Second order vibrational spectroscopy is nowadays an indispensable tool in surface science. Due to the special selection rules of second order nonlinear processes these laser-based techniques (e.g. sum frequency generation spectroscopy) are for many samples interface specific and allow for non-destructively studying interfacial processes by exclusively probing the vibrational fingerprint of interfacial molecular species. Despite the success of these techniques the obtained nonlinear spectra often contain insufficient information to unambiguously interpret the spectroscopic results and to obtain a clear physical picture of the interfacial region under investigation. This deficiency originates from the high complexity of interfacial systems which typically goes beyond the presence of a simple interfacial atomic layer.

In order to grasp this complexity, we have recently developed a new spectroscopic approach that combines the simultaneous acquisition of the phase resolved sum (SFG) and difference frequency responses (DFG) of a sample in the time domain. With this method we increase the number of complementary spectroscopic observables which allows us to get deeper insight into the complex physical properties of interfacial systems. By exploiting the symmetry properties of these two nonlinear signals in combination with highly accurate phase resolution we are able to disentangle different signal contributions, study local field effects and investigate the subsurface transition zone where the physical properties of the bulk approach the properties of the interface.