Abstract. In this contribution, nanosecond pulsed discharges in N 2 and N 2 /0.9% H 2 O at atmospheric pressure (at 300 K) are studied with time-resolved imaging, optical emission spectroscopy and Rayleigh scattering. A 170 ns high voltage pulse is applied across two pin-shaped electrodes at a frequency of 1 kHz. The discharge consists of three phases: an ignition phase, a spark phase and a recombination phase. During the ignition phase the emission is mainly caused by molecular nitrogen (N 2 (C-B)). In the spark and recombination phase mainly atomic nitrogen emission is observed. The emission when H 2 O is added is very similar, except the small contribution of H α and the intensity of the molecular N 2 (C-B) emission is less.The gas temperature during the ignition phase is about 350 K, during the discharge the gas temperature increases and is 1 µs after ignition equal to 750 K. The electron density is obtained by the broadening of the N emission line at 746 nm and, if water is added, the H α line. The electron density reaches densities up to 4 · 10 24 m −3 . Addition of water has no significant influence on the gas temperature and electron density.The diagnostics used in this study are described in detail and the validity of different techniques is compared with previously reported results of other groups.

The production of OH in a nanosecond pulsed filamentary discharge generated in pin–pin geometry in a He–H2O mixture is studied by time and spatially resolved laser-induced fluorescence. Apart from the OH density the gas temperature and the electron density are also measured. Depending on the applied voltage the discharge is in a different mode. The maximum electron densities in the low- (1.3 kV) and high-density (5 kV) modes are 2 × 1021 m−3 and 7 × 1022 m−3, respectively. The gas temperature in both modes does not exceed 600 K. In the low-density mode the maximum OH density is at the centre of the discharge filament, while in the high-density mode the largest OH density is observed on the edge of the discharge. A chemical model is used to obtain an estimate of the absolute OH density. The chemical model also shows that charge exchange and dissociative recombination can explain the production of OH in the case of the high-density mode.

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The new generation of lithography tools use high energy EUV radiation which ionizes the present background gas due to photoionization. To predict and understand the long term impact on the highly delicate mirrors It is essential to characterize these kinds of EUV-induced plasmas. We measured the electron density evolution in argon gas during and just after irradiation by a short pulse of EUV light at 13.5 nm by applying microwave cavity resonance spectroscopy. Dependencies on EUV pulse energy and gas pressure have been explored over a range relevant for industrial applications.Our experimental results show that the maximum reached electron density depends linearly on pulse energy. A quadratic dependence caused by photoionization and subsequent electron impact ionization by free electrons -is found from experiments where the gas pressure is varied. This is demonstrated by our theoretical estimates presented in this manuscript as well.PACS numbers: 52.70.Gw,81.16.Nd Submitted to: J. Phys. D: Appl. Phys. time ( s) 1.5 J 15 J 29 J 44 J 59 J 73 J 88 J 100 J 150 J

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