Hybrid Stimulated Raman Scattering -- Two Plasmon Decay Instability and 3/2 Harmonic in Steep-Gradient Femtosecond Plasmas

The hybrid Stimulated Raman Scattering - Two Plasmon Decay instability had shown to play the dominant role for plasma waves excitation and energy absorption at interaction of very intense femtosecond obliquely incident $p$-polarized laser pulse (intensity ${10}^{17}-5\cdot {10}^{19}$ W/cm${}^2$) with a steep gradient ($L\sim \lambda$) plasma near the quarter critical surface, i.e. within the typical for modern experiments conditions. The plasmons are excited as two wave packets confined near this surface with very wide $\approx \omega_0/c$ spatial spectra along its normal. Hence, phase matching conditions for 3/2 harmonic generation fulfill immediately and include new mechanism coming from high harmonics of plasma waves. The latter mechanism have been proved experimentally observing an additional 3/2 harmonic beam.