Femtosecond rotational dynamics of D${}_2$ molecules in superfluid helium nanodroplets

Rotational dynamics of D${}_2$ molecules inside helium nanodroplets is induced by a moderately intense femtosecond (fs) pump pulse and measured as a function of time by recording the yield of HeD${}^{+}$ ions, created through strong-field dissociative ionization with a delayed fs probe pulse. The yield oscillates with a period of 185 fs, reflecting field-free rotational wave packet dynamics, and the oscillation persists for more than 500 periods. Within the experimental uncertainty, the rotational constant BHe of the in-droplet D${}_2$ molecule, determined by Fourier analysis, is the same as Bgas for an isolated D${}_2$ molecule. Our observations show that the D${}_2$ molecules inside helium nanodroplets essentially rotate as free D${}_2$ molecules.