G. Ya. Lavrent’ev

Singly charged tungsten and tantalum ions observed during high-temperature field evaporation

High-temperature field evaporation of tungsten and tantalum emitters in the temperature range from room temperature to 2500 K is studied using a static magnetic mass spectrometer equipped with a field source of ions. At room temperature, triply charged W3+ and Ta3+ ions alone are observed in the mass spectra. However, as the emitter temperature grows, the charge of the ions decreases. At T ≈ 1000 K, doubly charged W2+ and Ta2+ ions dominate in the spectra, and singly charged W+ and Ta+ ions appear in the temperature range 1900 < T < 2500 K. The evaporation rate of the singly charged ions is one to two orders of magnitude lower than the evaporation rate of the doubly charged particles. The energy parameters of field evaporation for differently charged tungsten ions are found.

Detecting Singly Charged Ions during Field Evaporation of Tantalum at High Temperatures

The field evaporation of tantalum from point emitters in a broad range of temperatures from T = 300 to 2500 K was studied using a static magnetic mass spectrometer equipped with a special field ion source. The room-temperature mass spectrum of field-evaporated particles displayed only the peaks of triply charged ions (Ta3+). As the temperature was increased, the charge of field-evaporated ions exhibited a decrease: at T ∼ 1000 K, the peaks of doubly charged ions (Ta2+) prevailed. The peaks of singly charged ions (Ta+) were detected for the first time at temperatures in the interval 1900 K < T < 2500 K. The rate of evaporation of singly charged tantalum ions was several orders of magnitude lower than that of doubly charged ions.

Calculation of parameters of a multimolecular complex for nonequilibrium surface ionization of its fragments

Field surface ionization of radicals formed as a result of exothermal heterogeneous reaction in a vibrationally excited state is investigated. Such radicals are ionized in accordance with the mechanism of nonequilibrium surface ionization. The parameters of a multimolecular complex (the number of molecules in the complex and the number of its effective degrees of freedom) preceding the formation of radicals are calculated from the temperature dependences of ion currents. The values of these parameters coincide with analogous values of the quantities calculated for ions of the radicals formed in the same reaction, but having an equilibrium energy distribution.

Nonequilibrium surface ionization of water molecules in a high electric field in the presence of resonance absorption of near-infrared radiation

The results of experiments undertaken to establish the connection between the current of ions desorbed from the surface in the presence of an electric field and the frequency of the infrared radiation incident on that surface are described. It is shown that resonance absorption of infrared radiation by adsorbed molecules increases the efficiency of the surface ionization process.

Optical radiation and ionization of hydrogen atoms in heterogeneous exothermal reactions proceeding in an electric field

Optical radiation related to the Balmer series (Hα, Hβ, Hγ) of hydrogen atoms is discovered when studying the isothermal reaction of trimeric acetone peroxide decomposition on the surface of oxidized tungsten in a static electric field with a strength of up to 4 × 106 V/cm at T = 300 K. The distance from the surface over which desorbing excited hydrogen atoms radiate is determined from the Stark splitting of the lines. Electronically excited atoms remaining on the surface ionize according to the surface ionization mechanism.

Calculation of the parameters of polymolecular complexes-precursors in the process of associate formation

The parameters of a polymolecular complex (M + H)+ responsible for the formation of M + H associates, where M is a molecule (e.g., ethylamine or diethylamine) and H is a hydrogen atom, have been calculated using experimental temperature dependences of the ion currents of associated ions (M + H)+, assuming that these ions are formed according to the surface ionization mechanism. The parameters of the calculation include the number N of molecules in the complex under consideration, the activation energy for the surface dissociation of this complex, and the number of the effective degrees of freedom in the complex.

Field evaporation of tungsten in the presence of adsorbed water

The field evaporation of tungsten at T=300 and 600 K in the presence of water vapor in the residual atmosphere was studied using a static mass spectrometer with a moderate resolution (M/ΔM=200). The room-temperature mass spectrum of field-evaporated particles displayed only the peaks of triply charged ions; an increase in the temperature to 600 K added the peaks of doubly charged ions of approximately the same intensity. The presence of adsorbed water reduced the evaporating field strength by a factor of 2-2.5. At 300 K, an increase in the water content in the residual atmosphere markedly enhanced the field etching. No singly or quadruply charged tungsten ions were observed. The results are indicative of the activation mechanism of field evaporation at the temperatures studied and show evidence of a substantial decrease in the magnitude of evaporating field caused by the presence of water.

Surface ionization of products from monomolecular disintegration of multimolecular complexes

Mechanisms behind the narrow bell-shaped temperature dependences of the fluxes of ions and molecules being desorbed from the surface at low emitter temperatures have not been discovered so far. A model is suggested that allows the consideration of the narrow low-temperature peaks of various nature (associated ion peaks, dimer peaks, and those appearing under nonequilibrium surface ionization) from the unified viewpoint. It is believed that multimolecular complexes form on the surface and then disintegrate into fragments by the mechanism of monomolecular reactions with subsequent ionization of the fragments on the emitter surface.

Possible ionization of mercury atoms by elastic reflection in hyperthermal surface ionization experiments

An elastic collision model baed on data given by Danon et al. [Phys. Rev. Lett. 65, 2038 (1990)] was used to calculate the ionization potential of mercury atoms (V′) at the critical charge transfer distance as a function of the kinetic energy of the atoms (E). The linear dependence V′(E) obtained supports the model of surface ionization of mercury atoms by elastic reflection from a surface.

First experiments on field photodesorption IR spectroscopy

The ion current of adsorbed water molecules is studied experimentally as a function of the frequency of near-IR radiation incident on a surface at frequencies in the intrinsic vibrational bands of the water molecule. The ions are produced by nonequilibrium field surface ionization. The observed band (near one of the combination frequencies) has a width of 100 cm−1 and is shifted relative to the free molecular band by 130 cm−1. Estimates show that the cross section for absorption of the radiation by the adsorbed molecules is 3–4 orders of magnitude larger than for free molecules, as is typical of surface processes.