generally have activation energies.
The energy levels of the neutral species formed by near-
resonant neutralization from HCCH+, as shown in Fig. 5,
are much lower than those of CC + 2H and CH + CH, and a
few eV higher than that of CC + H2 . The excited neutrals
cannot dissociate into fragments with higher energy levels,
but dissociate spontaneously into those with lower energy
The present work has demonstrated that the dissociation
mechanism of highly excited neutral C2H2 isomers formed
from isomeric C2H2 cations involves dissociation into
+
C2H + H prior to isomerization. This is the same behaviour
as for ground-state vinylidene formed from the vinylidene
anion as reported by Hayes et al.5 and Levin et al.8
Acknowledgements
ꢀ
levels. The large intensity ratios of the C2 peak to the CHꢀ
peak for all of the targets indicates that C2 does not result from
two C–H bond cleavages but rather from loss of H2 .
Grants in aid for scientific research for the Ministry of
Education, Culture, Sports, Science and Technology under
Grant No.13640515 are gratefully acknowledged.
ꢀ
For both isomeric cations, the intensities of the C2 peak
relative to the C2Hꢀ peak decrease with increasing ionization
potential of the target, as shown in Fig. 3. The C2 + H2 level
is a little higher than that of CCH + H. Processes which
involve both bond dissociation and bond formation together
with structural rearrangement usually require higher activation
energy than that for a simple bond cleavage. Therefore, the
transition state in the formation of C2 + H2 from the C2H2 iso-
mers is expected to have a higher activation energy than that
for the simple cleavage into CCH + H. The higher internal
energy of excited neutrals formed using targets with lower ioni-
zation potential make it easier for these excited neutrals to dis-
sociate into fragments with higher activation energꢀy. The
observed dependence of relative intensities of the C2 peak
to the C2Hꢀ peak on target species is explained by this differ-
ence in activation energy for bond cleavage accompanied by
structural rearrangement into C2 + H2 compared with simple
bond cleavage to form C2H + H.
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3
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The heat of formation of the vinylidene cation has not been
determined experimentally, but is calculated to be 1.85 eV
higher than that of the acetylene cation.2 The energy levels
Phys. Chem. Chem. Phys., 2003, 5, 2386–2390
2389