Observing molecular spinning via the rotational Doppler effect

When a wave is reflected from a moving object, its frequency is Doppler shifted 1 . Similarly, when circularly polarized light is scattered from a rotating object, a rotational Doppler frequency shift may be observed 2 , 3 , with manifestations ranging from the quantum world (fluorescence spectroscopy, rotational Raman scattering and so on 3 , 4 ) to satellite-based global positioning systems 5 . 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 45° angle) causing a concerted unidirectional rotation of the ‘molecular propellers’ 6 , 7 . 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. The rotational Doppler frequency shift is observed for a circularly polarized lightwave propagating through a gas of synchronously spinning molecules by using a linearly polarized pulsed laser beam to align diatomic molecules and a linearly polarized pulse to induce concerted unidirectional rotation.