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Theme 1 colloquium: "High Harmonic Spectroscopy - A new Tool for Investigating Molecular Chirality" (Lecture)

Tuesday 5 March 2019Add to my calendar
from 16:00
dr. Raluca Cireasa (Institut des Sciences Moléculaires d’Orsay, Université Paris Sud, France/Solid State Institute, Technion, Haifa, Israel)

dr. Raluca CireasaThe high-order harmonic generation (HHG) is a strongly nonlinear process that occurs when focussing a laser at intensities of 1013 W/cm2 in a medium (atoms, molecules, clusters, nanoparticles, solids) which subsequently emits coherent radiation consisting of the high-order harmonics of the laser frequency. The spectrum, phase and polarization of the harmonics are sensitive to molecular information, as for example, the structures and symmetries of the highest occupied molecular orbitals, vibrational, rotational and electron dynamics, as well as, molecular alignment/orientation. Thus, the high harmonic spectroscopy (HHS) has been established as a new technique capable of providing structural and dynamical information on the systems used as generation medium with unparalleled temporal (attosecond) and spatial (Å) resolution .

We have explored the sensitivity of the HHG process to a subtle intrinsic orientational aspect of the generating medium: the handedness of chiral molecules. For this, we employed an elliptically polarised laser field (1800 nm) to generate HH in gas-phase enantiopure samples of Fenchone and of Propylene Oxide. The slight disparity in the laser-driven electron dynamics induced in the enantiomers is recorded and amplified by several orders of magnitude in their HH spectra which exhibit an elliptical dichroism (ED) of opposite sign. The ED is particularly large, reaching a magnitude of ~10%, even for ellipticities of albeit 1%! The absolute value of the ellipticity, for which the HH yield is maximum, reflects the chiral electron dynamics induced by the strong laser field. This dynamics occurs during the laser cycle and thus, it is mapped with 100 attosecond resolution, which corresponds to the difference between the emission times of individual HHs. The inherent high temporal resolution and extreme sensitivity to chirality of HHS provide a powerful tool for investigating ultrafast chiral dynamics.

Following new experimental and theoretical models, we are currently devising a HHS method for measuring enantiomeric excess using two colour bi-circular driving fields.

R. Cireasa, A. E. Boguslavsky, B. Pons, M.C.H. Wong, D. Descamps, S. Petit, H. Ruf, N. Thiré, A. Ferré, J. Surarez, J. Higuet, B. E. Schmidt, A. F. Alharbi, F. Legaré, V. Blanchet, B. Fabre, S. Patchkovskii, O. Smirnova, Y. Mairesse and R. Bhardwaj, Nat. Phys. 11, 654 (2015)

dr. Joost Bakker