Yu. N. Novoselov

Femtosecond laser writing of subwave one-dimensional quasiperiodic nanostructures on a titanium surface

One-dimensional quasiperiodic structures whose period is much smaller than the wavelength of exciting radiation have been obtained on a titanium surface under the multipulse action of linearly polarized femtosecond laser radiation with various surface energy densities. As the radiation energy density increases, the one-dimensional surface nanorelief oriented perpendicularly to the radiation polarization evolves from quasiperiodic ablation nanogrooves to regular lattices with subwave periods (100–400 nm). In contrast to the preceding works for various metals, the period of lattices for titanium decreases with increasing energy density. The formation of the indicated surface nanostructures is explained by the interference of the electric fields of incident laser radiation and a surface electromagnetic wave excited by this radiation, because the length of the surface electromagnetic wave for titanium with significant interband absorption decreases with an increase in the electron excitation of the material.

Ultrafast changes in the optical properties of a titanium surface and femtosecond laser writing of one-dimensional quasi-periodic nanogratings of its relief

One-dimensional quasi-periodic nanogratings with spacings in the range from 160 to 600 nm are written on a dry or wet titanium surface exposed to linearly polarized femtosecond IR and UV laser pulses with different surface energy densities. The topological properties of the obtained surface nanostructures are studied by scanning electron microscopy. Despite the observation of many harmonics of the one-dimensional surface relief in its Fourier spectra, a weak decreasing dependence of the first-harmonic wavenumber (nanograting spacing) on the laser fluence is found. Studies of the instantaneous optical characteristics of the material during laser irradiation by measuring the reflection of laser pump pulses and their simulation based on the Drude model taking into account the dominant interband absorption allowed us to estimate the length of the excited surface electromagnetic (plasmon-polariton) wave for different excitation conditions. This wavelength is quantitatively consistent with the corresponding nanograting spacings of the first harmonic of the relief of the dry and wet titanium surfaces. It is shown that the dependence of the first-harmonic nanograting spacing on the laser fluence is determined by a change in the instantaneous optical characteristics of the material and the saturation of the interband absorption along with the increasing role of intraband transitions. Three new methods are proposed for writing separate subwave surface nanogratings or their sets by femtosecond laser pulses using the near-threshold nanostructuring, the forced adjustment of the optical characteristics of the material or selecting the spectral range of laser radiation, and also by selecting an adjacent dielectric.

Formation of periodic nanostructures on aluminum surface by femtosecond laser pulses

One-dimensional periodic nanostructures have been produced on the surface of an aluminum specimen using femtosecond laser pulses at wavelengths of 744 and 248 nm. The nanostructurization of the specimen has been conducted in water and in air in the preablation regime. We investigate the dependence that the surface topology has on the parameters of laser radiation (wavelength, fluence, and number of pulses), as well as on the medium in contact with the specimen surface. A calculation of the optical characteristics of aluminum as they depend on the electron temperature is performed that is good at describing the dependence that the reflection of the p-polarized infrared femtosecond pulses of pumping has on the fluence. Using these optical characteristics of the photoexcited aluminum within the interferential model, periods of the aluminum surface nanogratings are estimated which are in good agreement with the periods measured experimentally.

Effect of electronegative impurities on the generation of ozone in air

A self-consistent numerical model is used to investigate the effect of electronegative impurities on the ozone yield in a dielectric barrier discharge with a pulsed voltage supply, and the range of impurity concentrations giving a substantial (two-or threefold) increase in the ozone yield is established. Sulfur hexafluoride is considered as a representative component having strong electronegative properties. It is shown that a tiny admixture [SF6]<0.1% can have an appreciable effect on the characteristics of an ozonator. The calculations are compared with published experimental data and given an interpretation.

Plasma-chemical oxidation of SO2 in air by pulsed electron beams

The results are given of detailed experimental studies of the model flue gas mixture irradiation by pulsed electron beams and electron-beam-initiated non-self-sustained discharges. The effects of the electron beam parameters, the external electric field and the composition of irradiated gas on the process of sulphur dioxide SO2 removal are analysed. We demonstrate the existence of optimum values of the electron beam current density and pulse duration, and of the external electric field strength, at which the energy required to remove one SO2 molecule is minimum. The energy cost and removal rate are given as functions of the concentration of SO2, oxygen and water vapour in the mixture. The concentration limit that divides the chain and radical mechanisms of SO2 removal is determined.

Losses in a pulsed electron beam during its formation and extraction from the diode chamber of an accelerator

The results of experimental studies of the current and charge balance in the diode unit of the  ЭpU-500 high-current pulsed electron accelerator (an accelerating voltage of 350–500 keV, a half-height pulse duration of 60 ns, and a total kinetic electron energy of 250 J/pulse) during generation of an electron beam are presented. Planar diodes with multipointed cathodes having diameters of 43–60 mm and manufactured from graphite, copper, and carbon felt were studied. It is shown that the electron-beam divergence in the anode-cathode gap caused by a distortion in the electric field at the periphery of the cathode is the main source of parasitic losses in planar diodes. The half-angle of divergence is 68° at small anode-cathode gaps and decreases to 60° with an increase in the gap. When the diode impedance is matched to the generator’s output impedance (at a gap of 10–12 mm), the charge loss is within 12%.

Efficiency of a planar diode with an explosive emission cathode under the conditions of delayed plasma formation

The energy and current balances in the diode unit of a high-current pulsed electron accelerator (350–500 keV, 60 ns, 250 J per pulse) are compared for an explosive emission cathode (made of graphite, copper, or carbon felt) and a multipoint (graphite or copper) cathode. The planar diode with the continuous cathode is shown to be more efficient in terms of conversion of the applied energy to electron energy (more than 90%) despite a delay in the plasma surface formation. With the impedance of the planar diode matched to the output resistance of the nanosecond generator, the efficiency of the diode does not depend on the time of plasma formation on the cathode. In the case of the graphite cathode, the plasma formation delay reduces the fraction of slow electrons in the forming electron beam and reduces electron losses in anode foil when the beam is extracted from the vacuum space of the diode chamber into the reactor.

Influence of the parameters of a pulsed electron beam on the removal of nitrogen oxides from flue gases

Results are presented of an experimental investigation of the oxidation of small quantities of nitrogen oxides in air irradiated by a pulsed electron beam. It is shown that the impurity removal process is strongly influenced by the pulse length and current density of the electron beam. It is noted that an adequate model is needed to describe the plasma-chemical oxidation processes of nitrogen oxides involving charged and excited particles.

Elimination of styrene vapor from air by a pulsed electron beam

Styrene vapor contained in air in small amounts is decomposed when the air is subjected to a pulsed electron beam and non-self-maintained space discharge. The physical laws of the process and the final products of styrene vapor conversion are found. Experimental data make it possible to consistently describe the styrene vapor elimination from air exposed to a pulsed electron beam and relate the beam parameters to the properties of the gas flow being irradiated.

Streamer corona removes styrene vapor from air flow

Decomposition of the styrene vapor in air induced by a 60-ns pulsed streamer corona discharge was studied. Empirical relationships were obtained that allow the necessary energy consumption to be evaluated depending on the required degree of purification, the initial styrene content in air, and the nitrogen-oxygen mixture composition. On this basis, a general equation is derived that provides systematization of the experience in air purification.