Chirality reveals
electronic/nuclear
motion couplings
Photoelectron Circular Dichroism (PECD)
is known to be very sensitive to static
molecular structures of chiral systems,
such as isomers or conformers. We show
here that it may also probe dynamical
structures, i.e. the vibrational motion
sampled upon photoionization, as revealed
by a spectacular inversion of the electron
angular distribution asymmetry according
to the final vibrational state of the resulting
cation.
The coupling of electronic and nuclear
motions is a striking manifestation
of the complex many-particle dynamics
involved in off-equilibrium systems, like
excited molecules. Such a coupling may
be revealed, in the case of molecular
electronic transitions, by the breakdown
of the so-called Franck-Condon (FC)
approximation, which assumes that the
probability of a given electronic transition
does not depend on the nuclear geometry
sampled during vibrational motion. In the
case of molecular photoionization, this
means that the electron continuum features
should be decoupled from the vibrational
energy content of the resulting ion.
In this context, chiral molecule systems
can offer an enhanced opportunity
for investigating molecular photoionization,
including vibrational dynamics, even for
randomly oriented samples. Indeed, for
such chiral systems the laboratory-frame
angular distribution of photoelectrons
produced by photoionization with circularly-
polarized light (CPL), can present a strong
forward-backward asymmetry with respect
to the light axis, called PhotoElectron
Circular Dichroism (PECD). Quantitatively,
the PECD asymmetry equals 2
b
1
, where
b
1
, the so-called dichroic parameter which
encapsulates the chiral contribution,
ATOMIC AND MOLECULAR PHYSICS, DILUTE MATTER, UNIVERSE SCIENCE
➊
PES and dichroic
parameter at 10.4 eV
for the HOMO orbital
of S-methyloxirane.
(
a
) raw and (
b
)
Abel-inverted difference
images, obtained with
the VMI spectrometer.
The photon beam
propagates from
the bottom to the top
of this image. (
c
)
VMI-PES. (
d
)
b
1
{+1}
dichroic
parameter curve.
The sticks correspond
to the calculated
vibrational spectrum.
82
SOLEIL
HIGHLIGHTS
2013
