Highly Sensitive Non-linear Probes of Chiral Structure and Dynamics
Final Report Abstract
The project developed the theoretical foundations of a novel approach to observing coherent excited state dynamics in chiral molecules, photoexcitation induced photoelectron circular dichroism (PXECD). It was shown that the photoionization of randomly oriented chiral molecules prepared, by a circular polarized pump pulse, in a coherent superposition of excited states can, upon ionization by a linear probe pules, give access to the underlying chiral coherent dynamics (see the joint experimental and theoretical work [S1], where the analytic theory of PXECD was developed, and the purely theoretical work in which it was further generalised to arbitrary polarization and propagation directions of the exciting and ionizing pulses). In photoelectron circular dichroism (PECD), which consists of single photon ionization using circular polarised light, chirality manifests as asymmetric photoelectron emission in the forwards/backwards direction (relative to the laser propagation direction). The same is true in PXECD, but the asymmetric photoemission is additionally contingent on coherence. This exclusive dependence on coherence can also be seen in a different part of the photoelectron angular distribution, where it is not contingent on the chirality of the molecule, thus allowing extension of PXECD’s sensitivity to tracking coherence to non-chiral molecules. The general theory clearly and simply separates chiral and non-chiral contributions to the time dependent photoelectron angular distribution. PXECD was shown to originate from an orientation imposed by the first pulse by inducing a net dipole in the ensemble that oscillates with angular frequency determined by the energy spacing of the excited states. This is in contrast to one-photon PECD where chiral asymmetric emission emerges as a result of the orientation imposed by the ionizing pulse. In the more general case, where both pulses have a circular component, there is a mixing of PXECD terms, that require coherent population of excited states, with PECD-like terms that do not rely on such coherencies. In the case of two linearly polarized pulses, forwards/backwards asymmetry can still be seen in chiral molecules when the polarization of two pulses are neither parallel nor perpendicular, and change sign with a change in angle between the polarizations of π/2. This does not require coherent population of multiple states. PXECD is a background-free probe of coherent bound dynamics providing individual access to its chiral and non-chiral contributions and allows for the possibility of the ultrafast probing of these dynamics with intense X-ray sources e.g. free-electron lasers, leading to few femtosecond time resolution of both chiral and non-chiral charge dynamics.
Publications
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General theory of photoexcitation induced photoelectron circular dichroism. The Journal of Chemical Physics, 149(6):064104, 2018
A. G. Harvey, Z. Mašín, and O. Smirnova
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Photoexcitation Circular Dichroism in Chiral Molecules. Nat. Phys., 14:484–489, 2018
S. Beaulieu, A. Comby, D. Descamps, B. Fabre, G. A. Garcia, R. Géneaux, A. G. Harvey, F. Légaré, Z. Mašín, L. Nahon, A. F. Ordonez, S. Petit, B. Pons, Y. Mairesse, O. Smirnova, and V. Blanchet
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Ultrafast imaging of laser-controlled non-adiabatic dynamics in NO2 from time-resolved photoelectron emission. Phys. Chem. Chem. Phys., 21:10038–10051, 2019
M. Richter, J. González-Vázquez, Z. Mašín, D. S. Brambila, A. G. Harvey, F. Morales, and F. Martín
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UKRmol+: A suite for modelling electronic processes in molecules interacting with electrons, positrons and photons using the R-matrix method. Computer Physics Communications, 249:107092, 2020
Z. Mašín, J. Benda, J. D. Gorfinkiel, A. G. Harvey, and J. Tennyson
