(
)
C.R. Bucher, K.K. LehmannrChemical Physics Letters 294 1998 173–180
177
1
did not change. In particular, no sharp structure in
the photodissociation cross-section was observed.
which has P linear symmetry. The electronic con-
figurations responsible for the states mixing are
1aX2 aX2 3aX24a2 5a2 6aX21aX 7aX for HCN 13 and
w
x
1s2 2s2 3s24s21p35s2 for CN 18 . Dissociation
w
x
Ž .
4. Discussion
energies below the barrier will predissociate to CN X
via coupling with either the 1 3A or the X 1 Sq state,
Y
In the linear configuration, HCN has three low
while a second predissociative channel opens up
lying singlet excited states that arise from p–p)
Ž .
leading to CN A for energies above the barrier. For
1
1
1
excitation. These are Sy, D, and P in order of
increasing energy. Only the last is dipole allowed
from the ground 1 Sq state. However, the lowest two
surfaces are known to have minimia at bent geome-
example, Eng et al. photodissociate HCN from the
Ž
.
Ž
.
Ž .
000 and 010 vibrational levels of HCN A at an
energy of 6.42 eV and only detect ground state CN
fragments, whereas West and Berry photolyze HCN
at energies of 8 eV, which is above the potential
1
Y
1 X
tries, where they are of A and A symmetries,
respectively, allowing dipole allowed transition from
the ground state. For these transitions the transition
moments are always small and go to zero at the
linear geometry of the ground state, and thus these
transitions are quite weak in absorption.
Ž .
barrier, and observe fluorescence from CN A . West
and Berry determine that the threshold energy for
Ž .
CN A production is 6.88 eV. Therefore, it could be
argued based on the West and Berry findings that the
Ž .
energy minimum for the CN A channel to open up
The observed LIF spectrum demonstrates that
is 0.32 eV instead of 0.4 eV as previously calculated
2
Ž
.
w
x
CN A P, Õs0 is the dominant photoproduct pro-
by Ref. 17 . The West and Berry chemical laser
experiments demonstrate that there is a population
inversion produced between the CN A and X states,
but they could not directly determine the fraction of
population produced directly in the two states.
This investigation deposits 7.2 eV of energy into
duced by 220 nm excitation from the 4n3 state. In
Ž2
.
Ž
.
2
the linear configuration, the H S qCN A P
products correlate with the upper 1,3P states. In a
bent configuration, these products correlate with
lower states of 1,3A and 1,3A symmetries. Previ-
ously, discrepancies appeared in the literature over
the symmetry and the number of excited electronic
states of HCN in the energy region of 6.6 eV
X
Y
Ž
.
HCN and finds that CN A, Õs0 is the dominate
Ž .
product. Based on the above reasoning, CN A would
be expected since the total energy is above the
w
x
Ž .
2,4,5,13,14 . It had been predicted that two states of
energy barrier for CN A production. In comparison
1A symmetry existed in the vicinity of each other
based on an anomaly in anharmonic constants. The
Y
to the isoenergetic studies of West and Berry, the
results are similar. Both studies produce CN A in
low vibrational levels unlike other studies where
photolysis occurs at higher energies and produces
vibrationally excited CN A 2,8,9 . Using a less
state selective technique of flash photolysis, West
and Berry account for the production of CN A as
being due to a predissociative mechanism since bound
to free transitions should have negligible Franck–
Condon density in this spectral region from the
ground vibrational state of HCN. Since the results of
this study are consistent with single photon isoener-
getic studies, it seems reasonable to assume that the
Ž .
1
Y
second assigned A state or ‘B’ state was later
reassigned to a bending progression in the A state
built on a quantum of C–H vibrational excitation
Ž . w
x
1
Y
w
x
15 . As a result, there is only one A state in this
energy region, and it adiabatically correlates to the
Ž .
Ž .
HqCN A products in nonlinear geometries.
1
Y
Ž .
Although the A correlates to the CN A state, it
has two predissociative channels which yield either
Ž .
Ž .
w
x
CN X or CN A products 5,16 . These two dissoci-
ation pathways are rationalized in terms of a poten-
tial energy barrier. Theoretical calculations by Peric
w
x
Ž .
et al. 17 predict that there is a potential barrier of
mechanism to produce CN A in this study is also
1
Y
Ž
.
0.4 eV measured from the A minimum to dissoci-
ation of the CH bond in the A state. This barrier
originates from a conical intersection between the
predissociative. However, we varied the photolysis
wavelength and did not observe structure in the
photodissociation cross-section, which suggests di-
rect dissociation. It is believed that the four quanta of
vibrational excitation extends the C–H bond length
1
Y
1
y
Ž
A A state of HCN which is S in linear geome-
tries and the potential curve leading to CN 2 P,
.