Valeriy N. Azyazov

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...

HACA's Heritage: A Free-Radical Pathway to Phenanthrene in Circumstellar Envelopes of Asymptotic Giant Branch Stars

The hydrogen-abstraction/acetylene-addition (HACA) mechanism has been central for the last decades in attempting to rationalize the formation of polycyclic aromatic hydrocarbons (PAHs) as detected in carbonaceous meteorites such as in Murchison. Nevertheless, the basic reaction mechanisms leading to the formation of even the simplest tricyclic PAHs like anthracene and phenanthrene are still elusive. Here, by exploring the previously unknown chemistry of the ortho-biphenylyl radical with acetylene, we deliver compelling evidence on the efficient synthesis of phenanthrene in carbon-rich circumstellar environments. However, the lack of formation of the anthracene isomer implies that HACA alone cannot be responsible for the formation of PAHs in extreme environments. Considering the overall picture, alternative pathways such as vinylacetylene-mediated reactions are required to play a crucial role in the synthesis of complex PAHs in circumstellar envelopes of dying carbon-rich stars.

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.

Analysis of CW Oxygen-Iodine Laser Performance Using Similarity Criteria

A simplified single-frequency two-level model for predicting power extraction from a CW oxygen-iodine laser (OIL) considering relaxation losses has been developed. The model predicts that the energy efficiency of CW OILs with stable resonators depends on three similarity criteria. Criterion γ_d is the ratio of the residence time of active medium in the resonator to the singlet oxygen O₂(¹\triangle) energy extraction time at an infinite intraresonator intensity. Criterion Π is small-signal gain to the threshold ratio. Criterion Λ is the relaxation to excitation rate ratio for the electronically excited iodine atoms I(²P_1/2). The analysis of the calculated and experimental data show that effective power extraction from the CW OIL active medium is achieved when the values of the similarity criteria are in the ranges: γ_d=4-6, Π = 3-8, and Λ ≤ 0.01.

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...

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.

VUV Photoionization Study of the Formation of the Simplest Polycyclic Aromatic Hydrocarbon: Naphthalene (C10H8)

The formation of the simplest polycyclic aromatic hydrocarbon (PAH), naphthalene (C10H8), was explored in a high-temperature chemical reactor under combustion-like conditions in the phenyl (C6H5)-vinylacetylene (C4H4) system. The products were probed utilizing tunable vacuum ultraviolet light by scanning the photoionization efficiency (PIE) curve at a mass-to-charge m/ z = 128 (C10H8+) of molecules entrained in a molecular beam. The data fitting with PIE reference curves of naphthalene, 4-phenylvinylacetylene (C6H5CCC2H3), and trans-1-phenylvinylacetylene (C6H5CHCHCCH) indicates that the isomers were generated with branching ratios of 43.5±9.0 : 6.5±1.0 : 50.0±10.0%. Kinetics simulations agree nicely with the experimental findings with naphthalene synthesized via the hydrogen abstraction-vinylacetylene addition (HAVA) pathway and through hydrogen-assisted isomerization of phenylvinylacetylenes. The HAVA route to naphthalene at elevated temperatures represents an alternative pathway to the hydrogen abstraction-acetylene addition (HACA) forming naphthalene in flames and circumstellar envelopes, whereas in cold molecular clouds, HAVA synthesizes naphthalene via a barrierless bimolecular route.

O2(b1Σg+) Quenching by O2, CO2, H2O, and N2 at Temperatures of 300-800 K

Rate constants for the removal of O2(b1Σg+) by collisions with O2, N2, CO2, and H2O have been determined over the temperature range from 297 to 800 K. O2(b1Σg+) was excited by pulses from a tunable dye laser, and the deactivation kinetics were followed by observing the temporal behavior of the b1Σg+-X3Σg- fluorescence. The removal rate constants for CO2, N2, and H2O were not strongly dependent on temperature and could be represented by the expressions kCO2 = (1.18 ± 0.05) × 10-17 × T1.5 × exp[Formula: see text], kN2 = (8 ± 0.3) × 10-20 × T1.5 × exp[Formula: see text], and kH2O = (1.27 ± 0.08) × 10-16 × T1.5 × exp[Formula: see text] cm3 molecule-1 s-1. Rate constants for O2(b1Σg+) removal by O2(X), being orders of magnitude lower, demonstrated a sharp increase with temperature, represented by the fitted expression kO2 = (7.4 ± 0.8) × 10-17 × T0.5 × exp[Formula: see text] cm3 molecule-1 s-1. All of the rate constants measured at room temperature were found to be in good agreement with previously reported values.

Deactivation and reaction of excited states of Rb in collisions with H2, CH4 and C2H6

Alkali vapor lasers commonly use methane (CH4) or ethane (C2H6) to induce energy transfer between the optically pumped level (n2P3/2) and the upper laser level (n2P1/2). A complication is that the alkali metal eventually reacts with the hydrocarbons. The reaction becomes exothermic for alkali atoms that have been excited by energy pooling processes such as Rb(5p)+Rb(5p)→Rb(*)+Rb(5s), where * indicates 6s, 6p or 4d. We have used laser pump-probe methods to examine the reactions of Rb(n2P) with CH4, and C2H6 for states with n=6 and 7. Pump-probe measurements indicated a loss of Rb due to a reaction. Surprisingly, the RbH product was not detected with CH4, and C2H6. High-level ab initio calculations have been used to study the reactive interactions between Rb, methane and ethane.