Laser-produced plasmas play an important role in plasma physics and fusion research. Parametric instabilities are the most important processes governing the laser-plasma interaction. Some parametric instabilities as two-plasmon decay or stimulated Raman scattering were investigated in this work using an optical signal, the so called 3/2 harmonics. As this harmonic originates from the plasma its characteristics provide important information about the plasma properties. The angular, spectral and temporal structure of this 3/2 harmonics were studied. Detailed theoretical analysis was made based on a new model to explain the measured results and to gain information about the generation process and the plasma.