N. A. Timofeev

Simulation of the high-pressure mercury discharge lamp during the middle phase of start-up (medium mercury pressure)

This paper deals with the modelling of the medium pressure Hg and Hg-Ar positive column (Hg pressure range 5-350 Torr). The aim of this work is to simulate, in a first approximation, the middle phase of the Hg high-pressure lamp warm-up. In this approach, the main assumption is that time evolution of the discharge can be divided into a succession of stationary sub-phases characterized by the Hg partial pressure. Thus, we present here a self-consistent steady-state collisional-radiative model describing the middle sub-phase. This model includes volume recombination of Hg atomic and molecular ions, as well as several atom-atom inelastic scattering mechanisms. Calculations are carried out for both pure Hg and Hg-Ar discharges. Our results, which are in good agreement with experimental data from the literature, confirm that plasma thermalization occurs in the middle start-up phase (electron and gas temperatures become equal during this phase). Furthermore, this simulation shows the importance of different elementary processes, like atom-atom inelastic scattering, for the medium pressure plasma description. In fact, these mechanisms cannot be neglected until LTE conditions are reached.

Low pressure water vapour discharge as a light source: I. Spectroscopic characteristics and efficiency

Spectral and electrical characteristics of a low pressure dc discharge formed from a mixture of one of the rare gases Ne, Ar or Kr plus water vapour are studied. Water vapour is only a minor additive to the rare gas. It has been shown that enhanced emission of the OH 306.4 nm band is registered from the discharge of Ar mixed with water vapour. Plasmas from the other investigated rare gases yielded considerably less OH 306.4 nm emission. Data about consumed electric power, spectra and relative efficiencies are presented.

Low pressure water vapour discharge as a light source: II. Electrical characteristics

The electric field strength, electrode fall voltage, light emission characteristics and efficiency of a (Ar + H2O) dc discharge as functions of water vapour content, argon pressure and electric current are presented. The data show that the main processes of 306.4 nm OH band generation are (1) a collision between an excited argon atom and a water molecule with simultaneous excitation of OH into the A 2Σ+ state and (2) electron excitation of a ground state hydroxyl molecule produced by a quenching process from a water molecule. Electric field strength measurements make it possible to conclude that the light production efficiency of the plasma under study can reach 35 lm W−1. It is possible, with these data, to propose a model of the plasma in question having reasonable accordance with the experiment and show the way to further increase the efficiency.

Electronic spectra of XeNe molecules in the range 77100-90100 cm-1 near Xe* (6p, 5d, 6p’, 7s, 7p, 6d) obtained by the (3 + 1) REMPI and (2 + 1) REMPI methods

The electronic spectra of XeNe molecules in the range of 77100-90100 cm-1 are measured by the method of laser resonance multiphoton ionization in a supersonic jet. The photoionization spectra are obtained upon two- and three-photon excitations of molecules and their ionization by the next photon. In the range of 80300-90100 cm-1 near Xe*(5d, 6p’, 6d, 7s, and 7p), the spectra are obtained for the first time. A whole number of vibrational systems are measured in this range. The majority of vibrational systems near Xe* (5d, 6d, 7p, and 7s) are located in the red range with respect to their dissociation limits. In the blue range with respect to the dissociation limits, continua corresponding to transitions of molecules from the ground state to repulsive potential curves of excited states are detected. For a number of excited states of XeNe molecules, the vibrational analysis is performed and molecular constants are estimated.

The study of ungerade electronic states of the Xe2 molecules in the region of Xe*(5p56p, 5d, 7s, 6d) by the resonance multiphoton ionization method

Electronic spectra of the Xe2 molecules in the energy range of 77700–89300 cm−1 are recorded. The method of resonance enhanced multiphoton ionization of molecules in a supersonic molecular beam was used, in which excitation of the molecules by three photons was followed by ionization caused by a fourth photon (the (3+1) REMPI method). Analysis of the vibrational structure of observed systems of bands yielded information about the dissociation energy and the molecular constants for ungerade states of molecules. On the basis of the Franck-Condon principle, the equilibrium distances for potential curves were estimated from the relative intensities in vibrational progressions. Data on 16 new electronic states of diatomic xenon molecules with the dissociation limits Xe2* → XE(5p61S0) + Xe*(5p56p,5d, 7s, 7p) were obtained.

Investigation of a -discharge plasma under an increased pressure of Ar and in narrow tubes

The electrokinetic characteristics (the electron energy distribution function, the strength of the longitudinal electric field and the concentration and average energy of electrons) are measured and calculated in a -discharge plasma under a high pressure of argon (up to 30 Torr) and in narrow tubes (the tube radius is less than 1.0 cm). A simple method of treating the second derivative of the probe current with respect to the probe potential is proposed for a straightforward way to obtain the electron energy distribution function under a high pressure of a gas. It is shown that the main assumptions on which modelling of the plasma of mercury luminescent lamps is based are also valid for the plasma in question. This leads to the existence of special similarity laws and gives a new possibility for diagnostics based on the similarity properties of the plasma. The approach proposed in the work can be easily extended to the mixtures of mercury vapour and other rare gases.

Discharge plasma in an argon–water vapor mixture as a gas laser medium

The possibility of obtaining the inverse population of the excited states of hydroxyl molecules generated in low-pressure discharge plasma in an Ar–H2O mixture was studied. The possibility of inversion of both the A2Σ+ state relative to the X2Π ground state and inside the vibrational-rotational structure of the levels of the A2Σ+ excited state of the OH molecule was analyzed.

Relaxation channels of multi-photon excited xenon clusters

The relaxation processes of the xenon clusters subjected to multi-photon excitation by laser radiation with quantum energies significantly lower than the thresholds of excitation of atoms and ionization of clusters were studied. Results obtained by means of the photoelectron spectroscopy method showed that desorption processes of excited atoms play a significant role in the decay of two-photon excited xenon clusters. A number of excited states of xenon atoms formed during this process were discovered and identified.

A new similarity law and its application to plasma research

A new similarity law for the discharge plasma in gas mixtures has been obtained-its main feature being the smaller number of independent external parameters which are necessary to describe the plasma in question. This law is proposed for use in plasma research and some examples of such an application are presented.

Relaxation times measurement in single and multiply excited xenon clusters

Direct measurement of the rates of nonradiative relaxation processes in electronically excited xenon clusters was carried out. The clusters were created in a pulsed supersonic beam and two-photon excited by femtosecond laser pulses with a wavelength of 263 nm. The measurements were performed using the pump-probe method and electron spectroscopy. It is shown that relaxation of light clusters XeN (N < 15) predominantly occurs by desorption of excited xenon atoms with a characteristic time constant of 3 ps. Heavier electronically excited clusters (N > 10) vibrationally relax to the lowest electronically excited state at a rate of about 0.075 eV/ps. Multiply excited clusters are deactivated via energy exchange between excited centers with the ionization of one of them. The production of electrons in this process occurs with a delay of ∼4 ps from the pump pulse, and the process is completed in 10 ps.