Chemistry Reference
In-Depth Information
partial electron-yield spectra for the electron kinetic energy of
384 eV to identify
the doubly excited state in N
2
[67]. The 384-eV band, which was assigned to the
atomic-like Auger decay after ultrafast fragmentation in a dissociative doubly
excited state [65], has been assigned to the molecular Auger decay during the
vibration around a long equilibrium bond length of a weakly bound, doubly excited
state [68]. The atomic or molecular decay is dependent on the relative energy
position between the vertical excitation energy and the dissociation limit, as well as
on the dissociation speed; that is, the atomic and molecular Auger decays take place
in the case of the excitation energy that is higher and lower than the dissociation
limit, respectively. In any case, it is obvious that the CFS spectrum of the 384-eV
band provides the information on the doubly excited states.
In Fig. 5, features labeled from A to F near and above
E
th
, which may arise
from multiple excitations, are clearly seen in the
- and
-symmetry spectra. In
the
-symmetry spectrum (
I
0
), a very weak feature B just above
E
th
andalow-
energy shoulder structure E of the shape resonance are distinctly observed. On the
other hand, the
-symmetry spectrum (
I
90
) shows feature A just above
E
th
and
clear enhancement F just at the
σ
∗
shape resonance position, in addition to the
well-known double-excitation feature C and D at
∼
∼
415 eV [69, 70]. Note that the
∼
415-eV feature in the photoabsorption spectrum of Fig. 2 is composed of not only
the
-symmetry components C and D, but also the
-symmetry component E.
In the 384-eV electron yield spectrum [67], the three broad peaks assigned to the
doubly excited states show intensity maxima at
410
.
5, 414, and 416 eV. The
energy positions of the
-symmetry features A, C, and D are in good agreement
with them. In addition, two weak and broad features with the
symmetry, B
and E, which have been predicted theoretically, are detected [32]. Thus, the three
((1s
∼
→
π
∗
)(
π
→
π
∗
) doubly excited states with main CSFs,
(2
1
u
),
(3
1
u
),
and
(4
1
u
), are assigned to A, C, and D, and the two (1s
σ
)(3
σ
g
/
2
σ
u
)
(
π
∗
)
2
doubly excited states,
(1
1
u
) and
(2
1
u
), are assigned to B and E. There is
no other doubly excited state involving the 2
σ
u
,3
σ
g
, and 1
π
u
electrons, and the
doubly excited state involving the 2
σ
g
electron is very high in energy.
Note that fine structures are observed in C and D in the
-symmetry resolved
spectrum
I
90
in Fig. 5. The repulsive state cannot give fine structures, such as
molecular vibrations. As discussed by using the potential energy curves [62, 64]
and by comparing with the optical data of core-equivalent molecule NO [17], the
vibrational side bands of the Rydberg shake-up states converging to the lowest
shake-up ionized state at
→
419 eV are the most probable candidate for the fine
structures. Based on the potential energy curves [64, 67], the lowest shake-up state
associated with the N 1s ionization and simultaneous
π
→
π
∗
∼
excitation has an
1.3 A, and the Rydberg shake-up states involving
equilibrium bond length of
∼
Rydberg and simultaneous
π
→
π
∗
theN1s
→
excitation can be mixed with the
→
π
∗
and simultaneous
π
→
π
∗
excitation
double excitations involving the N 1s
to gain their intensities.
