  The surface of an insulating material irradiated by a beam of low energy
electrons charges positively if the yield of secondary electron is greater than
unity. For such a dynamical equilibrium, the thermodynamic properties have been
investigated by measuring the surface potential in response to a temperature
oscillation of the material. It is shown that an oscillation amplitude of 0.4 K
at 530 K induces an oscillation of the surface potential of about 0.5 volts.
The frequency dependence indicates a monotonous decrease in the response with
decreasing frequency, extrapolating to zero at zero frequency. We propose that
this modification of the surface charge is driven by the temperature dependence
of a gas of charged particles in equilibrium with the vacuum level.
