Orientation of jet-cooled polar molecules with an intense single-cycle THz pulse

Abstract

We report the direct observation of the orientation of jet-cooled molecules induced by a THz pulse. The intense THz field ($\ensuremath{\sim}$30 kV/cm) creates a superposition between the rotational ground state ($J=0$) and the excited state ($J=1$) of HBr molecules. The rotational dynamics is probed by Coulomb explosion imaging of the fragment ions produced by a near-infrared femtosecond laser pulse. Comparing with the simulations based on a time-dependent Schr\"odinger equation, the rotational wave packets are completely reconstructed, which reveal that $\ensuremath{\sim}$$7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ of the ground-state population is transferred to the $J=1$ state at an initial temperature of 8 K, achieving a degree of orientation of $\ensuremath{\langle}cos\ensuremath{\theta}\ensuremath{\rangle}\ensuremath{\sim}0.017$.