  Analyzing the first equation in the BBGKY chain of equations for an
equilibrium liquid-gas system, we derived the analytical expression for the
atom work function from liquid into gas. The coupling between the atom work
function from liquid into vacuum and the stability criterion of liquid in
limiting points of the first type was shown (using I.Z. Fisher classification).
As it turned out, Fisher/s criterion corresponds to the condition of
atomization. We have expressed the state equation in terms of the atom work
function from liquid into vacuum and performed calculations of the limiting
line of stability composed of limiting points of the first type for argon. Our
model discovers an interesting effect of the negative atom work function: at a
constant volume of liquid, on a temperature rise (and also at a fixed
temperature and decreasing specific volume of liquid) the atom work function
drops and takes a negative value with a modulus that is significantly larger
than the atomic thermal energy. We propose a new two-stage mechanism of
sonoluminescence based on non-thermal processes in liquid in a state with a
negative atom work function. The first stage includes the emission of atoms
from the interior of the bubble into gas at hyper-thermal velocities. At the
second stage a collision of emitted flow takes place between the gas atoms
along with the implosion of the central part of the bubble. As a result of the
impact excitation, ionization and the subsequent recombination, the flash of
electromagnetic radiation that can be seen in sonoluminescence experiments
develops.
