Real-time probing of chirality during a chemical reaction

Abstract

Chiral molecules interact and react differently with other chiral objects, depending on their handedness. Therefore, it is essential to understand and ultimately control the evolution of molecular chirality during chemical reactions. Although highly sophisticated techniques for the controlled synthesis of chiral molecules have been developed, the observation of chirality on the natural femtosecond time scale of a chemical reaction has so far remained out of reach for isolated molecules.Here, we demonstrate a general experimental technique, based on high-harmonic generation in tailored laser fields, and apply it to probe the time evolution of molecular chirality during the photodissociation of 2-iodobutane. These measurements show a change in sign and a pronounced increase in the magnitude of the chiral response over the first 100 fs, followed by its decay within less than 500 fs, revealing the photodissociation to achiral products. The observed time evolution is explained in terms of the variation of the electric and magnetic transition-dipole moments between the lowest electronic states of the cation as a function of the reaction coordinate. These results open the path to investigations of the chirality of molecular reaction pathways, light-induced chirality in chemical processes and the control of molecular chirality through tailored laser pulses. SIGNIFICANCE STATEMENTChiral molecules interact and react differently, depending on their handedness (left vs. right). This chiral recognition is the basic principle governing most biomolecular interactions, such as the activity of drugs or our perception of scents. Inspite of this fundamental importance, a real-time (femtosecond) observation of chirality during a chemical reaction has remained out of reach for isolated molecules. In the present work, we report this fundamental breakthrough with a seemingly unlikely technique: high-harmonic generation (HHG) in tailored intense near-infrared laser fields. Combining the powerful transient-grating technique with HHG in counter-rotating circularly-polarized laser fields, we follow the temporal evolution of molecular chirality during a chemical reaction from the unexcited electronic ground state through the transition-state region to the final achiral products.

Multichromatic Polarization-Controlled Pulse Sequences for Coherent Control of Multiphoton Ionization

“…Commensurable CRCP bichromatic fields exhibit unique propeller shapes with distinct rotational symmetry. Currently, such fields play a prominent role in HHG [67, 68, 70-72, 181, 182], strong-field ionization [158,[183][184][185][186], and were employed for the time-resolved probing of molecular chirality [187]…”

“…Polarization-shaped bichromatic fields are particularly suitable to chiral applications. Recent examples comprise the control of optical chirality [206], fundamental investigations on the PECD of chiral molecules [179,180] and time-resolved studies of molecular chirality [187,207]. Motivated by our experimental findings on the CEP-control of lateral asymmetries in the atomic MPI of sodium atoms by (3ω: 4ω) [73] and xenon atoms by (7ω: 8ω) [189] bichromatic PLP pulses (cf.…”

High-order Nonlinear Dipole Response Characterized by Extreme-Ultraviolet Ellipsometry

“…Moreover, HHG can also be driven in molecular or solid targets, in which the emitted EUV radiation encodes unique information about the electronic and geometric arrangements of the radiating molecules or solid systems. This gives rise to self-probing schemes that allow the performance of molecular tomography [11,12], chirality assignment [13][14][15], and retrieval of the electronic band structure in solids [16].…”

Machine Learning‐Based Optimization of Chiral Photonic Nanostructures: Evolution‐ and Neural Network‐Based Designs

“…While chirality is a fundamental structural property that tells whether or not a structure can be superimposed on its mirror image, the existence of such property gained attraction for applications. Chirality is understood to play an important role in chemical reactions of certain molecules, [ 25 ] characteristic excitation pathways driven by circularly polarized light fields [ 26–28 ] and in the manipulation as well as control of electromagnetic radiation through tailored micro‐ and nanostructures (see literature examples below), which are commonly realized as metamaterials.…”