Structural Markers of the Frenkel Line in the Proximity of Widom Lines

We
have performed a neutron scattering experiment on supercritical
fluid nitrogen at 160 K (1.27 TC) over
a wide pressure range (7.8 MPa/0.260 g/mL–125 MPa/0.805 g/mL).
This has enabled us to study the process by which nitrogen changes
from a fluid that exhibits gaslike behavior to one that exhibits rigid
liquidlike behavior at a temperature close to, but above, the critical
temperature by crossing the Widom lines followed by the Frenkel line
on pressure (density) increase. We find that the Frenkel line transition
is indicated by a transition to a regime of rigid liquidlike behavior
in which the coordination number remains constant within error, in
agreement with our previous work at 300 K. The Frenkel line transition
takes place at approximately the same density at 160 and 300 K. The
data do not conclusively show an additional transition at the location
of the known Widom lines. We find that behavior remains gaslike until
the Frenkel line is crossed and our data support the hypothesis that
Widom line transitions are density increase-driven.