The use of a broad band tuneable infrared FEL in non-linear optical spectroscopies, allows to overcome the limited spectral range accessible by benchtop ir lasers. The gain in sensitivity achieved for sum-frequency generation studies when using the infrared beam provided by the CLIO FEL is discussed and compared to one of the existing benchtop sum-frequency generation set-ups, accounting for the surface damage by laser heating. The performance of the CLIO FEL-SFG set-up is illustrated by the different research projects.