Spectroscopic investigations of chemical reactions in liquids plasma C. Miron 1 , M. A. Bratescu 2 , N. Saito 3,4 , O. Takai 2,4 1 Department of Materials, Physics and Energy Engineering, Nagoya University, Nagoya 464-8601, Japan 2 EcoTopia Science Institute, Nagoya University, Nagoya 464-8601, Japan 3 Department of Molecular Design and Engineering, Nagoya University, Nagoya 464-8601, Japan 4 CREST/JST, Nagoya 464-8603, Japan 1. Introduction Pulsed electrical discharges in liquids have been intensively studied for synthesis of advanced materials [1,2]. Complex physical and chemical phenomena are involved in these processes which are not yet completely understood. Our purpose is to investigate the chemical properties of electrical discharges generated in ultrapure water and hydrogen peroxide spectroscopic methods. The time evolution of the reactive species generated in the process was also investigated. Hydrogen and oxygen bubble gases were also added in the discharge process in order to evaluate their influence on the chemical processes in plasma. 2. Experimental set-up Electrical discharges were generated between two pairs of tungsten and tantalum electrodes immersed in different liquids, such as ultrapure water and hydrogen peroxide. The diameter of the electrodes was 1mm, and the interelectrodes distance was 0.5 mm. Liquids were circulated using a pumping system in order to avoid the temperature, pH and conductivity increasing. Bipolar voltage pulses were applied to the plasma, the pulse width was 2 µs and the repetition frequency was 25 KHz. The emission spectra were acquired using a spectrograph AvaSpec-3648. A gate Vg of a boxcar integrator (SRS250) can be set at variable time delays (in the range of 0 – 100 µs) related to the voltage pulse applied to the electrodes in the water plasma. 3. Results When voltage was applied across the electrodes, small bubbles are primarily formed at the tip of the electrodes and the electrical discharges are initiated. The spectra acquired showed the presence of atomic lines and molecular bands, several of them being emitted at the same wavelength for both types of electrodes. Therefore, excited states of atomic hydrogen of the Balmer series H α (n’ = 3 → n = 2), H β (n’ = 4 → n = 2), H γ (n’ = 5 → n = 2), and atomic oxygen could be detected. Molecular bands of hydroxyl radicals and molecular hydrogen were also detected in the emission spectra: OH (3064Å System, A 2 Σ + → X 2 Π), OH + (3565 Å System, X 3 Σ → A 3 Π), H 2 dissociation continuum (a 3 Σ + g → b 3 Σ + u ), the O 2 Schumann-Runge system (B 3 Σ u → X 3 Σ g - ). In the case of tantalum electrodes, a very broad hydrogen dissociation continuum was formed when the electrical discharge was initiated in ultrapure water, so that one of the conditions (three-body recombination) for a good LTE [3] is not satisfied in this system. The hydrogen dissociation continuum was not formed when the discharges were realized in the hydrogen peroxide system using both types of electrodes. Fig.1. Time-resolved optical emission spectroscopy of an electrical discharge initiated between tantalum electrodes immersed in ultrapure water Time-resolved optical emission spectra showed that the time evolution of the reactive species depends on the pulse polarity. The deexcitation time of different reactive species was measured. The very long lifetime of some reactive species, such as hydroxyl or the electronic state a 1 ∆ g of the oxygen molecule provide an important energy reservoir for the chemical reactions. Further investigations will be carried out in order to elucidate the mechanism of chemical reactions in liquid plasma. References [1] M. A. Malik, A. Ghaffar, S. A. Malik, Plasma Sources Sci. Technol. 10 (2001) 82. [2] R.P. Joshi, J. Qian, K.H. Schoenbach, J. Appl. Phys. 96 (2004) 3617. [3] W. Lochte-Holtgreven, Plasma Diagnostics, Amsterdam: North-Holland, 1968. View publication stats View publication stats