Starting from the general momentum and energy balance equations for ch
arged particle swarms in a gas, as furnished by momentum-transfer theo
ry, we obtain expressions for mean velocity and mean energy of an elec
tron swarm in an r.f. electric field under spatially uniform condition
s, in the frequency range omega tau(e) > 1, where tau(e) is the energy
collisional relaxation time. If tau(e) is a decreasing function of en
ergy, it is shown that the cycle-averaged mean energy reaches a maximu
m at a certain frequency. Physical arguments are provided to support t
his result and the prediction is verified for a constant elastic cross
section model by direct numerical solution of Boltzmann's equation.