Time-dependent dynamics of nuclear spin symmetry and parity violation in dichlorodisulfane (ClSSCl) during and after coherent radiative excitation

Abstract

Parity and nuclear spin symmetry are approximate symmetries, which for many primary processes in molecular spectroscopy and kinetics lead to almost exact constants of the motion. The symmetric isotopomer dichlorodisulfane (disulfur dichloride) is chiral with two enantiomers, each of which has also two nuclear spin isomers (ortho and para isomers). Electroweak quantum chemistry predicts a small ‘parity violating’ energy difference between the ground states of the enantiomers (M being more stable than P by about ). Furthermore recent microwave spectroscopy has indicated ‘forbidden’ transitions between ortho and para isomers. These phenomena allow for a comparative study of the time-dependent quantum dynamics of parity and nuclear spin symmetry violation in a chiral molecule for the first time. We report quantum dynamical calculations of coherent radiative excitation of the complex manifold of polyads with hyperfine structure in ClSSCl. We demonstrate almost complete (near 100%) transfer of population from an almost pure para-ground state to an almost pure excited ortho state. Short-pulse excitation from the para-ground state to an excited time-dependent para chromophore state shows intramolecular interconversion between para and ortho nuclear spin isomers under isolation after the pulse on a time scale of a few nanoseconds to a few microsecond depending on the excited polyad. We also demonstrate the preparation of exotic ‘chameleon states’, which are superpositions of energy eigenstates corresponding to ortho- and para- nuclear spin symmetry isomers at the same time. The results are discussed in relation to the strong separation of time scales between the primary processes of nuclear spin symmetry isomerisation (nanoseconds to microseconds) and parity change (seconds) and in relation to possible experiments to measure in this molecule. We also compare with the rate processes of nuclear spin symmetry isomerisation induced by thermal black body radiation, depending on temperature, occurring on time scales from less than seconds to more than 100 years.