Soft Report - Dipartimento di Fisica - Sapienza

Tackling the λ-transition in liquid Sulphur by InfraRed PhotonCorrelation SpectroscopyDeveloping a novel experimental technique, we appliedInfrared Radiation Photon Correlation Spectroscopy(IRPCS) to liquid sulphur in the λ transition region, i.e.where an abrupt increase in viscosity by four orders ofmagnitude has been observed upon heating within fewdegrees [1]. According to basic paradigms, thisincrease in viscosity around T λ=159 °C should inprinciple trigger an equivalent increase of thestructural relaxation time τ α from the ps to the nsrange, according to the Maxwell equation η=G ∞·τ α, G ∞being the limiting short wavelength shear modulus.This should imply, in turn, the detection of a relaxationeffect crossing the GHz frequency domain by means oftechniques, like Brillouin light scattering (BLS), thatmeasure the longitudinal elastic modulus, and a largeincrease in attenuation at ultrasonic frequencies. BLShas shown [2], on the contrary, a continuous smoothdecrease in the real and imaginary part of themodulus, testifying a decrease of τ α. Ultrasonicexperiments performed in the 1-10 MHz frequencywindow [3] did non evidence, as well, any criticalvariation of the acoustic attenuation around T λ;sulphur thus escaped the established viscoelasticframework for over a century, warranting the title of “viscous but not viscoelastic liquid” [3]. IRPCS wasperformed using a diode pumped solid state lasersource operating at λ = 1064 nm. The detector was aPerkin Elmer avalanche photodiode retaining a 2%quantum efficiency at the probe wavelength, with a 50counts/s dark count and an after pulse probabilitylower than 3%. The 90° scattered radiation wascollected by a lens and collimated by means of an IRoptimized collimator from OZ-optics. The sample waskept in a home-made furnace whose temperature wasmeasured by a RTD and stabilized by a software PIDfeedback. The digital signal coming out from the• Fig. 1: Reduced homodyne IR correlationfunctions at selected temperatures. Opensymbols are for TT λ. The temperature behaviour of thedetector was acquired by a PC and used to performreal time autocorrelation. Correlation functions werecollected at constant temperatures during an upscan inthe range 145