The temporal and spectral evolution of soft x‐ray pulses (40–100 A˚) emitted from Al plasma produced by a pair of femtosecond Ti:sapphire laser pulses at normal incidence was studied. Both the soft x‐ray emission and the pulse duration increased with increase in the scale length of the preformed plasma. Prepulse enhanced soft x‐ray emission about 100 times with a pulse duration of 100–130 ps. A spectrally resolved time history revealed that the emission at shorter wavelengths started and decayed relatively more quickly, and emissions at longer wavelengths built up and decayed more slowly. ©1996 American Institute of Physics.