Kinetic mechanisms of initiating hydrogen-oxygen mixture combustion through the excitation of electronic degrees of freedom of molecular oxygen by laser radiation

Kinetic mechanisms resulting in the enhancement of combustion of H₂+O₂ mixtures when O₂ molecules are excited to the a ¹Δ_g and b ¹Σ _g ⁺ states with laser radiation (λ=1.268 and 0.762 μm) are analyzed. It is shown that the excitation of O₂ molecules by the laser radiation leads to the appearance of new O, H, and OH formation channels; promotes the ignition of the starting mixture; and reduces the self-ignition temperature. With initial pressures in the range P ₀=10³–10⁴ Pa, the self-ignition temperature can be reduced to 300 K even at relatively low energies of the laser radiation with λ=0.762 μm.     

Initiation of combustion of a methane-air mixture in a supersonic flow behind a shock wave during laser excitation of O<Subscript>2</Subscript> molecules

The possibility of initiating detonation of CH₄ + air in a supersonic flow behind an oblique shock wave under the exposure of the mixture to laser radiation with wavelengths λ_I=1.268 μm and 762 nm is analyzed. It is shown that this irradiation leads to excitation of O₂ molecules to the a ¹Δ_g and b ¹Σ _g ⁺ states, which intensifies the chain mechanism of combustion of CH₄/O₂ (air) mixtures. Even for a small value of the laser radiation energy absorbed by an O₂ molecule (∼0.05–0.1 eV), detonation mode of combustion in a poorly inflammable mixture such as CH₄/air can be realized at a distance of only 1 m from the primary shock wave front for relatively small values of temperature (∼1100 K) behind the front under atmospheric pressure.     

Kinetics of processes in the middle atmosphere during the laser excitation of O2 molecules

Kinetic processes taking place in the atomic-molecular system O-O₂-O₃ in the middle atmosphere with the participation of oxygen molecules in the excited electronic states O₂(a ¹Δ_g) and O₂(b ¹Σ _g ⁺ ) are analyzed in detail. The possibility of increased ozone production under the influence of solar radiation during the laser excitation of O₂ molecules in the a ¹Δ_g state is demonstrated on the basis of numerical modeling. Upper and lower bounds are determined for the densities of O₂(a ¹Δ_g) molecules at which the ozone concentration increases in the irradiated zone. Zh. Tekh. Fiz. 68, 15–23 (August 1998)     

Molecular relaxations in mixtures of O2 with CO2 observed on?laser-induced gratings

Laser-induced gratings in mixtures of O₂ with CO₂ were formed by excitation of the O₂ molecules to the singlet state b ¹ Σ _g ⁺(v′=0). Density changes from heat release and from electrostriction, and variation of electric polarizability by excitation of molecules contribute to the grating. For modeling the relaxation of the excited O₂ molecules, a three-step process is assumed: Fast heat release with respect to the rotational states first, then medium fast electronic de-activation with excitation of vibrational states of the O₂ and CO₂ molecules, and then the final slow heat release processes. The observed temporal evolution of the diffraction efficiency of the grating agrees rather well with modeling. Average rates of the final relaxations and the value of the polarizability of the singlet state b ¹ Σ _g ⁺(v′=0) are determined in this way.     

Hyperfine interactions in molecules through laser spectroscopy

The infrared and millimeter wave spectroscopy, laser Stark spectroscopy, and beam maser spectroscopy of CH₃CN and its isotopic species will be discussed. The beam maser spectroscopy and hyperfine structure of molecules like NCCCD, ClCCD and CH₃CCH are reviewed. The laser magnetic resonance and hyperfine structure in CF, CH and CH₂ free radicals will be discussed. The Lamb dip spectroscopy and laser-induced fluorescence in I₂ involving theB ³Π(0 _u ⁺ ) state are reviewed with special reference to its hyperfine structure. The splitting of the rotational levels of N ₂ ⁺ in itsX ²Σ _g ⁺ andB ²Σ _u ⁺ states due to hyperfine interactions (along with the hyperfine structure) in laser-induced fluorescence in theB−X transition is discussed. Recent results obtained in the laser photo-acoustic spectrum of ICl in the transitionX ¹Σ⁺ −A ³Π₁ will be presented and the possibility of the use of this technique in studying the hyperfine structure will be discussed.     

