Chemistry Reference
In-Depth Information
total
I
I
90
C
2
H
2
C 1s
1s-
π
*
1s-
σ
*
1s-3s
×
4
Figure 9.
The ARPIS of
C
2
H
2
in the 1s
→
π
∗
excitation re-
gion. The 1s
→
π
∗
excited state has
a bent stable structure due to the
Renner-Teller effect.
284
285
286
287
288
Photon Energy (eV)
B. Core-Valence Excitation
−
π
∗
)
1. Renner-Teller Effect (
1s
→
π
∗
→
π
∗
Figure 9 shows ARPIS of the C 1s
excitations of C
2
H
2
.IntheC1s
0
◦
, as well
as 90
◦
. That is, fragment ions with a momentum orthogonal to the linear molecular
axis are emitted upon fragmentation following the Auger decay. If the core-to-
π
∗
excited states are stabilized by having a bent geometry, the bending modes are
greatly excited, and if the fragmentation takes place during the bending motion,
the fragment ions can have a rather large momentum orthogonal to the molecular
axis [96]. This finding is rationalized by the Renner-Teller effect [27, 29, 97, 98].
The Renner-Teller effect in the C 1s
perpendicular (
=+
1) transition, fragment ions are observed at
θ
=
→
π
∗
excited state is investigated in detail
for isovalent molecules with a center carbon atom: CS
2
, OCS, and CO
2
[29, 34],
where the twofold degeneracy of the
π
∗
excited state in the linear polyatomic
molecule is removed by the vibronic coupling with bending vibrations, and the
core-to-
π
∗
excited state is split into the core-to-
π
in
(in-plane
π
∗
) and
π
out
(out-of-
plane
π
∗
) excited states with bent and linear equilibrium geometries, respectively.
The Renner-Teller splitting is, however, not observable in most cases of inner-
shell absorption spectroscopy due to the lifetime broadening. Generally speaking,
