[en] Detailed measurements of the angular distribution of 3 ω₀/2 radiation are presented in short scale length plasmas (0.8-7 μm) generated by laser radiation at intensities reaching the relativistic level (10¹⁶-6x10¹⁸ W/cm²). The experimental results are in very good agreement with theoretical predictions based on two-plasmon decay and stimulated Raman scattering instabilities. New three-halves harmonic generation mechanisms are an identified characteristic of femtosecond laser induced parametric instabilities. These are the joint interaction of incident and reflected laser beams as well as stimulated Raman scattering. It is shown both experimentally and theoretically that the three-halves harmonic radiation is a useful preplasma diagnostic tool