Coherent control of molecular rotation in superfluid helium

We experimentally demonstrate control of molecular rotation in bulk superfluid ${}^4\mathrm{H}\mathrm{e}$. Metastable helium dimers, ${\mathrm{H}\mathrm{e}}_2^{\ast }$, are rotationally excited by a periodic train of linearly polarized femtosecond laser pulses. We show that the degree of rotational excitation of ${\mathrm{H}\mathrm{e}}_2^{\ast }$ can be enhanced or suppressed by varying the period of the pulse train, whereas the directionality of molecular rotation can be controlled by the relative angle between the polarization vectors of pulses in the train. The experimental results are in agreement with numerical calculations, based on a simple model, in which ${\mathrm{H}\mathrm{e}}_2^{\ast }$ molecules do not interact with the superfluid.