An experiment was designed to measure sum-frequency generation (SFG) spectr
a in both reflection and transmission. The result from an octadecyltrichlor
osilane monolayer on a substrate reveals that while the resonant spectra in
the two geometries both originate from the surface monolayer, the bulk sub
strate has a significant contribution to the nonresonant background in the
transmitted spectra. Study of a polyethylene film also shows that the bulk
contribution to the SFG spectra is only significant for the transmission ge
ometry, exhibiting a resonant mode not observable in the reflected spectra.