Chemistry Reference
In-Depth Information
1.0
N
2
1s-
π
g
0.8
0.6
I
0
0.4
I
90
0.2
0.0
401
402
Photon Energy (eV)
Figure 4.
The ARPIS of N
2
in the 1s
→
π
∗
excitation region.
→
π
∗
and
1s
→
σ
∗
2.
1s
The lowest 401-eV peak in Fig. 2 corresponds to the excitation from the ground
state to
(1
1
ux
)
S
(1
σ
u
,
x
∗
), based on a single electron picture. Figure 4
presents vibrationally resolved ARPIS spectra for the N 1s 1
σ
u
→
=
1
π
g
excita-
tions of N
2
. This transition is perpendicular and the 90
◦
ion yield spectrum I
90
is
dominant. A small contribution in the 0
◦
ion yield
I
0
comes from its acceptance
angle and the imperfection in the linear polarization of the incident light. The rel-
ative energy of the
π
∗
excitation from the ionization threshold
E
th
is
∼
9 eV. This
means the
π
∗
state is well separated from the Rydberg region, as shown in Fig. 3,
where, in general, the lowest Rydberg state does not exist
>
5 eV below
E
th
. The
1
σ
u
→
1
π
g
excited state is of pure valence character.
Figure 5 indicates the
and
symmetry resolved spectra near the K-shell
ionization threshold of N
2
(
E>
405 eV) above the
π
∗
energy region (
E
=
401 eV)
in the photoabsorption spectrum shown in Fig. 2. In Fig. 5, a very strong and
broad enhancement is found at
419 eV in the
-symmetry spectrum, and is
definitely assigned to the
σ
∗
shape resonance. The 3
σ
u
orbital with a positive
orbital energy is not uniquely defined without any boundary condition, as shown
in Fig. 3. There are some theoretical methods to get PECs even for continuum
resonances and to evaluate the interaction between the valence and the continuum
states; for example, the R-matrix approach [63]. The
σ
∗
resonance has two features:
∼
