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Spectroscopy, Quantum Dynamics and Electroweak Parity Violation in Chiral Molecules

Final Report Summary - SPEQUACHIRAL2 (Spectroscopy, Quantum Dynamics and Electroweak Parity Violation in Chiral Molecules)

This project approaches one of the most fundamental questions of molecular physics, concerning the role of parity violation in chiral molecules, with important consequences for physical stereochemistry. The traditional point of view assigns exactly equal ground state energies to the enantiomers of chiral molecules. However, with the discovery of parity violation in physics we expect a small “parity violating” energy difference D, corresponding to an in principle measurable reaction enthalpy. Because D is expected typically in the subfemto eV range these energies were in the past considered to be immeasurably small. Recent theoretical and experimental progress initiated by our group has led to order of magnitude larger predicted values for D and has also made rovibrational line assignment of optical spectra of chiral molecules possible (Quack, Stohner, Willeke, Ann. Revs. Phys. Chem. 2008). Thus the outlook to carry out successful experiments on D is now good, following a scheme previously published by us. The present project follows in detail several logical steps for these experiments, which have been carried out as described in the following.

1. We have successfully built a new experimental set up for the preparation of well-defined “parity states”, which in chiral molecules would correspond to a new kind of “parity isomers” existing as time dependent superposition states of other-wise stable R and S (or P and M) isomers never before prepared. The working of the experiment using two powerful OPO’s (lasers) and sensitive REMPI (laser) detection was demonstrated with the achiral molecule ammonia, for which new hyperfine couplings in excited vibrational states were measured. The sensitivity achieved allows an estimate that parity violating energy differences D of about 100 aeV should be measurable.

2. Detailed theoretical simulations were carried out for the simple chiral molecule ClOOCl including for the first time both the very small parity violation from the electroweak interaction, multidimensional tunneling and the influence of electromagnetic fields designed for the population transfer in the preparation of the well-defined “parity states”. The frequencies necessary for experiments on this molecule are, however, not available to our lasers.

3. We have developed an efficient and accurate theoretical method to predict electroweak parity violation in chiral molecules in terms of the coupled cluster linear response approach.

4. We have developed experimental and theoretical techniques to measure and analyse spectra of moderately complex achiral and chiral molecules including the interplay of tunneling and rovibrational motion.

5. We have studied numerous chiral molecules by theory with the goal to select suitable molecular candidates, of which only two were, however, available by synthesis.

6. We have synthesized these two candidate molecules, studied them by detailed theory and preliminary high resolution spectroscopic analysis. Future more detailed spectroscopic analysis should allow us to identify the tunneling-rotational quantum states suitable for the parity selection and measurement of the parity violating energy difference D in these molecules predicted to be between 0.1 feV and 1 feV (femto electron Volt).