Pavel A. Mikheyev

Production of iodine atoms by dissociating CH3I and HI in a dc glow discharge in the flow of argon

The production of iodine atoms by means of glow discharge in gas flow, with a view to using them in chemical oxygen?iodine lasers, was studied. A dc glow discharge was sustained between coaxial electrodes in the vortex flow of argon, used as the carrier gas, with the addition of iodine-containing molecular precursors CH3I and HI.In the experiments a high atomic iodine concentration of more than 1016?cm?3 was achieved at a pressure of the Ar carrier of up to 30?Torr. The dissociation degree of the precursor molecules was 80?100% for CH3I and 100% for HI.The loss of iodine atoms in volumetric processes and due to diffusion to the walls of the transporting duct from the discharge region to the active medium was estimated. For the existing designs of iodine injectors of oxygen?iodine lasers, the loss of iodine atoms during transportation can be reduced.

Properties of O2(Δ1)–I(P21/2) laser medium with a dc glow discharge iodine atom generator

Experiments were carried out in a flow cell apparatus under conditions corresponding to those of a typical oxygen-iodine laser. The cell was equipped with a chemical jet type singlet oxygen generator and an electric discharge for the production of iodine atoms. The properties of the discharge generator and the active medium were studied using laser-induced fluorescence and emission spectroscopy. I2 or CH3I entrained in a carrier flow of Ar were used as atomic iodine precursors. About 50% of the iodine contained in CH3I molecules was extracted in the generator. 2.6% of the electric power loaded into the discharge was used in CH3I dissociation. Right after the discharge 80%–90% of the iodine flow consisted of atoms. However, due to recombination during transport, only 20%–50% of atoms remained at the point of injection into the oxygen flow. A straightforward comparison of two methods of oxygen-iodine medium production—conventional, by means of I2 dissociation in the singlet oxygen flow and with iodine atoms...

Study of iodine atom production in Ar/CH3I dc glow discharges

A transverse flow dc glow discharge in Ar/CH3I mixtures was modeled using a plug-flow approximation and plasma evolution was computed within a frame of reference moving with the gas. Reduced electric field as a function of transportation length was determined using the condition of a constant power density in the discharge. Cathode fall (CF) was found by comparing modeled and experimental discharge voltages. It was shown that conditions in the plasma column are nearly optimal for iodine atoms production with the lowest required energy and larger energies observed in the experiment are due to energy loss in the CF. In the experiment, concentrations of iodine molecules were measured downstream of the discharge using laser-induced fluorescence. Iodine flow at the exit of the discharge generator consisted of 80–90% of iodine atoms and only of 10–20% of iodine molecules. A good agreement between modeled and measured concentrations was found.

On the O 2 ( a 1 Δ) quenching by vibrationally excited ozone

The development of a discharge oxygen iodine laser (DOIL) requires efficient production of singlet delta oxygen (O2(a)) in electric discharge. It is important to understand the mechanisms by which O2(a) is quenched in these devices. To gain understanding of this mechanisms quenching of O2(a) in O(3P)/O2/O3/CO2/He/Ar mixtures has been investigated. Oxygen atoms and singlet oxygen molecules were produced by the 248 nm laser photolysis of ozone. The kinetics of O2(a) quenching were followed by observing the 1268 nm fluorescence of the O2 a → X transition. Fast quenching of O2(a) in the presence of oxygen atoms and molecules was observed. The mechanism of the process has been examined using kinetic models, which indicate that quenching by vibrationally excited ozone is the dominant reaction.

Formation of I2(B3Π0) in the presence of O2(a1Δ)

The mechanism by which I2(B3Π0) is excited in the chemical oxygen-iodine laser was studied by means of emission spectroscopy. Using the intensity of the O2(b1Σ,υ′=0)→O2(X3Σ,υ″=0) band as a reference, I2(B3Π0) relative number densities were assessed by measuring the I2(B3Π0,υ′)→I2(X1Σ,υ″) emission intensities. Vibrationally excited singlet oxygen molecules O2(a1Δ,υ′=1) were detected using infrared emission spectroscopy. The measured relative density of O2(a1Δ,υ′=1) for the conditions of a typical oxygen-iodine laser medium amounted to ∼15% of the total O2(a1Δ) content. Mechanisms for I2(B3Π0) formation were proposed for both the I2 dissociation zone and the region downstream of the dissociation zone. Both pumping mechanisms involved electronically excited molecular iodine I2(A′3Π2u, A3Π1u) as an intermediate. It is proposed that in the dissociation zone the molecular iodine A′3Π2u and A3Π1u states are populated in collisions with vibrationally excited singlet oxygen molecules O2(a1Δ,υ′), whereas in the dow...

