Heating of Thin Foils with a Relativistic-Intensity Short-Pulse Laser

K-shell x-ray spectroscopy of sub-100 nm Al foils irradiated by high contrast, spatially uniform, 150 fs, Iλ²=2×10¹⁸ W μm²/cm², laser pulses is obtained with 500 fs time resolution. Two distinct phases occur: At ≤500 fs a broad feature comparable to the resonance transitions occurs due to satellites, and at ≥500 fs the resonance transitions dominate. Initial satellites arise from a large area, high density, low temperature (~100 eV) plasma created by fast electrons. Thus, contrary to predictions, a short, high intensity laser incident on a thin foil does not create a uniform, hot dense plasma.