Threshold conditions for the formation of surface plasmas by HF and DF laser radiation

Energy thresholds for the formation of HF and DF laser-supported absorption waves at target surfaces have been measured for several target materials. Chemical laser pulses with a FWHM of 4 μsec and energies up to 150 J were focused to an effective target spot diameter of 0.94 cm. Thresholds were determined by observing the cutoff of incident laser radiation transmitted through a pinhole in the target. The observed variation of the plasma ignition time with incident pulse energy Ei showed that neither the threshold fluence es nor the threshold irradiance qm is invariant under variation of Ei. The measured dependence of es and qm on Ei is compared to calculations based on two simple threshold models. The minimum values of fluence es (in J/cm2) for which surface plasmas are formed are as follows: in the format ’’material [es (HF), es (DF)]’’: aluminum (65, 71); titanium (51, 43); stainless steel (65, 51); nickel (94, 43); copper (110, 46); Grafoil (94, 43); Lucite (100, 100).