Tunable diode-laser spectroscopy (TDLS) of 811.5nm Ar line for Ar(4s[3/2]2) number density measurements in a 40MHz RF discharge

A hardware and a computative technique for tunable laser spectroscopy was developed for simultaneous measurement of Gaussian and Lorentian components of line broadening by fitting Voigt line profile. The technique was tested in measurements of pressure broadening coefficient for 811.5 nm Ar absorption line in a 40 MHz discharge in the pressure range 15-75 Torr with the help of tunable diode laser with a short external resonator. The obtained values for this coefficients reduced to 300 K are: ξAr-Ar = (2.85±0.1)×10-10 s-1cm3 for broadening in the parent gas and ξAr-He = (3.3±0.1)×10-10 s-1cm3 for broadening in helium. A good agreement with published results is observed. Measured Ar(1s5) number density amounted to 10-11cm-3 for the discharge power density ~10 W cm-3.

Properties of a DC glow discharge iodine atom generator

Concentration of iodine molecules at the outlet of an electric discharge iodine atoms generator was measured using laser-induced fluorescence. Methyl iodine was used as an iodine atom precursor. Fraction of iodine extracted from CH3I in the discharge generator was about 50%. Optimal mode of operation at which 80-90% of total extracted iodine was in the form of iodine atoms was found. Iodine atom content in the gas flow decreased during transportation down to 20-30% at the point of iodine injection into the oxygen flow. Fraction of power load spent on CH3I dissociation amounted to ≈3%.

Oxygen-iodine active medium with external production of iodine in a DC glow discharge

Experiments with a flow cell apparatus imitating conditions of oxygen-iodine laser, equipped with a chemical jet singlet oxygen generator and an electric discharge iodine generator have been performed. I2 and CH3I in the flow of Ar were used as atomic iodine precursors. The distributions of the electronically excited species along the flow were examined detecting their optical emissions. A straightforward comparison of two methods of oxygen-iodine medium production - conventional, by means of I2 dissociation in the singlet oxygen flow and with iodine atoms produced externally in the electric discharge - was performed. It was found that stored electron energy lifetime had been about 30% longer, when iodine was produced from CH3I in the discharge, compared to the conventional I2 dissociation in the singlet oxygen flow. It was observed that maximums of the I(2P1/2) and I2(B) concentrations had shifted to the nozzle plane, when I2 in Ar carrier was subjected to the glow discharge, pointing to a nearly twofold increase in the I2 dissociation rate. Contrary to the known results for low iodine and singlet oxygen concentrations, squared dependence of the amplitude of the I2(B) luminescence maximum with I(2P1/2) concentration was observed in the dissociation region for both methods of iodine production.

Production of Ar metastables in a dielectric barrier discharge

The results of experiments with a dielectric barrier discharge (DBD) are presented, where the production of metastable argon atoms was studied. The recently proposed optically pumped all-rare-gas laser (OPRGL) utilizes metastable atoms of heavier rare gases as lasing species. The required number density of metastables for efficient laser operation is 1012÷1013 cm-3 in an atmospheric pressure of He buffer gas. Recent experiments had shown that such densities are easily produced in a nanosecond pulsed discharge, even at pressures larger than atmospheric, but problems appear when one is trying to produce them in a CW regime. The reason for difficulties in the CW production of metastables at an atmospheric pressure seems to be the low value of the E/N parameter (<5-6 Td). In our experiments a 20 KHz DBD in 2-5% Ar mixture with He at an atmospheric pressure was studied. [Ar(1s5)] number density of the order of 1012 cm-3 was readily achieved. Temporal behavior of [Ar(1s5)] throughout the DBD cycle was obtained. The results demonstrate the feasibility of DBDs for OPRGL development.

Influence of molecular oxygen on iodine atoms production in an RF discharge

The results of the experiments and modeling of CH3I dissociation in a 40 MHz RF discharge plasma are presented. A discharge chamber of an original design, consisting of quartz tubes between two planar electrodes, permitted us to produce iodine atoms with a number density up to 2 × 1016 cm−3. In this discharge chamber, contrary to the previous experiments with a DC discharge and RF discharge with bare planar electrodes, contamination of the walls of the tubes did not disturb discharge stability, thus increasing iodine production rate. A substantial increase in CH3I dissociation efficiency due to the addition of oxygen into Ar(He) : CH3I mixtures was observed. Complete CH3I dissociation in the Ar : CH3I : O2 mixture occurred at 200 W discharge power, while a fraction of discharge power spent on iodine atoms production at 0.17 mmol s−1 CH3I flow rate amounted to 16%. Extensive numerical modeling showed satisfactory agreement with the experiments and permitted us to estimate a previously unknown rate of constants for the processes: Ar* + CH2I2 → Ar + CH2 + I + I – 1.5 × 10−11 cm3 s−1; Ar* + CH2I2 → Ar + CH2I+ + I + e – 10−11 cm3 s−1. Also, the cross section for the process CH2I2 + e → CH2 + I + I + e was estimated to be five times smaller than for the analogous process with CH3I.