THEORETICAL-STUDY OF THE SUM-FREQUENCY GENERATION IN MOLECULAR ADSORBATE - NH3 MGO(100)/

We present a theoretical study of the sum frequency generation (SFG) f or the ammonia molecule adsorbed on the MgO(100) surface. This particu lar system could be a model for experimentalists in order to test this nonlinear technique, mainly for two reasons. First, the ammonia molec ules can be considered in a first approximation as isolated on the sur face due to repulsive lateral interactions. Second, the symmetry of am monia and its particular equilibrium configuration on this surface all ow us to separate the SFG response in a part which depends only on the non linear susceptibility of the molecules and an other part that cha racterizes the experimental setup. As a consequence, the bandshifts an d widths of the SFG and infrared (IR) signals can be directly compared . Within the accuracy of the vibrational dependence of the molecular p arameters, our calculations predict that the SFG signals connected to the symmetric vibrational modes should be more intense than the antisy mmetric ones, in consistency with the IR and Raman spectroscopies and that the two symmetric modes should be observable while only the bendi ng vibration has a significant intensity in IR. Finally, we discuss in terms of beam polarizations, the geometry of the experimental setup w hich optimizes the detected intensities. (C) 1998 American Institute o f Physics, [S0021-9606(98)03015-3].