Bulk contribution from isotropic media in surface sum-frequency generation

We present systematic conceptual and experimental investigations of the bulk contribution to surface sum-frequency generation (SFG) from an isotropic medium. A practical method for the analysis of spectra obtained in a transmission geometry is presented which allows to estimate the bulk contribution in any future application. A systematic degeneracy due to the molecular nature of the material is found which makes a subtle interpretation of transmission experiments necessary. Practical consequences for the analysis of typical SFG spectra are discussed.