Coplanar surface barrier discharge ignited in water vapor—a selective source of OH radicals proved by (TA)LIF measurement

Abstract

Coplanar dielectric barrier discharge (DBD) was ignited in pure water vapor at atmospheric pressure in order to generate highly oxidizing plasma with one specific type of reactive radicals. In order to prevent water condensation the used plasma reactor was heated to 120 • C. The composition of the radical species in the discharge was studied by methods based on laserinduced fluorescence (LIF) and compared with analogous measurements realized in the same coplanar DBD ignited in air. Fast collisional processes and laser-surface interaction were taken into account during LIF data processing. It was found that coplanar DBD ignited in water vapor produces hydroxyl (OH) radicals with concentration in the order of 10 20 m −3 , which is 10 higher than the value measured in discharge in humid air (40% relative humidity at 21 • C). The concentration of atomic hydrogen radicals in the DBD ignited in water vapor was below the detection limit, which proves that the generation of oxidizing plasma with dominance of one specific type of reactive radicals was achieved. The temporal evolution, spatial distribution, power dependence and rotational temperature of the OH radicals was determined in the DBD ignited in both water vapor and air.

Laser-Induced Fluorescence of Hydroxyl (OH) Radical in Cold Atmospheric Discharges

“…By means of two-photon absorption LIF of atomic hydrogen and by optical emission spectrometry, it was verified that plasma in pure water vapor generated almost solely OH radicals, concentration of other reactive radicals was negligible. As a result, the LIF method proved that the investigated discharge can be used as a selective source of oxidizing OH radicals with relatively high concentration [33].…”

“…The influence of collisional partners on fluorescence spectra can be demonstrated by comparison of spectra measured in air and in water vapor when OH radicals were excited to the v 0 ¼ 1 state: in air, most of excited OH radicals were transferred to the v 0 ¼ 0 state prior to photon emission and the 0 -0 band with head located at 306 nm dominated to the fluorescence spectrum. By contrast, the OH(A) lifetime in water vapor was so short that the OH radicals did not have time for VET; most of photons were emitted directly from the v 0 ¼ 1 state and mainly radiation of the 1 -1 band around 315 nm was observed [33].…”

“…In such a case, each vibrational state can be treated as only one effective level, which simplifies the LIF data processing considerably. When RET is not so fast, it is often sufficient to take out the one directly excited rotational level and to suppose that all other rotational levels are in rotational equilibrium [33,35]. In general, when RET is not fast enough, rotational levels may not reach equilibrium during the OH * lifetime and each vibrational state may be described by its own rotational temperature.…”

“…An example of a challenging LIF experiment is the OH measurement in a surface coplanar DBD ignited in water vapor at atmospheric pressure [33]. The coplanar DBD is made of a system of coplanar strip electrodes that are covered by a ceramic and plasma is created in the gas on the opposite side of the ceramic.…”

Solution to the perturbation of LIF measurements via photodissociation, OH measurement in atmospheric‐pressure multihollow DBD

“…Consequently, OH belongs to the plasma species that are most frequently studied by means of LIF. [16][17][18][19][20][21][22][23][24] In this study, OH LIF is used to study multihollow surface dielectric barrier discharge (MSDBD). MSDBD is a planar plasma source, which generates atmosphericpressure plasma in an array of holes penetrating a ceramic plate.…”