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(3d)
4s
(4d)
5s
(5d)
6s
(6d)
7s
Σ
+
3s
2
σ
g
δ
= 1.00
δ
= 1.08
1.02
1.11
1.24
(0.00)
3p
4p
σ
u
+
2
Σ
g
0.74
δ
=0.72
ΔΛ
=0(I
0
)
409.94 eV
Δ
gu
~0.10 eV
3p
2
5p
6p
7p
4p
Σ
+
π
u
g
δ
= 0.80
ΔΛ
=1(
I
90
)
0.78
0.80
0.79
0.78
Σ
+
2
3d
4d
5d
6d
π
g
N
2
0.00
0.02
δ
= 0.01
409.84 eV
N1s Ry
406
407
408
409
410
Photon energy (eV)
Figure 7.
High-resolution ARPIS of N
2
in the 1s
→
Rydberg excitation region. Here
δ
is the
quantum defect.
triple bond in N
2
and C
2
H
2
is shorter than the single and double bonds and the C
and N 1s cores are shallower than the O and F 1s cores.
The resonant photoionization following the (1s
σ
local
g
)
−
1
(
σ
u
)
+
1
shape resonance
in N
2
and C
2
H
2
enhances the (1s
σ
local
)
−
1
ionization channel [60, 90], where
g
(1s
σ
local
g
g
and
loca
u
. On the other hand,
photoexcitation to the core-to-Rydberg excited state should indicate two ioniza-
tion thresholds, though conventional photoabsorption spectra show rather com-
plicated and unresolved Rydberg features. Figures 7 and 8 display ARPIS of the
1s-Rydberg excitation region of N
2
[31, 34, 84, 91] and C
2
H
2
[31, 34, 84, 91, 92].
We can distinguish between parallel (
=
)
−
1
and (1s
σ
local
u
)
−
1
correspond to
local
1
,
)
transitions, that is, between the
σ
- and
π
-type Rydberg states. In these symmetry-
resolved spectra
I
0
and
I
90
, the two ionization thresholds
0
,
) and perpendicular (
=
2
g
2
u
and
with
∼
0.10 eV are determined by using the Rydberg formula for a hydrogen-like system
