Overheating of a femtosecond plasma in freely suspended ultrathin carbon films

Overheating of a femtosecond laser plasma was observed when carbon films 10 — 50 nm thick were irradiated with pulses of 200 fs duration and of intensity in excess of 10¹⁵ W cm^–2. An increase in the plasma temperature from 150 to 700 eV enhanced almost threefold the x-ray yield of the plasma. This effect was caused by limited heat exchange with the bulk of a target during a time of the order of the laser pulse duration.