Excitation spectra arising from A 30 ← X10+ and B31 ← X101 vibronic transitions in Hg-rare gas (RG) van der Waals molecules are calculated using the newly obtained theoretical potential curves for these species. In the molecular structure calculations, Hg20+ and RG8+ cores are simulated by energy-consistent pseudopotentials which also account for scalar-relativistic effects and spin-orbit interaction within the valence shell. Potential energies in the AS coupling scheme have been obtained by means of ab initio CASSCF/CASPT2 calculations with a total 28 valence electrons, while the spin-orbit matrix has been computed in a reduced CI space restricted to the CASSCF level. The final Ω potential curves are obtained by diagonalizing the modified spin-orbit matrix (its diagonal elements before diagonalization substituted for the corresponding CASPT2 eigenenergies). The spectroscopic parameters for the ground and several excited states of the Hg-RG species deduced from the calculated potential curves exhibit very good agreement with available experimental data. The radial Schrödinger equation for nuclear motion was solved numerically with the calculated potentials to evaluate the corresponding vibronic levels and radial wavefunctions. The latter have been used in the calculation of the appropriate Franck-Condon factors to yield information on relative intensities of the vibrational bands of the Hg-RG complexes. The theoretical vibrational bands are discussed in the context of available experimental spectra.