The “Doppler effect”, a phenomenon the name of which is part of everyday language, especially for its acoustic implications: the frequency of the sound wave emitted by a moving object - a vehicle, for example - and seen by an observer will be different depending on the speed and direction of movement of the object. Also, in the case of acoustic waves, this phenomenon is used, for example, in medicine for measuring blood flows. On a considerably larger scale, and in the case of electromagnetic waves, it is used to measure the velocity of celestial bodies (by analyzing the emitted light).
Atoms and molecules in the gas phase also move. As with light emitted by stars, the value of the kinetic energy of the emitted electrons, measured by photoelectron spectroscopy, will vary, depending on the speed and direction of movement of the atoms or molecules emitting them. In the gas phase, the thermal motion of molecules is random and thus leads to a broadening of the peaks, called translational Doppler broadening.
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Going through the motions. A rotating nitrogen molecule (N2) emits an electron when hit by an x-ray photon. The rotation can either boost or detract from the electron energy, which leads to a wider range of energies for detected electrons than a stationary molecule would give. This effect has now been detected in the electron energy spectrum.
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This effect is known and has been taken into account for decades by spectroscopists when analyzing their results. This was not the case, however, for another Doppler broadening effect, rotational this time, i.e. related to the rotation of molecules. An extremely weak effect compared to the high kinetic energies measured when shining the samples with X-rays. The resolution available on the PLEIADES beamline is such that it has been possible, for the first time in the world, to show clearly this rotational Doppler broadening on the nitrogen N2 molecule. This study, carried out in collaboration with groups from Universities in Oregon and New York (USA), Turku (Finland) and Sendai (Japan), is a small revolution (!) in the spectroscopy world. The study has been published in Physical Review Letters.
Illustration of the Doppler effect
© Charly Whisky – Wikipedia
References:
Experimental observation of rotational Doppler broadening in a molecular system
T. D. Thomas, E. Kukk, K. Ueda, T. Ouchi, K. Sakai, T.X. Carroll, C. Nicolas, O. Travnikova and C. Miron, Phys. Rev. Lett. 106, 193009 (2011)
Rotational Doppler Effect in X-ray Photoionization
Y.-P. Sun, C.-K. Wang and F. Gel'mukhanov, Phys. Rev. A82, 052506 (2010)
Phys. Rev. Focus: "Tumbling of molecules affects experiments"
American scientist : Miniscule Speed Traps
Science Daily: Doppler Effect Found Even at Molecular Level -- 169 Years After Its Discovery
Futura Sciences: L'effet Doppler rotationnel moléculaire enfin observé
1Photoemission: ejection of an electron by photoelectric effect, resulting from the interaction between light and matter. See also the following article about Professor Svante Svensson
