Chemistry Reference
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presented that the photoion angular distribution is represented by the same formula
as Eq. (3), and the net photoion asymmetry parameter is expressed in terms of the
partial transition strengths for the dipole-allowed
←
and
←
transitions.
The authors have measured the angular distribution of energetic protons from three
repulsive states (2p
σ
u
,2p
π
u
, and 2s
σ
g
)ofH
2
produced by the photoionization with
the 304- A radiation, and have demonstrated that the measurements of the photoion
angular distribution make it possible to partition the excitation probability between
degenerated ionization channels [38]. Their theoretical and experimental results
strongly suggest that the photoion angular distribution measurements of diatomic
molecules can be used for an identification of the symmetry of discrete excited
states, as well as a symmetry decomposition of continuum states, as long as the
axial-recoil condition is satisfied.
Molecular K-shell photoabsorption processes leading to both discrete and con-
tinuum states are intrinsically anisotropic, since photoexcited states have definite
symmetries and degenerate ionization channels with different symmetries can be
enhanced by shape resonances. Such an anisotropy is reflected in the angular distri-
bution of the products created either from the initial core hole state or from states
following the instantaneous decay of the initial state. Processes, such as photo-
electron ejection, Auger electron emission, fluorescence, and photodissociation,
all potentially provide the information about the symmetry of the initial excited
state as discussed in early publications by Dill and co-workers [39-41].
The asymmetry parameter for the angular distribution of fragment ions is de-
fined as the difference between the photoabsorption strengths (
D
2
) for molecular
orientation parallel (
=
0
,
) and perpendicular (
=
1
,
) to the electric vector
of the light:
2
D
2
−
D
2
D
2
+
β
=
(5)
·
D
2
2
Because the photoabsorption cross-section
σ
is the sum of the photoabsorption
strengths, that is,
σ
=
·
D
2
)
/
3(
α
is the fine structure constant
and
hν
is the photon energy), the differential cross-section for the direction parallel
to the electric vector gives the
-symmetry component of the photoabsorption
cross-section [as derived from Eqs. (3) and (5)]:
4
π
2
αhν
(
D
2
+
2
θ
=
0
◦
=
dσ
d
3
4
π
σ
(6)
and the differential cross-section for the direction perpendicular to the polarization
vector gives the
-symmetry component of the photoabsorption cross-section:
θ
=
90
◦
=
dσ
d
3
4
π
σ
2
(7)
