Chemistry Reference
In-Depth Information
spectrum. Thus, the initial increase of the cross-section is followed by a sudden
decrease, after which the cross-section grows again. The broad tail to the lower
energy is mostly due to the
σ
∗
(D) state of dissociative character. The intensity
for the excitation to the dissociative state or repulsive state
σ
∗
(Q) is now reduced
∼
540 eV due to the intensity transfer from the
σ
∗
(Q) to the 3p(Q) state near
the crossing point S1 (Fig. 17). When tuning the photon energy at feature B, the
1s
2p
3
P excited state
with no curve crossing, as shown in Fig. 15, resulting in atomic Auger decay [103,
114, 115].
→
σ
∗
(D) state is possibly dissociated to the atomic O 1s
→
2. Higher
σ
∗
Region
In contrast to the mixing related to the D channel, the higher and stronger 1s
→
σ
∗
(Q) state can interact with several Rydberg states with the same ion core Q,
especially,
n
p
σ
(Q) series with the same orbital character. In the
I
0
spectrum,
the
n
p
σ
(Q) Rydberg and
n
p
σ
(D) series show single peaks similarly to the
n
p
π
(Q) and
n
p
π
(D) Rydberg series, except that the 3p
σ
(Q) Rydberg transition
cannot be assigned to a single peak. The potential energy curves shown in Fig. 15
[26] indicate that the 1s
→
σ
∗
(Q) repulsive state can strongly interact with the
1s
3p
σ
(Q) Rydberg state in the FC region. Then, a bound state (upper state)
is produced through the avoided curve crossing between
σ
∗
(Q) and 3p
σ
(Q) and
several vibrational excitations become allowed. Thus, the 3p
σ
(Q) Rydberg state
is strongly perturbed by the repulsive
σ
∗
(Q) state with the same ion core, and
has an unusual vibrational enhancement as clearly observed in the
I
0
spectrum
in Fig. 14.
A similar Rydberg-valence mixing is more or less expected in other
n
p
σ
Ryd-
berg states; in fact, the
n
p
σ
(Q) Rydberg series is much stronger than the
n
p
σ
(D)
Rydberg series. Due to strong Rydberg-valence mixings through the avoided curve
crossings in the Q channel, the repulsive 1s
→
→
σ
∗
(Q) excited state cannot be iso-
→
σ
∗
lated from the Rydberg states. The 1s
(Q) excitation disperses its intensity
→
n
p
σ
(Q) Rydberg series, which has bound-state character, and
reduces dissociative character. Therefore, the molecular Auger decay is observed
when tuning the photon energy at feature C.
mainly to the 1s
VI. ARPIS OF SO
2
A. Core-Valence Exchange versus Spin-Orbit Interactions
The spin-orbit (SO) splitting of the core electrons is directly observed in XPS.
Even in the 2p photoabsorption spectra, the 2p SO splitting of third-row and heavier
elements is large and is easily distinguishable. The singlet and triplet 2p excited
states are strongly and indistinguishably mixed with each other through SO or jj
