  The process in which light is absorbed by an ensemble of molecules obeys the
fundamental law of conservation of energy - the energy removed from the light
resides in the molecular degrees of freedom. In the process of coherent
emission from excited molecules known as free-induction decay (FID),
spectroscopic measurements of the emitted radiation are often conducted in
order to gain insight into molecular structure and behavior. However, the
direct influence of the FID emission on its molecular source is not measured
directly. In this work we present experimental evidence from the molecular
rotor perspective of the consequences of terahertz-frequency FID emission from
rotationally excited molecules. We show that when gas phase molecules
transiently orient under field-free conditions, the energy radiated via FID is
manifest as an abrupt reduction in excited rotational populations. The
connection between coherent FID emission and stored energy leaves a
particularly distinct signature in our measurements, but the results are
generalizable throughout coherent spectroscopy and coherent control.
