Chemistry Reference
In-Depth Information
Ry
N
2
1s-
σ
*
1.0
I
90
N1s
-1
E
I
0
C
0.5
D
F
A
B
0.0
410
420
430
Photon Energy (eV)
Figure 5.
The ARPIS of N
2
in the 1s
→
σ
∗
excitation region. Features A-F are assigned to
double and triple excitations.
3
σ
u
excited state is well defined below the
E
th
region, namely, for the region of a longer bond length.
valence and continuum. The 1
σ
g
→
3. Double Excitation
It is of fundamental importance to reveal what types of multielectron excitations
are accessible through the single photon excitation and what dynamics follow
the multielectron excitation in free, condensed, and chemisorbed molecules. An
important aspect of the multiply excited states in adsorbed molecules is the local-
ization of positive charges after the subsequent Auger-type decay, which may lead
to particularly efficient ion desorption from the solid surface. The doubly or triply
excited state is much more repulsive than the ionized and singly excited state [64],
since the excited electrons in antibonding orbitals can increase dissociative char-
acter. However, it is generally hard to identify the multiple excitations embedded
in the ionization continuum.
The photoabsorption cross-section close to
E
th
(N 1s) contains a significant
contribution from double excitations [65]. The double excitation is essential from
the viewpoint of electron correlation in molecules [66]. Neeb et al. [65] distinctly
found a double-excitation feature at only 0.6 eV above
E
th
(
N
1
s
)
=
409
.
9eV in
addition to the well-known double excitations at
415 eV, as shown in Fig. 2. They
initiated the constant final state (CFS) spectroscopy applied to repulsive states not
easily accessible in conventional photoabsorption; that is, they have measured the
∼
