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Probing molecular chirality using high harmonic generation


Probing molecular chirality using high harmonic generation

Summary:

Molecules driven out of equilibrium by intense, ultrashort laser pulses are of central importance in many areas of science and technology. In such a non-equilibrium situation, a non-adiabatic coupling exists between electrons and ions which can induce charge and energy transfer across the molecule on a femtosecond timescale. However, since the interaction itself commences with the coupling between the electrons and the laser pulse, a process that occurs on the attosecond timescale, recent work has concentrated on controlling the electron dynamics directly through the use of attosecond pulses. Understanding and controlling these intra-molecular electron transfer processes is therefore fundamental to many chemical processes and key to future ultrafast technologies: examples include the design of electronic devices, probes and sensors, biological repair and signalling processes and development of optically-driven ultrafast electronics. Of particular interest in many of these processes and technologies is the effect of chirality. Chiral molecules lack an internal plane of symmetry and thus have non-superimposable mirror images (one form is called left-handed while the other is called right-handed). Enantiomers (pairs of chiral molecules) often interact with biological systems differently. A classic example of this is Thalidomide. Thalidomide is chiral and one form can cure morning sickness while the other causes birth defects. It is therefore important to know which isomer is present in a particular sample of chiral molecules. The goal of this work is to understand if the interaction of ultrashort laser pulses with chiral molecules can be used for the identification of enantiomers. We aim to study this using high-harmonic spectroscopy. High-harmonic generation (HHG) is a highly nonlinear process in which an atom or molecule absorbs photons from a laser field and emits high-energy photons at multiples of the laser frequency. In this project we will use a non-adiabatic quantum molecular dynamic (NA-QMD) method for studying the response of the molecules to intense laser pulses. At the heart of the NA-QMD approach is a time-dependent density functional theory (TDDFT) treatment of the electronic dynamics together with a classical description of the nuclear dynamics. This apprach is implemented in a parallel code called EDAMAME (Ehrenfest DynAMics on Adaptive MEshes) [1]. This code has been recently used to study HHG in benzene [2].

[1] D. Dundas, J. Chem. Phys. 136: 194303 (2012)
[2] A. Wardlow and D. Dundas, Phys. Rev. A 93: 023428 (2016)