## 2 Measurements of the _THz_ Spectrum of Noble Gases up to the 1990s
### 1964-1967: Seeking for a Triplet Shape at Mesoscopic Scales  
The use of a bare hydrodynamic description of the spectrum becomes questionable when approaching the mesoscopic regime, as therein the matter is no longer continuous, nor stationary. However, there are solid reasons to assume a suitably generalized hydrodynamic theory to hold validity even in this time-space domain. To illustrate this point, it is useful to recognize that for a system at densities typical of the liquid phase, the mean free path spans the \(10^{-10}\) m window, potentially becoming much smaller than the atomic size. Under these conditions, the movements permitted to the atoms are mainly short, vibration-like, cage oscillations (\(\approx\)\(10^{-13}\) s). Consequently, even in the (\(Q\approx\) nm\({}^{-1}\), \(\omega\approx\)_Th_z_) mesoscopic range, the response of the system is still \"averaged\" over a large number of elementary dynamic interactions, which is a necessary pre-requisite for a suitably generalized hydrodynamic description. In this perspective, the persistence of spectral features in the _Th_z_ spectrum reminiscent of Brillouin hydrodynamic peaks did not appear to be a groundless expectation. Indeed, several _INS_ experiments performed during the mid-1960s aimed at validating this hypothesis by examining inelastic peaks in the spectrum of simple fluids. For instance, this is the case of an _INS_ measurements on dense Ne, Ar and D\({}_{2}\)[31], which clearly documented the presence of \"extended hydrodynamic\" modes in the spectrum. Due to the largely coherent cross section of these samples, both inelastic and quasi-elastic spectral features appeared in the measured spectrum. In particular, it was observed that at low \(Q\) values the inelastic shift of the side peaks approached from above the linear hydrodynamic law \(c_{s}Q\) predicted by Equation (3a). This urged the authors to interpret these peaks as the finite-\(Q\) extensions of the acoustic Brillouin peaks to mesoscopic scales. Conversely, [PERSON] _et al._[32] reported no direct evidence of inelastic structures, however they inferred the existence of phonon like excitations indirectly from a comparison between liquid and solid phase spectra. The discrepancy between the results of [PERSON] and those of [PERSON] is a likely consequence of the different incident wavelength used in the two experiments (4.1 A and 5.3 A respectively), which made the two sets of data hardly comparable.  
In a successive _INS_ work, [PERSON] _et al._[33] measured the spectrum of Ar in liquid (at \(T\) = 94 K and 102 K) and solid (at \(T\) = 68 K and 78 K) phases. The authors observed a linear \(Q\)-dependence of the inelastic peak at the lowest \(Qs\), with a slope consistent to the adiabatic sound velocity. Furthermore, they showed that, at higher \(Qs\), the sound dispersion curve of the liquid sample vaguely resembles the phonon dispersion curve of the solid, thus suggesting that the local pseudo-periodicity of the liquid structure gives rise to quasi-periodic zones reminiscent of the Brillouin zones of a crystal. Since clear signatures of acoustic excitations in the liquid were found in a \(Q\) range equivalent to the second pseudo-Brillouin zone, the authors concluded that at least two somehow \"loose\" pseudo-Brillouin zones could be identified in liquid argon. A similar conclusion was reached by an earlier _INS_ investigation on liquid Pb [34], as well as a computer simulation on Rb [35]. The latter work used an inter-particle potential model characterized by an oscillatory decay to zero [36] instead of the Lennard-Jones potential routinely used to simulate noble gases. These differences in the interatomic interaction explain why in liquid metals inelastic modes are well-resolved up to almost the position of first diffraction peak (\(Q_{m}\)), whereas in noble gases they become overdamped for \(Q>Q_{m}\)/2.