  A moderately intense $450$ fs laser pulse is used to create rotational wave
packets in gas phase $\rm{I_2}$ molecules. The ensuing time-dependent
alignment, measured by Coulomb explosion imaging with a delayed probe pulse,
exhibits the characteristic revival structures expected for rotational wave
packets but also a complex non-periodic substructure and decreasing mean
alignment not observed before. A quantum mechanical model attributes the
phenomena to coupling between the rotational angular momenta and the nuclear
spins through the electric quadrupole interaction. The calculated alignment
trace agrees very well with the experimental results.
