We have studied the resonant two-photon ionization (R2PI) spectra of three van der Waals (vdW) complexes p, m, o-C6H4F2... NH3(ND3) through the S-1<--S-0 transition with mass selectivity. The stretching frequencies of the three vdW complexes were found to be quite close (86 cm(-1)). From the photodissociation mechanism and the relationship between anharmonicity of the stretching vibration and the dissociation energy, we estimated the dissociation energies of all the three vdW complexes in the S-0 and S-1 states. A quantum ab initio calculation on p-C6H4F2... NH3 at the MP2/6-31G** level gave the following geometry: The N atom of NH3 is located on the symmetry axis (Z-axis) and 3.53 Angstrom above the benzene ring; the C-3 axis of NH3 is at an angle of 52.5 degrees with the Z-axis of p-C6H4F2 with one of the hydrogen atoms pointing towards the benzene ring; the rotation of NH3 around the Z-axis is nearly free. The calculated bond dissociation energies and the expectation of internal rotation are consistent with our experimental results.