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 suc h that the surface expanded no more than the peak electron quiver amplitude during excitation. Under these experimentally verified conditions, oblique ly incident, p-polarized pulses uniquely experienced anomalous absorption, proportional to (I lambda(2))(0.64), and as high as 20%. This extra absorpt ion was distinguished from competing pump-induced linear mechanisms by fs-t ime-resolved reflectivity, and agreed quantitatively with essential feature s of Brunel's "vacuum heating," in which light is absorbed by drawing elect rons into the vacuum and sending them back into the plasma with approximate ly the quiver velocity.