Experimental Identification of “Vacuum Heating” at Femtosecond-Laser-Irradiated Metal Surfaces

Aluminum and iron targets were irradiated by intense ( I≤10¹⁵W/cm²), 120 fs laser pulses with sufficiently high contrast such that the surface expanded no more than the peak electron quiver amplitude during excitation. Under these experimentally verified conditions, obliquely incident, p-polarized pulses uniquely experienced anomalous absorption, proportional to (Iλ²)0.64, and as high as $20%$. This extra absorption was distinguished from competing pump-induced linear mechanisms by fs-time-resolved reflectivity, and agreed quantitatively with essential features of Brunel's ``vacuum heating,'' in which light is absorbed by drawing electrons into the vacuum and sending them back into the plasma with approximately the quiver velocity.