Chemistry Reference
In-Depth Information
the axial recoil mechanism is valid even for stretching mode couplings, but fails
for bending mode couplings, and the angular distribution of the fragmentation
following the inner-shell excitation should be related to the molecular vibration,
but not to the equilibrium geometry [99]. Thus, the ARPIS technique is powerful to
investigate vibronic couplings in inner-shell excited states as summarized below:
1. Stretching mode coupling:
Molecular symmetry unchanged.
Anisotropic ion fragmentation (
,
) for linear and planar molecules.
Isotropic ion fragmentation for highly symmetric molecules (e.g.,
T
d
and
O
h
).
2. Bending mode coupling:
Molecular symmetry lowered due to vibronic couplings, such as Renner-
Teller, Jahn-Teller, conical intersection,
...
Incomplete anisotropic ion fragmentation for linear and planner
molecules. Incomplete isotropic ion fragmentation for highly symmet-
ric molecules.
−
σ
∗
)
2. Conical Intersection
(1s
The lowest feature at 288 eV observed in the
I
0
spectrum cannot be assigned
only to the 1s
σ
u
→
3s
σ
g
Rydberg transition, considering that the 1s
σ
u
→
3s
σ
g
excited state should show nearly the same
ν
2
progression as the 1s
σ
g
→
3p
π
u
and
3p
σ
u
states. Below the ionization threshold of C
2
H
2
, however, we must consider a
transition to the lowest
σ
∗
orbital of the three
σ
∗
molecular orbitals, 3
σ
u
,4
σ
g
, and
4
σ
u
, whereas N
2
and CO have no
σ
∗
state below the thresholds. Thus, it is probable
that the other contribution at 288 eV may arise from the C 1s
σ
g
→
3
σ
u
valence-
type excitation. The 3
σ
u
orbital is of
σ
CH
antibonding character, but not of
σ
CC
character. As demonstrated in Fig. 10, the vibrational fine structure in the 1s
σ
g
→
3
σ
u
state is assigned to the symmetric C-H stretching mode (
ν
1
), considering
that the 3
σ
u
orbital is of the C-H antibonding character and the vibrational fine
structure at 288 eV in C
2
H
2
shows an apparent isotope shift in C
2
D
2
[20, 100].
Furthermore,
→
3s
σ
g
/
3
σ
u
(
1
u
) region. The
I
90
peak energy at 288 eV is larger by 80 meV than the
corresponding
I
0
peak energy. The most probable mechanism is the vibronic cou-
pling in the dipole-forbidden C 1s
σ
u
→
it
is
interesting
to
observe
the
I
90
yield
in
the
C
1s
3
σ
u
valence state (
1
g
) coupled with the
1
π
g
excited state (
1
u
) through the
ν
5
(
π
u
) mode [92, 98], where
lowest 1s
σ
u
→
1
π
g
excited state has a stable bent geometry due to the Renner-Teller
effect and the potential energy curve has a double well for the bent angle. In the case
of the 3
σ
u
and 1
π
g
excited states, they will fall into the b
2
symmetry through the
ν
5
cis bending motion (
π
u
). Why can the excited state of C-H antibonding character
the 1s
→
