Determination of chiral asymmetries in the valence photoionization of camphor enantiomers by photoelectron imaging using tunable circularly polarized light
L. Nahon, G. A. Garcia, C. J. Harding, E. A. Mikajlo and I. Powis J. Chem. Phys. 125(2006), 114309. An electron imaging technique has been used to study the
full angular distribution of valence photoelectrons produced
from enantiomerically pure molecular beams of camphor when these
are photoionized with circularly polarized light. In addition to
the familiar beta parameter,
this
provides a new chiral term, taking the form of an additional
cosine function in the angular distribution which consequently
displays a forward-backward electron ejection asymmetry.
Several ionization channels have been studied using
synchrotron radiation in the 8.85 – 26 eV photon energy
range. With alternating left and right circularly polarized
radiations the photoelectron circular dichroism (PECD) in the
angular distribution can be measured and shows some strong
dynamical variations with the photon energy, depending in sign
and intensity on the ionized orbital.
For
all orbitals the measured PECD has a quite perfect antisymmetry
when switching between R and S enantiomers, as expected from
theory. In the HOMO-1 channel the PECD chiral asymmetry curves
show a double maxima reaching nearly 10% close to threshold, and
peaking again at ~20% some 11 eV above threshold. This is
attributed to a resonance that is also visible in the beta
parameter curve.
Newly optimized CMS-Xα photoionization dynamics calculations are also presented. They are in reasonably good agreement with the experimental data, including in the very challenging threshold regions. These calculations show that PECD in such randomly oriented samples can be understood in the electric dipole approximation and that, unlike the case pertaining in core-shell ionization – where a highly localized achiral initial orbital means that the dichroism arises purely as a final state scattering effect – in valence shell ionization there is a significant additional influence contributed by the initial orbital density. ©2006 American Institute of Physics
Chiral signatures in angle-resolved valence photoelectron spectroscopy of pure glycidol enantiomers
Gustavo A. Garcia, Laurent Nahon, Chris J. Harding and Ivan Powis Phys. Chem. Chem. Phys., 10(2008), 1628-1639.Photoionization of the chiral molecule glycidol has been
investigated in the valence region. Photoelectron circular
dichroism (PECD) curves have been obtained at various photon
energies by using circularly polarized VUV synchrotron
radiation and a velocity map imaging technique to record
angle-resolved photoelectron spectra (PES). The measured
chiral asymmetries vary dramatically with the photon energy as
well as with the ionized orbital, improving the effective
orbital resolution of the PECD spectrum with respect to the
PES. Typical asymmetry factors of 5% are observed, but the
peak values measured range up to 15%.
The experimental results are
interpreted by continuum multiple scattering (CMS-Xα)
calculations for several thermally accessible glycidol
conformers. We find that a nearly quantitative agreement
between theory and experiments can be achieved for the
ionization of several molecular orbitals. Owing to the
sensitivity of PECD to molecular conformation this allows us
to identify the dominant conformer. The influence of
intramolecular hydrogen bond orbital polarization is found to
play a small yet significant role in determining the chiral
asymmetry in the electron angular distributions.
Whats So Special?
A new form of CD — Photoelectron Circular Dichroism (PECD) — has been pioneered in Nottingham by the development of experimental and theoretical methods. PECD is already present in the pure electric-dipole approximation for the radiation-matter interaction approximation (unlike the "normal" absorption CD that requires higher order, and weaker, electric quadrupole or magnetic interaction terms) leading to unprecedentedly large chiral asymmetries in the photoelectron angular distributions that range up to several tens of percent — many orders of magnitude greater than encountered in absorption CD spectroscopy.
But there are other new features — PECD develops from the continuum electron phase in a significantly different manner than the dipolar β parameter, and so provides a very much more sensitive probe of the molecular photoionization dynamics as well as of the molecular potential, leading to a very high sensitivity to molecular conformation and to the chemical environment.
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