The paper deals with measurements of excitation spectra of forbidden transitions in di- and poly-atomic molecules in a supersonic beam. The molecules were excited using pulsed tunable dye lasers directly in the following forbidden transitions: B'2 (Delta) $IMP X2(Sigma) in the case of CaF and CaCl molecules, and S0(1A1g) yields T1 (3B3u), S0(1A1) yields T1 (3B1) and S0(1A1g) yields T1(3Au) transitions in the case of pyrazine, pyrimidine and glyoxal, respectively. The spectral resolution of our set-up allows determination of energy values and rotational constants for the two spin-orbit levels of the CaF and CaCl molecules. For the CaCl molecule the rotational constants of the vibrational v" equals 0 level of the B' (2(Delta) ) state have been determined for the first time. The excitation spectra of pyrazine, pyrimidine and glyoxal in the range between the S1(00) 0 T1(00) yielded frequencies of fundamental vibrations, which for pyrazine and glyoxal are in a good agreement with literature values, but for pyrimidine were determine for the first time. Both types of phosphorescence spectra exhibit many hot bands besides the fundamental, totally symmetric, vibrations. The hot bands are very helpful in supporting the fundamental mode assignments.