  The decay of field-free rotational dynamics is experimentally studied in two
complementary methods: laser-induced molecular alignment and
terahertz-field-induced molecular orientation. Comparison between the decay
rates of different molecular species at various gas pressures reveals that
oriented molecular ensembles decay faster than aligned ensembles. The
discrepancy in decay rates is attributed to the coherent radiation emitted by
the transiently oriented ensembles and is absent from aligned molecules. The
experimental results reveal the dramatic contribution of coherent radiative
emission to the observed decay of rotational dynamics and underline a general
phenomenon expected whenever field-free coherent dipole oscillations are
induced.
