  We have studied in detail the transition from gas-like to rigid liquid-like
behaviour in supercritical $N_2$ at 300 K (2.4 $T_C$). Our study combines
neutron diffraction and Raman spectroscopy with empirical potential structure
refinement and ab-initio molecular dynamics simulations. We observe a narrow
transition from gas-like to rigid liquid-like behaviour at ca. 150 MPa, which
we associate with the Frenkel line. Our findings allow us to reliably
characterize the Frenkel line using both diffraction and spectroscopy methods,
backed up by simulation, for the same substance. We clearly lay out what
parameters change, and what parameters do not change, when the Frenkel line is
crossed.
