Spatiotemporal rotational dynamics of laser-driven molecules

Under normal conditions, molecules in the gas phase rotate freely and their orientation in space is random. Molecules can be of two kinds – symmetric without a preferred direction; or they can have a preferred direction, namely there is a head and a tail which аrе distinguishable.

The scientists from the State Key Laboratory of Precision Spectroscopy at ECNU Shanghai (China), Weizmann Institute of Science (Israel) and Laboratoire Interdisciplinaire CARNOT de Bourgogne (France) reviewed recent developments in the effort to control molecular rotations with ultrashort, femtosecond lasers. The review article is published in Advanced Photonics, Vol.2, Issue 2, 2020 (Kang Lin, Ilia Tutunnikov, Junyang Ma, et al. Spatiotemporal rotational dynamics of laser-driven molecules[J]. Advanced Photonics, 2020, 2(2): 024002).

In the first set of experiments, the scientists applied laser fields to a molecular gas and achieved "molecular alignment". The electric field of the laser first disturbs the electrons, displacing them back and forth from their equilibrium positions in the molecule. Being attached to the nuclei, the latter are forced to lean towards the polarization direction of the field. In order to detect the molecular directionality in space, the scientists applied an additional intense laser field, which completely ripped the electrons off the molecule, followed by the nuclei flying apart due to repulsive forces and then being individually detected. This "Coulomb explosion", when done in high vacuum, provides accurate information on the position of each nucleus at the moment of explosion, and therefore enables visualization of the molecular alignment or orientation. Furthermore, if the polarization of the laser changes with time, the molecules will follow. This enabled the scientists to initiate one-way motion, or "molecular unidirectional rotation" which is also reviewed.

The ability to control the spatial orientation of molecules allows for further studies on the ordered collection of molecules. Just like a patient is required to stay still during, for example, CT or MRI scanning, it is preferred that the molecules are ordered and "still" during molecular imaging. Besides the potential practical utility of the molecular laser control, molecules driven by laser fields represent an interesting and rich system having surprising relations with other fields of physics like solid state physics.

Two additional interesting developments which are discussed are the selective laser control of chiral molecules, and the observation of molecular echoes. Echoes are a fascinating physical phenomenon, and their manifestation in the laser controlled rotational motion is overviewed in this article. Chiral molecules exist in two versions, which are mirror images of each other, just like the palms of our hands. Chirality has been known since the beginning of the 19th century, and Louis Pasteur studied it by observing the specific optical activity of the two different "enantiomers". Since in biology practically all 'life-related' molecules come in only one specific chiral form, chirality plays a major role in modern pharmacology. Different enantiomers of a chiral drug may have strikingly different biological effects, one may cure the disease while the other may be poisonous. The ability to selectively control chiral molecules at the level of individual molecules may pave the way towards extremely sensitive analysis techniques.

While the studies covered here were carried out with relatively small molecules composed of a small number of atoms, when implemented to larger, biologically relevant molecules, many fundamental as well as practical current topics may be addressed.

Molecular fragments detected after Coulomb explosion of sulfur dioxide (SO2). Atoms are color coded: sulfur (S) in yellow, oxygen (O) in red. (a) Schematic representation of the experimental setup. (b) Before the laser pulse the molecules are randomly oriented. (c) After the laser pulse, oxygen atoms are aligned along the X axis, while sulfur atoms are oriented against the Y axis (down).