The photoluminescence emitted by porous silicon under conditions of co
ntinuous (Ar+ laser) and pulsed (N2 laser) photoexcitation at temperat
ures 4.2-300 K have been studied. The spectrum of the photoluminescenc
e generated by continuous excitation (''static'' spectrum) contained a
wide structure-free band. The time-resolved spectra (time resolution
20 ns) differed fundamentally from the static spectra. Two luminescenc
e bands were emitted by porous silicon. The short-wavelength band A1 w
ith a maximum at 2.5 eV was observed in the nanosecond range of delay
times (0-70 ns). The long-wavelength band A2 shifted with increasing d
elay time toward the red part of the spectrum and became dominant afte
r > 100-ns delays. The observed transformation of the spectra with tim
e was due to a strong dependence of the carrier lifetime on the diamet
er of a quantum wire, because porous silicon could be regarded as a se
t of such wires; it was also due to migration of excitations.