The mechanism of thermal ionization of methenamine on the surface intermetallic compound NaAux

Abstract

Thermal ionization of methenamine (C6H12N4) on the surface of the NaAux intermetallic compound has been studied. It has been established thatthe processes of decomposition, desorption and ionization of adsorbedcompounds, thermally stimulated on the surface, proceed due to theaccumulation of energy at the degrees of freedom of the adsorption complex,including the adsorbed compound and a solid, by the mechanism ofmonomolecular decomposition reactions. In this case, the decomposition ofthe adsorption complex is accompanied by the desorption of ions that are notin thermal equilibrium with the solid. The uniformity of the temperaturedependences of the ion current and their distribution over two groupsallowed us to conclude that ions are desorbed from the surface, whichcorrespond to the decays of individual adsorbed molecules, as well as thedecays of dimers formed on the surface. The decay of methenamine moleculesduring thermal ionization occurs in the same way as their decay in vacuumduring electron ionization, which indicates the preservation of the bulkstructure of methenamine molecules during adsorption and a significantlifetime of the excited state of compounds on NaAux. Keywords: thermal ionization, methenamine, intermetallic compound, NaAux.\

Current-voltage characteristic of the thermal ion emission of Na from the Na-=SUB=-x-=/SUB=-Au surface

“…According to (1), the ratio of α on Na x Au with ϕ = 5.1 eV to α on W oxide with ϕ = 6.8 eV [2] at T em ∼ 1000 K should be much lower than 10 −8 . This contradicts the results reported in [3,4].…”

“…According to (1), this is possible only at high values of ϕ [2]. Record-high estimates of ϕ were obtained in the study of TI of hexamethylenetetramine molecules on Na x Au: ϕ ∼ 7.6−7.9 eV [4]. However, thermal equilibrium between desorbing particles and the emitter is not established in TI of hexamethylenetetramine, making Eq.…”

“…(1) inapplicable. High α values in TI of organic compounds on Na x Au may be induced by non-equilibrium processes due to reactions between coadsorbed particles [4]. The determination of ϕ in accordance with (1) for TI of atoms is made difficult by their dissolution in Na x Au [5].…”

“…i.e., the ϕ value of Na x Au is insufficient for efficient TI of organic compounds at operating Na x Au emitter temperatures T em < 1200 K [2][3][4]. According to (1), the ratio of α on Na x Au with ϕ = 5.1 eV to α on W oxide with ϕ = 6.8 eV [2] at T em ∼ 1000 K should be much lower than 10 −8 .…”

“…(I-V) measurements were performed in a high-vacuum diode with its cathode being a 2.5-mm-wide gold ribbon coated with Na x Au in accordance with the procedure detailed in [3][4][5][6]. A metal plate with a width of 20 mm and a gold layer deposited onto it was the anode, which was positioned 8 mm away from the ribbon.…”