Collisional versus collisionless resonant and autoresonant heating in laser-cluster interaction.

When a hot cluster expands, a transient matching between the plasma frequency and the laser frequency has been predicted, observed, and analyzed recently. The associate energy transfer to the electrons has been described as an enhanced collisional absorption. However, for hot plasmas the collision frequency is small and a collisionless resonant heating is more efficient. We set up and solve the problem of resonant collisional and collisionless cluster heating taking into account cluster expansion, laser pulse duration and pulse chirping. Moreover, we identify an efficient autoresonant mechanism of collisionless heating with a chirped laser pulse when the crossing between the plasmon frequency and the laser frequency is degenerate and the time derivatives of these two frequencies are equal at the crossing time. Transition between collisional regime of cluster heating and collisionless one is discussed.