Threshold of the 2ωpe instability in a laser produced plasma

$\text{3}\text{2}$ω_L emission from various plane targets irradiated by a 20 J, 5 ns neodymium laser pulse has been investigated. The onset of $\text{3}\text{2}$ω_L emission and an increase in intensity by five orders of magnitude is observed at a laser intensity of ≌2×10¹³W cm^-2, the threshold intensity predicted for excitation of the 2ω_pe instability.