On the influence of singlet oxygen molecules on characteristics of HCCI combustion: A numerical study

Mechanisms of homogeneous charge compression ignition (HCCI) combustion enhancement are investigated numerically when excited O₂(a ¹Δ_g) molecules are produced at different points in the compression stroke. The analysis is conducted with the use of an extended kinetic model involving the submechanism of nitric oxide formation in the presence of singlet oxygen O₂(a ¹Δ_g) or O₂(b ¹Σ_g ⁺) molecules in the methane-air mixture. It is demonstrated that the abundance of excited O₂(a ¹Δ_g) molecules in the mixture even in a small amounts intensifies the ignition and combustion and allows one to control the ignition event in the HCCI engine. Such a method of energy supply in the HCCI engine is much more effective in advancement of combustion timing than mere heating of the mixture, because it leads to acceleration of the chain-branching mechanism. The excitation of O₂ molecules to the a ¹Δ_g electronic state makes it possible to organise the successful combustion in the cylinder at diminished initial temperature of the mixture and increase the effective energy released during HCCI combustion. The advance in the value of this energy is much higher than the energy needed for the excitation of oxygen molecules. Moreover, in this case, the output concentration of NO and CO can be reduced significantly.     

Study of collisional deactivation of O2(b 1Σ g + ) molecules in a hydrogen-oxygen mixture at high temperatures using laser-induced gratings

Collisional deactivation of O₂(b ¹Σ _g ⁺ ) molecules resonantly excited by a 10 ns pulse of laser radiation with a wavelength of 762 nm in H₂/O₂ mixtures is experimentally studied. The radiation intensity and hence the molecule excitation efficiency have a spatially periodic modulation that leads to the formation of laser-induced gratings (LIGs) of the refractive index. The study of LIG temporal evolution allows collisional relaxation rates of molecular excited states and gas temperature to be determined. In this work, the b ¹Σ _g ⁺ state of O₂ molecules deactivation rates are measured in a 4.3 vol % H₂ mixture at the number density of 2 amg in the temperature range 291–850 K. The physical deactivation is shown to dominate in the collisions of H₂ with O₂(b ¹Σ _g ⁺ ) and O₂(a ¹Δ _g ) up to temperatures of 780–790 K at time delays up to 10 μs after the excitation pulse. The parameters of the obtained temperature dependence of the (b ¹Σ _g ⁺ state deactivation rate agree well with the data of independent measurements performed earlier at lower temperatures (200–400 K). Tunable diode laser absorption spectroscopy is used to measure the temperature dependence of the number density of the H₂O molecules which appear as the mixture, as the result of the dark gross reaction with O₂ molecules in the ground state, O₂ + 2H₂ → 2H₂O. The measurements show that this reaction results in complete transformation of H₂ into H₂O at temperatures of 790–810 K.     

Tunable KrF laser-induced fluorescence of C2 in a sooting flame

2 in a flame, excited by a tunable KrF laser near 248 nm. The first comprises several P and R lines of the (1,0) band of the e ³Π_g-a ³Π_u Fox–Herzberg system, with fluorescence bands extending past 350 nm. The second is the band head region of the (7,1) band of the D ¹Σ_u ⁺←B^′ 1Σ_g ⁺ system, with fluorescence at 232 nm from D to the X ¹Σ_g ⁺ ground state. Neither band has been previously observed in any environment. The flame in these experiments is highly sooting, and the C₂ seen here is likely produced by laser vaporization of the soot with subsequent laser photolysis of a C₂ precursor. In a rich flame, this fluorescence could cause interferences in other studies such as KrF laser Raman scattering. Moreover, signal level calculations suggest native C₂ near 10 ppm could be readily observed using the Fox–Herzberg excitation. Raman measurements of major species (X≥0.01) in the same flame, using the KrF laser, are in good agreement with a model prediction. Received: 2 April 1998/Revised version: 8 June 1998     

