Experimental identification of "vacuum heating" at femtosecond-laser-irradiated metal surfaces

Aluminum and iron targets were irradiated by intense (I less than or equal to 10(15) W/cm(2)), 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 (I lambda(2))(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.