Observing molecular spinning via the rotational Doppler effect

When a wave is reflected from a moving object, its frequency is Doppler shifted. Similarly, when circularly polarized light is scattered from a rotating object, a rotational Doppler frequency shift may be observed, with manifestations ranging from the quantum world (fluorescence spectroscopy, rotational Raman scattering and so on) to satellite-based global positioning systems. Here, we observe for the first time the Doppler frequency shift phenomenon for a circularly polarized light wave propagating through a gas of synchronously spinning molecules. An ensemble of such spinning molecules was produced by double-pulse laser excitation, with the first pulse aligning the molecules and the second (linearly polarized at a 45angle) causing a concerted unidirectional rotation of the 'molecular propellers'. We observed the resulting rotating birefringence of the gas by detecting a Doppler-shifted wave that is circularly polarized in a sense opposite to that of the incident probe.