Parity violation in chiral molecules – PVCM

The objective of the PVCM proposal is to make the first observation of parity violation (PV) in chiral molecules. Although the weak nuclear force is known to violate parity, its effects have never been observed in molecular systems. It is expected to cause a minute difference in energy levels between two enantiomers of a chiral species. Using laser spectroscopy, we intend to measure the resulting frequency shift in their vibrational spectra.

Theoretical chemistry predicts PV shifts in chiral heavy metal complexes to be on the order of 1 Hz for transitions at 30 THz. Although this level of sensitivity is challenging, it can be attained by 2-photon Ramsey interferometry on a molecular beam. Several promising candidate species have been synthesized in solid form. However, our current beam setup is ill-suited for vaporizing solid-phase molecules, the tuning range of our high precision spectrometer is too narrow for studying the variety of molecules of interest and our current detection sensitivity is rather poor.

I therefore propose to develop a new cutting-edge experiment specifically designed for ultra-high resolution vibrational spectroscopy of new complex molecules. The experimental setup will comprise an ultra-stable quantum cascade laser (QCL) based Ramsey interferometer calibrated against primary frequency standards, a new detector which measures state populations by free induction decay of rotational transitions and a buffer-gas-cooled molecular beam. The use of QCLs allows the study of any species showing absorption between 10 and 100 THz. The new detector will increase our sensitivity while opening new possibilities for coherently manipulating chiral molecules. Buffer-gas-cooled molecular beams formed using laser ablation of solid-state molecules exhibit both low velocity and some of the highest beam fluxes to date, making them very attractive for high resolution spectroscopy.

There are many reasons for attempting this difficult measurement. A successful PV measurement will undoubtedly shed light on the origins of biomolecular homochirality. It can also constitute a test of the standard model of the universe in the low-energy regime or, in fact, a probe of physics beyond it. It could help elucidate unanswered questions in nuclear physics and will serve as a stringent benchmark for computational protocols used in relativistic quantum chemistry. Parity violation aside, the technological developments proposed in this project are, in and of themselves interesting and important. They will allow complex polyatomic molecules to be studied at an unprecedented level of precision. They are at the forefront of cold molecule research, in terms of beam sources, coherent manipulation techniques, sensitive detection schemes and spectroscopic tools. With such techniques, we envision new possibilities for using polyatomic molecules to perform further precision measurements of importance for fundamental physics, such as testing other fundamental symmetries and postulates as well as measurements of fundamental constants or their variation in time. They also constitute a generic set of technologies of interest for atmospheric or interstellar physics, chemistry, biology, as well as for medical or industrial diagnostics.