Mapping the Evolution of Optically-Generated Rotational Wavepackets in a Room Temperature Ensemble of D${}_2$

A coherent superposition of rotational states in D${}_2$ has been excited by nonresonant ultrafast (12 femtosecond) intense (2 $\times$ 10${}^{14}$ Wcm${}^{-2}$) 800 nm laser pulses leading to impulsive dynamic alignment. Field-free evolution of this rotational wavepacket has been mapped to high temporal resolution by a time-delayed pulse, initiating rapid double ionization, which is highly sensitive to the angle of orientation of the molecular axis with respect to the polarization direction, $\theta$. The detailed fractional revivals of the neutral D${}_2$ wavepacket as a function of $\theta$ and evolution time have been observed and modelled theoretically.