We report high pressure measurements of the half-width and shift of the vibrational Raman spectrum of nitrogen in the homogeneous helium-nitrogen fluid binary mixture. The measurements have been performed using a diamond anvil cell in a large range of pressure p, temperature T, and composition x including the critical region. In approaching the critical line a considerable broadening has been observed which is shown to be due to critical concentration fluctuations. This proves that the order parameter distribution can explicitly be determined by a general experimental method.
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