Free rotation of D₂ molecules in superfluid helium nanodroplets

Laser-induced molecular alignment was recently extended to molecules in superfluid helium nanodroplets. Studies on in-droplet OCS, CS₂ and I₂ molecules show that the effective rotational constant B_He is smaller, and the centrifugal distortion constant D_He is much larger than those of the gas-phase molecules. On the other hand, the IR spectroscopy studies show that the D_He and B_He values have an empirical correlation D_He = 0.038B_He^1.88. For lighter molecules, the B_He/B_gas ratio tends towards 1, notably when B_gas > 1 cm^-1. How do light molecules rotate inside superfluid helium nanodroplets? We answer this question by investigating the rotational dynamics of in-droplet D₂ molecules induced by a moderately intense femtosecond pump pulse. By measuring the time-dependent yield of HeD⁺, created through strong-field dissociative ionization with a probe pulse, the rotational coherence was traced to persist for longer than 100 ps (500 rotational periods). Our results show that the B_He and D_He values of the in-droplet D₂ molecules are the same as those for the isolated D₂ molecules, and the empirical correlation between B_He and D_He does not apply to D₂ molecules. Our experiment demonstrates that D₂ molecules inside superfluid helium nanodroplets essentially rotate as free D₂ molecules.