is anti-symmetric with the swapping
of either the enantiomer or the light
handedness.
At DESIRS, we investigated the case
of methyloxirane, recording the mass-
filtered photoelectrons on a position
sensitive detector via a Velocity Map
Imaging (VMI) spectrometer that provides
in a multiplex way the Angle-Resolved
Photoelectron Spectrum (AR-PES).
From the radial distribution in the total
electron image we obtain the PES, while
from the lCPL-rCPL difference images
the dichroic parameter,
b
1
, can be
obtained. The difference images (panel
a and b of the figure) readily reveal
a marked vibrational structure in the
radial (i.e. electron energy) distribution,
associated with strong forward-backward
asymmetries in the angular distribution.
Dramatic changes in the sign of the latter
for some adjacent vibrational ring patterns
can also be seen in the alternating color
mapping.
The vibrational structure identified in the
PES (panel c) - the origin band (peak “a”)
and a series of single quantum excitation
of skeletal deformations (peaks “b” to “f”)
- exhibit very distinctive and remarkable
dichroic behavior, with
b
1
values (panel d)
ranging from a null value for peak “a”, to
a negative value of -0.03 for “c”, and then
switching sign within a few tens of meV to
a positive ~ +0.02 value for peaks “d” and
“e”. Phenomenologically, this sign change
in
b
1
means that the forward/backward
asymmetry reverses direction for adjacent
vibrational modes. This striking behavior
of asymmetry flipping upon vibrational
(adjacent) excitation is not influenced
byany continuum resonance, and has
never been observed in any molecular
photo-ionization experiment. Moreover,
such a dependence of PECD with the
vibrational energy content of the residual
cation is completely unexpected within
the usual FC assumptions.
This dramatic vibrational effect is probably
due the known sensitivity of PECD to static
geometric structures, which would be
somehow replicated in dynamic structural
changes occasioned by the vibrational
motion sampling different regions
of the nuclear configuration space.
This photoionization vibrational effect
in PECD appears more remarkable than
vibrational effects measured in the past
on fixed-in-space molecules [1], yet it has
been achieved without requiring molecular
orientation because of the
intrinsically
chiral nature of the targets. In this context,
PECD appears to offer a powerful, and
(for chiral species) universally applicable
probe of vibrational dynamics in molecular
photoionization, even from randomly
oriented targets. This, of course, is the
“natural” situation for the ubiquitous chiral
molecules in the biosphere.
Besides its fundamental interest, this
finding may have important consequences
for the interpretation of PECD in an
analytical context, with the quickly-
growing field of laser-based multi-photon
PECD experiments whose outcomes may
be driven by vibrational dynamics in both
the intermediate and final state.
DESIRS Beamline
ASSOCIATED PUBLICATION
Vibrationally induced inversion of photoelectron
forward-backward asymmetry in chiral molecule
photoionization by circularly polarized light
G. Garcia, L. Nahon*, S. Daly and I. Powis
Nature Communications 4 (2013) 2132
*Synchrotron SOLEIL, l’Orme des Merisiers,
St Aubin BP48, 91192 Gif sur Yvette Cedex,
France
CORRESPONDING AUTHOR
REFERENCE
[1] T. Jahnke et al., Phys. Rev. Lett. 93 (2004),
083002
➋
The (S) Methyloxirane molecule
83
SOLEIL
HIGHLIGHTS
2013
