The spectrum of second harmonic emission from a laser-produced plasma

Observations show a second harmonic spectrum which is broadened and shifted towards the low-frequency side of the true harmonic. A theoretical description of this effect is given based on the assumption that plasma waves generated by resonant absorption at the critical surface are coupled to a series of side-bands via an ion sound wave, and the resultant broadening of the plasma wave spectrum is reflected in the second harmonic emission. If the ion sound wave is propagating outwards, it is shown that, for suitable parameters, a large number of side-bands on the low-frequency side can be excited, giving a spectrum in good agreement with experimental results. (author)