Hydrogen Balmer-Line Broadening in a Water-Vapor Microwave Plasma Source

Emission spectroscopy was used for the diagnostic of a large-scale, slot-antenna excited microwave (2.45 GHz) plasma source operating in water vapor at low-pressures (1 mbar). The Doppler temperatures corresponding to the broadening of the $H_\beta $ line at 486.1~nm are in the range 2,500-3,000 K, and much higher than the rotational temperatures ($\sim $500 K) determined from the Q-branch (in the wavelength range 600-610~nm ) of the Fulcher-$\alpha $ band [$d^2\Pi _u (v=0)\to a^3\Sigma _g^+ (v=0)$]. Kinetic considerations demonstrate that the electron-ion and ion-ion recombination processes, respectively $H_3 O^++e\to H_2 O+H_{hot}^\ast +\Delta E_1 $ and $H_3 O^++OH^-\to H_2 O+H_2 +O_{hot}^\ast +\Delta E_2 $ can be the source of ``hot'' hydrogen and oxygen atoms provided H$_{3}$O$^{+}$ is the main positive ion in the water plasma. ``Hot'' atoms were also detected in the far remote plasma zone of the source up to 30 cm away from the slot-antennas. Acknowledgment: This study was funded by FCT/FEDER in the framework of the project ``Ecological Plasma Engineering Laboratory'' POCI/FIS/61679/2004.