Abstract
A fresh perspective on the interaction of electron and nuclear motions in photon induced dynamical processes can be provided by the coupling of photoelectron angular distributions and cation vibrational states in the photoionization of chiral molecules using circularly polarized radiation. The chiral contributions, manifesting as a forward-backward asymmetry in the photoemission, can be assessed using Photoelectron Circular Dichroism (PECD), which has revealed an enhanced vibrational influence exerted on the outgoing photoelectron. In this paper, we investigate the PECD of a rigid chiral monoterpene, 3-carene, using single-photon vacuum ultraviolet ionization by polarized synchrotron radiation and selecting energies from the ionization threshold up to 19.0 eV. By judicious choice of these photon energies, two factors that influence PECD asymmetry values, electron kinetic energy and ion vibrational level, can be effectively isolated, allowing a clear demonstration of the very marked vibrational effects. A slow photoelectron spectrum is used to examine the vibrational structure of the isolated outermost valence (HOMO) photoelectron band, and peak assignments are made with the aid of a Franck-Condon simulation. Together, these provide an estimate of the adiabatic ionization energy as 8.385 eV. The reported chiral asymmetry from the randomly oriented 3-carene enantiomers reaches a maximum of over 21%. Theoretical PECD calculations, made both for the fixed equilibrium molecular geometry and also modeling selected normal mode vibration effects, are presented to provide further insight.
Highlights
Photoionization of randomly-oriented enantiomers of a chiral molecule by circularly polarized light (CPL) opens up a rich source of information on the dynamics of electrons in the field of the molecular nuclei when the laboratory frame photoelectron angular emission is recorded
The HOMO-1 band of the 3-carene photoelectron spectrum presents very well differentiated peaks that are readily ascribed to vibrational excitation of the cation produced
The measured Photoelectron Circular Dichroism (PECD) spectra show a strong correlation with the position of these photoelectron spectrum (PES) peaks, suggesting a vibrational dependence in the chiral angular distribution of photoelectrons as measured by the PECD technique
Summary
Photoionization of randomly-oriented enantiomers of a chiral molecule by circularly polarized light (CPL) opens up a rich source of information on the dynamics of electrons in the field of the molecular nuclei when the laboratory frame photoelectron angular emission is recorded. Further strong modulations of the PECD asymmetry with vibrational structure of the corresponding molecular photoelectron spectrum (PES) has been noted in several monoterpene (C10H16) isomers,[8] and has been most extensively studied for one of these terpenes, limonene.[23] In this molecule strong fluctuations of the PECD signal over a wide photon energy range were recorded, with peaks that correlate with the major vibrational peaks in the PES HOMO band. Variations in the PECD associated with passage across the eKE axis of a photoelectron spectrum may be attributable both to the simple direct increase in electron energy or to more subtle changes in the dipole matrix elements influenced by a varying vibrational level population. While full vibrational PECD simulations are not currently feasible for systems of this size (with 72 normal modes) extended calculations are reported that consider excitation of a few selected vibrational modes in order to provide a more qualitative insight into the potential influence of electron-nuclei coupling on PECD asymmetries
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