The origin of the luminescence from porous Si (PS) is currently under intense scrutiny. Characterisation of this material on the atomic scale is difficult and optical techniques offer certain advantages. Ultrahigh vacuum studies of PS by optical second-harmonic generation (SHG) are presented. Excitation wavelengths corresponding to surface state and a suspected strain resonance from clean Si surfaces are used. Hydrogen-terminated and clean surfaces of Si(100) and PS are compared. The H-terminated surfaces give no signal under the excitation conditions used. On desorbing the hydrogen, a stable SH signal is obtained from the PS which has polarisation dependences different to those of clean Si(100), and which also differ between the surface and strain resonances.
