Excitation of water molecules by electron impact with formation of OH-radicals in the A²Σ⁺state

Abstract

The excitation cross-sections of the OH-radical band A2Σ+ → X2 (v′ = 0 → v″ = 0, v′ = 1 → v″ = 1) were measured. OH-radicals were formed during dissociation of water molecules by electron impact in the conditions of crossing of supersonic molecular and electron beams in the energy range 10–120 eV. Measurements were conducted at temperatures of 50, 80 and 200 K. It was shown that the excitation function had a sharp maximum in the region of low energies (at 16 eV) and an extended plateau up to 120 eV. It is proved that there are two channels of molecule dissociation with formation OH (A2Σ+) through excitation of either the triplet b3A1 or the singlet B1A1 states of H2O molecules. The form of the excitation function essentially depends on the temperature of water vapours in the beam. With the decrease of the water molecule temperature the height of the plateau in the region 30–120 eV decreases in comparison with that of the peak at 16 eV. The absolute value of the excitation cross-section of the OH band at the temperature 50 K has been measured. It is equal to (1.6 ± 0.5) × 10−18 cm2 in the maximum at 16 eV. The ratio of cross-sections of bands 1–1 and 0–0 weakly depends on the energy of the exciting electron in the range 12–120 eV and is equal to 0.28 ± 0.05. The appearance threshold is equal to (9.1 ± 0.5) eV.

Cross Sections for Electron Collisions with H2O

“…All measurements agree that the (sharp) maximum of the OHà 2 Σ + → X 2 Π optical emission is situated at about 15-20 eV. As recalled by Khodorkovskii et al, 116 the OH radical in theà 2 Σ + excited state may come from the dissociation of two excited H 2 O molecular electronic states: 1 A 1 and 3 A 1 . According to Ralph et al, 100 the cross section for the excitation of the H 2 O molecule to these states amounts at 20 eV to about 20 3 10 −18 cm 2 .…”

“…The optical emission from the OH radical formed in electron-induced dissociation was reviewed in detail by Itikawa and Mason. 21 The two more recent measurements of the OH optical emission bandà 2 Σ + → X 2 Π disagree: the data of Schappe and Urban 114 confirm the earlier experiment performed by Beenakker et al, 115 with the maximum of 0.9 3 10 −18 cm 2 , and those by Khodorkovskii et al 116 are by few folds lower. All measurements agree that the (sharp) maximum of the OHà 2 Σ + → X 2 Π optical emission is situated at about 15-20 eV.…”

Influence of ambient air on the flowing afterglow of an atmospheric pressure Ar/O2 radiofrequency plasma

“…Dissociative excitation of H 2 O molecules by electron impact is another channel to produce OH(A) radicals, this requires electrons of energies greater than 9 eV. 32 As it will be detailed in Sec. III D, RET through collisions with argon atoms is very efficient, and for most cases the effective lifetime of OH(A 2 R þ ; 0 ¼ 0) state is greater than the RET time constant by at least an order of magnitude, so we will consider that the state is in thermal equilibrium.…”

Pretreatment of polypropylene films for following technological processes, part 2: The use of low temperature plasma method

“…Peyrous et al26 have modeled the elementary processes in the plasma of water vapor resulting in the formation of OH radicals and hydrogen atoms in the collision of water molecules with electrons as well as by means of atomic oxygen: e* + H 2 O → OH + H + e; O( 1 D) + H 2 O → OH + OH. The formation of OH radicals in plasma has been considered in several articles 27–30…”

“…The identification of the atomic and molecular plasma species was carried out on the basis of data base 22. and the articles23–30…”