Coherent spin–rotational dynamics of oxygen superrotors

Abstract

We use state- and time-resolved coherent Raman spectroscopy to study the rotational dynamics of oxygen molecules in ultra-high rotational states. While it is possible to reach rotational quantum numbers up to $N\approx 50$ by increasing the gas temperature to 1500 K, low population levels and gas densities result in correspondingly weak optical response. By spinning ${{{\rm O}}_{2}}$ molecules with an optical centrifuge, we efficiently excite extreme rotational states with $N\leqslant 109$ in high-density room temperature ensembles. Fast molecular rotation results in the enhanced robustness of the created rotational wave packets against collisions, enabling us to observe the effects of weak spin–rotation coupling in the coherent rotational dynamics of oxygen. The decay rate of spin–rotational coherence due to collisions is measured as a function of the molecular angular momentum and its dependence on the collisional adiabaticity parameter is discussed. We find that at high values of N , the rotational decoherence of oxygen is much faster than that of the previously studied non-magnetic nitrogen molecules, pointing at the effects of spin relaxation in paramagnetic gases.

Keywords

• molecular superrotors
• spin–rotation coupling
• optical centrifuge
• quantum decoherence
• rotational spectroscopy