Clusters were produced as a result of argon gas cooling during expansion through a supersonic nozzle. A two-dimensional model was set up in order to calculate gas expansion and partial condensation into clusters. Calculations were validated by experimental measurements using Mach-Zehnder interferometry and Rayleigh scattering, and performed with two types of nozzles ͑Laval and conical nozzles͒. These optical diagnostics together with numerical simulations led to the cluster size and density determination with spatial resolution through the gas and cluster jet. Cluster production was observed to be very sensitive to the nozzle geometry. Homogeneous gas and cluster jets were produced and characterized using conical nozzle geometry, with cluster density about 10 12 per cm 3 . Due to the fast valve-nozzle connecting geometry, shock waves have been observed at the Laval nozzle throat that strongly affected cluster production on the jet axis. Averaged cluster radius was observed to be easily tunable from 180 to 350 Å by varying the upstream gas pressure P 0 from 20 to 60 bars. A different scaling law, versus P 0 , has been observed for this regime of large cluster, compared to Hagena's predictions for the small cluster regime.

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In this work, we report the study of bimolecular and termolecular charge transfer and Penning ionization reactions in He–N2 mixtures excited by repetitive 3 kA, 7 ns duration discharges. He(2 3S) destruction frequencies have been determined from measurements of the optical absorption of the 2 3S→3 3P helium transition and were found to correlate with the late-time decay rate of the N2+(B 2Σu→X 2Σg) fluorescence. The much more intense early-time fluorescence was ascribed to charge transfer from He2+. The values found for the bimolecular and termolecular charge transfer reactions were, respectively, (1.1±0.1)×10−9 cm3 s−1 and (1.36±0.20)×10−29 cm6 s−1, while the corresponding values for the Penning reactions were (7.6±0.4)×10−11 cm3 s−1 and (3.3±0.3)×10−30 cm6 s−1. Branching ratios for producing the N2+(B 2Σu) state in these reactions are discussed. A comprehensive kinetic model of the He–N2 afterglow has been formulated which was able to reproduce all the experimental data obtained as functions of helium pressure over the range 310–760 Torr and as functions of the partial pressure of nitrogen from 5–500 mTorr.