Study of the density and temperature dependences of the vibrational Raman transition in compressed liquid N2

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Abstract
Accurate values of linewidth and line shift in the isotropic vibrational Raman spectrum of compressed liquid N2 have been obtained by using inverse Raman spectroscopy. Experiments have been performed for eight isotherms, from the normal boiling point to the critical point temperatures of N2, the number density varying between the value on the coexistence line, and the maximum value of 2.1×1022 cm−3. Minima of the linewidth have been observed above 86 K, showing the increasing influence of vibration–rotation coupling (motional narrowing) competing with the broadening due to pure vibrational dephasing. Moreover, for the first time, maxima of the red line shifts have been found, exhibiting increasing values as temperature increases. As far as the linewidth is concerned, the comparison with two existing models shows that negative interference of the attractive and repulsive forces must be taken into account for obtaining reasonable agreement between the experimental and theoretical parameters. The experimental density dependence of the linewidth can be described well enough with the Schweizer–Chandler model by taking only into account a soft-core, hard-sphere approximation for the calculation of the radial distribution functions. In the case of the frequency shifts, there is only qualitative agreement between the experimental and theoretical density dependences for the effective repulsive contributions calculated within the framework of the Schweizer–Chandler model.