Chemistry Reference
In-Depth Information
4. Triple Excitation
The double excitations are all assigned to the features A-E. What is the feature F?
The lowest 1
π
u
−
420 eV might get some
intensity through the conjugate shake-up mechanism and contribute to band F.
However, the intensity of F seems to be too strong to come only from the conjugate
process. The lowest triply excited state
(5
1
u
)
,
{
1
π
g
shake-up state with the threshold
∼
→
π
∗
}{
(
π
)
2
(
π
∗
)
2
1s
→
}
,or
→
π
∗
}{
(3
σ
g
)
2
(
π
∗
)
2
{
→
}
, is a more probable candidate [32, 71].
The existence of the triple excitation is also discussed not only in recent CFS
spectra of N
2
, but also in recent photon stimulated ion desorption (PSID) of atomic
and molecular ions from diatomic molecules chemisorbed on metal surfaces [72].
Photon stimulated ion desorption is a very sensitive tool to identify dissociative
states, that is, multiply excited states, with small photoabsorption cross-sections.
Feulner et al. [72] reported that the N
+
1s
and N
2
+
PSID signals from chemisorbed
20 eV above the
π
∗
resonance, where there
is no corresponding feature in the total electron yield spectrum that is thought to
be equivalent to the photoabsorption spectrum. The rise in the atomic PSID signals
is interpreted as the onset of the formation of a three-hole state, on the basis of the
previous results for CO/Ru(001) and CO/Cu(111) in the O 1s ionization region
obtained by the same research group [73]. Considering a possible core-level energy
shift in the chemisorbed system, it is plausible that the enhancement of the atomic
PSID yields is attributed to the triple excitations of the feature F having the
1
u
symmetry, which is
N
2
molecules on Ru(001) rise up at
∼
→
π
∗
resonance.
The theoretical calculations [32, 64] have shown that all the double and the
lowest triple excitations involving the 2
σ
u
,3
σ
g
, and 1
π
u
electrons are converging
to the same dissociation limit,
2
D and
2
D (1s hole), and have the minimum potential
energies below the dissociation limit. The dissociation limit is located at 1.8 eV
above the vertical ionization threshold from the ground state; therefore, the lowest
and
doubly excited states A and B, which have the vertical transition energies
of 0.7 eV and the minimum potential energies at
R
∼
∼
18 eV above the lowest 1s
1.3 A, respectively,
are not dissociative. On the other hand, the other doubly and triply excited states
have much shallower potential energy curves and are dissociative after the vertical
excitation. The uppermost state F gives the most efficient repulsive force to the
N-N bond.
∼
1.4 and
B. Core-Rydberg Excitation
Figure 6 illustrates ARPIS spectra for inner-shell excitations in the Rydberg region
near the ionization threshold of N
2
(N 1s). The core-to-Rydberg excited states in
N
2
demonstrate clearly that the
I
0
and
I
90
ion-yield spectra can be regarded as
the
and
polarized spectra, respectively. The Rydberg states have almost the
same molecular geometries as the ionized state to which the Rydberg series is
converging, as shown in Fig. 3. The inner-shell ionized state has a hole in one of
