Chemistry Reference
In-Depth Information
→
σ
∗
excitation in S
2
[110] and Se
2
[111] belongs to Case B,
where the exchange interaction involving the core electron, K(
c, π
∗
) and K(
σ
∗
,c
),
are much smaller than the intravalence exchange interaction K(
σ
∗
,π
∗
).
The exchange splitting in the 1s
hand, the deeper 1s
→
σ
∗
excited state of O
2
is greatly different
from that in the core-ionized states (1.1 eV). The reported exchange splitting ranges
from
0.4 eV [21, 26, 112, 113], depending on theoretical methods used,
where the negative value means that
σ
∗
(Q) is higher in energy than
σ
∗
(D). This
energy inversion arises from relatively large exchange interactions both between
the 1s
σ
and
σ
∗
electrons K(
σ
∗
,c
) and between the intravalence electrons K(
σ
∗
,π
∗
)
as discussed above; whereas the exchange splitting in the ionized state arises from
a weak interaction between the 1s
σ
and
π
∗
electrons K(
c, π
∗
).
−
1.6 to
−
B. Rydberg-Valence Mixing
Figure 14 shows symmetry-resolved ion-yield spectra of high resolution in energy
around features B and C of O
2
, as shown in Fig. 12. The
π
-type Rydberg series has
no evidence of this valence
π
∗
1)
spectrum. On the other hand, the
σ
-type Rydberg series is obscured by strong
σ
∗
resonance features, B and C, in the
I
0
ion-yield (
,
=
contribution in the
I
90
ion-yield (
,
=+
0) spectrum. We must
take account of the Rydberg-valence mixing or avoided curve crossing between
the same symmetry states, as shown in Fig. 15.
In the
I
90
(
) spectrum, several Rydberg peaks converging to the
4
−
(D)
and
2
−
(D) core-ionized states, Rydberg (Q) and Rydberg (D), are resolved.
Considering that the exchange splitting in the Rydberg series is nearly the same
1.2
1.0
0.8
0.6
0.4
0.2
0.0
3s'
σ
3p'
σ
4p'
σ
1.2
O
2
3s
3p
4p
3d
σ
σ
σ
σ
-
2
Σ
-
4
Σ
1.0
I
90
I
0
0.8
B
C
3p'
π
3d'
π
4p'
π
0.6
3p
3d
4p
4d
π
π
π
π
-
2
Σ
0.4
-
4
Σ
0.2
0.0
538
540
542
544
546
Photon Energy (eV)
High-resolution ARPIS of O
2
in the 1s
→
3
σ
u
and Rydberg excitation region.
Figure 14.
