Unravelling coupled
electronic and nuclear
angular momenta
In this highlight we show how isotopically
resolved photoelectron imaging can be used
to isolate the pure electronic dynamics
of a photoionization process. For atoms
with nuclei possessing a magnetic moment
(nonzero nuclear spin) the interaction
of the nucleus with the electrons leads
to a “blurring” of the photoelectron image
which can cloud our view of the electron
dynamics. Detecting electrons in coincidence
with spin zero ions allows us to circumvent
this problem which, when combined with
theory, allows a greater understanding
of the complex electron dynamics going
on during ionization.
Measurement of Photoelectron Angular
Distributions (PADs) provides information
on the electron dynamics occurring during
a photoionization process. One way
to increase the amount of information
contained in the experiment is to first
prepare the target in a well-defined
photoexcited state that will then be ionized
with a second photon, leading to more
information rich PADs than observed
in single photon ionization. Comparison
of these PADs to theoretical predictions
provides a very stringent test of the theory
used. However, for atoms with nuclei with
a magnetic moment (nonzero nuclear spin,
I
≠0), the interaction of the nucleus with the
magnetic field generated by the electrons,
as illustrated in Figure
➊
, leads to a very
significant blurring of the PAD in spite
of the small size of the nucleus’ magnetic
moment. The result is that our picture
of the electronic dynamics is obscured.
Here we have applied an electron/ion
coincidence scheme which allowed
us to observe isotope dependent PADs
for the first time. Using this method
we selected only electrons emitted
from atoms with no nuclear spin (
I
=0)
by filtering out the other isotopes, thus
allowing us to see an unclouded picture
of the PADs.
The system chosen to demonstrate this
technique is the two-photon ionization
of the Xenon atom, which has been
performed by combining synchrotron
radiation (h
ν
SR
) from the DESIRS beamline
with the light from a visible dye laser
(h
ν
Laser
). The process can be described
as follows:
(1)
Xe 5p
6
(
J
=0) + h
ν
SR
→
Xe 5p
5
5d (
J
=1) +
h
ν
Laser
→
Xe 5p
5
4f (
J
=2)
→
Xe
+
+
ε
p
1/2
(
J
=1,2)/Xe
+
+
ε
p
3/2
(
J
=0,1,2)/Xe
+
+
ε
f
5/2
(
J
=1,2)/ Xe
+
+
ε
f
7/2
(
J
=2)
where
ε
p and
ε
f are possible outgoing
electron waves and the subscripts are
the angular momenta of outgoing
electrons. The complex PADs are formed
by the interplay of all of the open channels
shown above. For
129
Xe (
I
=1/2) and
131
Xe
(
I
=3/2), which form 26% and 21% of
the natural isotope composition of Xe,
the situation is further complicated by
the coupling of
I
and
J
in the Xe 5p
5
5d
(
J
=1) state, which leads to a very strong
distortion of the PAD. An example of this
is shown in Figure
➋
where the PAD from
the
129
Xe atom is compared with that
of the
132,134,136
Xe atoms (all
I
=0 nuclei)
for electrons emitted in the process
described in eq. (1). It can clearly be seen
that the PAD from the nonzero nuclear spin
atoms is much more isotropic than the
unperturbed zero spin isotopes.
These PADs can be described
quantitatively by the following formula:
(2)
I(
θ
) = n (1 +
β
2
P
2
(cos
θ
) +
β
4
P
4
(cos
θ
))
where
θ
is the angle between the electric
field vector of the light and the direction
of emission of the photoelectron and
P
n
are the Legendre polynomials. The
measurements of
β
2
and
β
4
by recording
photoelectron images while tuning the
optical laser energy across the Xe 5p
5
5d (
J
=1) + h
ν
Laser
→
Xe 5p
5
4f (
J
=2)
resonance are shown in Figure
➌
together
with the results of a number of theoretical
models. Only the multiconfiguration
Hartree Fock (MCHF) model including
J
=0 and 2 channels are able to describe
the experimental data while simpler
models fail.
ATOMIC AND MOLECULAR PHYSICS, DILUTE MATTER, UNIVERSE SCIENCE
76
SOLEIL
HIGHLIGHTS
2013
