Nanosecond time-resolved microscopic spectroscopy for diagnostics of an atmospheric-pressure discharge plasma formed in aqueous solution

Abstract

Glow discharge plasma formed in solution under atmospheric pressure has been expected to provide reaction fields with characteristic physical and chemical properties owing to the frequent collisions and reactions of reactive particles inside and the rapid quenching of the products by the surrounding cold solutions. In particular, when an aqueous solution is utilized as the surrounding solution, the atmospheric-pressure in-solution glow (ASG) plasma contains hydrogen and hydroxyl radicals showing large activities for reduction and oxidation, respectively. In addition, because the ASG plasma is formed under atmospheric pressure, the collision frequencies between the particles contained in the plasma are higher than those in other plasmas ordinarily formed under low pressure. This feature should result in rapid energy redistribution among particles contained in the plasma. In the present study, time-resolved optical emission spectroscopy with nanosecond time resolution was applied for the diagnostics of the ASG plasma with chemical species selectivity. The time-resolved measurements revealed that the temporal evolutions of the temperatures of blackbody, hydrogen radical, and hydroxyl radical contained in the ASG plasma consist of two stages: initial rise within 0.15 µs (rising stage) and fluctuation around certain values for about 1 µs (fluctuating stage). In the time region corresponding to the rising stage, the electron number density is about ten times larger than the value temporally averaged during the plasma emission. The initial rise should result from frequent collisions between charged particles accelerated by the applied voltage and unexcited particles. In the fluctuating stage, the electron number density strongly correlates with the increase in the radical temperatures. It is concluded that the electron number density, rather than the electron temperature, is a key parameter determining the temperatures of reactive species in the ASG plasma.

Time-and Space-resolved Optical Diagnostics for Discharge Plasmas Separately Formed in Aqueous Solution

“…The detailed information on the system is described elsewhere. 3,5 The m-ASS plasmas were generated in a gap between two opposed tungsten electrodes with the diameters of 0.25 mm. The electrodes were confined in the cell.…”

“…The shape of the continuum emission is well explained by Planck's function for the spectral density of the blackbody radiation, as well as that for the ASG plasma. 3 From these assignments, it is concluded that the plasmas commonly contain the four components, namely blackbody, ·OH, ·H, and Na.…”

“…Alternatively, we suggested that the temperature estimated from the blackbody continuum emission should correspond to the lower limit of the electron temperature because the blackbody radiation should be derived from the gas heated by the collisions with the accelerated electrons. 3 Here, we estimate the timeaveraged blackbody temperature for the +plasma and -plasma.…”

“…The temperature of the plasma generally reaches up to several thousand Kelvin or higher, but that of the surrounding solution can be kept at room temperature. 3 Thus, the released hot products are immediately cooled down and the reactions are suddenly quenched in the cold solution. 4 Due to this rapid quenching process, it is expected that thermodynamically semi-stable products, which are hardly synthesized under general gradual cooling conditions, can be synthesized.…”

“…We have studied and analyzed the emission spectrum from the ASG plasma with the time resolution of nanoseconds. 3 The ASG plasma contains ·H and ·OH, as well as the streamer generated in aqueous solution. 6 Owing to the efficient generation of ·H and ·OH due to the dissociation of the abundant water molecules deriving from the surrounding aqueous solutions, the ASG plasma can be applied to degradations of organic chemicals, 7,8 sterilizations, 9 and syntheses of metal nanoparticles, 10,11 called solution plasma processing (SPP).…”