Determination of Ar 2 + and N 4 + recombination coefficients by subpicosecond multiphoton ionization at 248 nm and microwave interferometry

Rate coefficients (αD) for the dissociative recombination of Ar2+ and N4+ in low-temperature plasmas have been determined by multiphoton ionization of the parent gas (Ar or N2) at 248 nm, in combination with microwave interferometry at 9.2 GHz. A subpicosecond, Ti:Al2O3-KrF hybrid laser system generating 40 mJ pulses at 248 nm serves as a photoionization source within one arm of an interferometer having a bandwidth of ∼800 MHz, thereby providing an ultrafast (δ-function) plasma channel generator and a noninvasive, microwave probe of the plasma that together decouple the ionization mechanism (and source) from the electron detection process. Comparisons of measurements of the temporally resolved electron density with numerical simulations find αD to be (1.2-6.0)×10-6 cm3 s-1 for Ar2+ and (2±1)×10-6 cm3 s-1 for N4+ for background pressures in the 150-600 Torr Ar and 10-400 Torr N2 intervals, respectively. Both sets of constants are consistent with values reported previously in other ranges of gas pressure. The data and simulations indicate the cross section for four-photon ionization of Ar at 248 nm to be (5±3)×10-118 cm8 s3, or approximately 1.4 orders of magnitude lower than the single value in the literature [Uiterwaal, Phys. Rev. APLRAAN1050-2947.