  Localized heating of a gas by intense laser pulses leads to interesting
acoustic, hydrodynamic and optical effects with numerous applications in
science and technology, including controlled wave guiding and remote atmosphere
sensing. Rotational excitation of molecules can serve as the energy source for
raising the gas temperature. Here, we study the dynamics of energy transfer
from the molecular rotation to heat. By optically imaging a cloud of molecular
superrotors, created with an optical centrifuge, we experimentally identify two
separate and qualitatively different stages of its evolution. The first
non-equilibrium "gyroscopic" stage is characterized by the modified optical
properties of the centrifuged gas - its refractive index and optical
birefringence, owing to the ultrafast directional molecular rotation, which
survives tens of collisions. The loss of rotational directionality is found to
overlap with the release of rotational energy to heat, which triggers the
second stage of thermal expansion. The crossover between anisotropic rotational
and isotropic thermal regimes is in agreement with recent theoretical
predictions and our hydrodynamic calculations.