Combined analysis of the PFOODR data on the a 3Σ u+ , 23Πg, 23Σ g+ , 33Πg, and 43Σ g+ states of the K2 molecule

The known and new heterogeneous spectral data on the triplet states a ³Σ _u ⁺ , 2³Π_g, 2³Σ _g ⁺ , 3³Π_g, and 4³Σ _g ⁺ of the K₂ dimer are simultaneously fitted. The data published in J. Mol. Spectrosc. 234, 41 (2005) are refined. The new information used in the analysis contains the data on the 2³Σ _g ⁺ state, which have not been considered previously. The range of internuclear distances where the potential function of the lowest triplet state a ³Σ _u ⁺ is defined is extended. Original Russian Text ? V.B. Sovkov, V.S. Ivanov, D. Li, F. Xie, Li Li, 2007, published in Optika i Spektroskopiya, 2007, Vol. 103, No. 5, pp. 747–751.     

Spectroscopy of the a3Σu + state and the coupling to the X1Σg + state of K2

We report on high resolution Fourier-transform spectroscopy of fluorescence to the a³Σ_u ⁺ state induced by two-photon or two-step excitation from the X¹Σ_g ⁺ state to the 2³Π_g state in the molecule K₂. These spectroscopic data are combined with recent results of Feshbach resonances and two-color photoassociation spectra for deriving the potential curves of X¹Σ_g ⁺ and a³Σ_u ⁺ up to the asymptote. The precise relative position of the triplet levels with respect to the singlet levels was achieved by including the excitation energies from the X¹Σ_g ⁺ state to the 2³Π_g state and the frequencies of the fluorescence down to the a³Σ_u ⁺ state in the simultaneous fit of both potentials. The derived precise potential curves allow for reliable modeling of cold collisions of pairs of potassium atoms in their ²S ground state. Electronic supplementary material Supplementary Online Material     

Determination of the helium-4 mass in a Penning trap

The determination of the rotational quadrupole alignment of diatomic molecules via REMPI detection is investigated. In this process a high focal intensity usually increases the detection probability. At high intensities the AC Stark effect may cause a splitting of the normally degenerate m_J sublevels of a rotational state J beyond the spectral width of the exciting radiation. This leads to a selective detection of only certain m_J states with the consequence that deduced alignment factors can be misleading. From the theoretical considerations line profiles are explicitly calculated for dynamic polarizabilities which represent the B ¹Σ⁺ _u←X ¹Σ⁺ _g transition of H₂, in order to fit an experimental (3+1) REMPI spectrum and to predict (1+1') line shapes as a function of laser intensity. It is further shown that the deduced quadrupole alignment factor A ₀ ⁽²⁾ is significantly changed by the second order AC Stark effect when the intensities are chosen high enough to observe asymmetric broadened line profiles. Different combinations of relative linear polarizations of the exciting and ionizing laser beams are discussed. Received 1st August 2000 and Received in final form 2 May 2001     

Wavelength stabilization of an Ar+ laser with an external129I2 absorption cell

Wavelength stabilization of the green Ar⁺ laser line at 514.5 nm by¹²⁹I₂ absorption was found to be more favourable than by¹²⁷I₂ absorption because the maximum of¹²⁹I₂ absorption is closer to the center of the 514.5 nm gain curve. A simple method for stabilizing a short air cooled laser with only one servo loop is given, yielding a stability of 10⁻⁸ λ.     

Studies on the full vibrational energy spectra for some electronic states of diatomic molecular ions XY+

The first accurate studies on the vibrational spectroscopic constants and the corresponding full vibrational energy spectra of some electronic states of diatomic molecular ions XY⁺ were performed using algebraic method(AM). The AM is applied on the X¹Σ⁺ state of BeH⁺, the X²Σ⁺ state of CO⁺, the X²Π_g state of F ₂ ⁺ , the A²Π_u state of O ₂ ⁺ and the X²Σ _g ⁺ state of Li ₂ ⁺ . The results show that AM can generate accurate vibrational spectroscopic constants as well as accurate full vibrational energy spectra by using some accurate experimental vibrational energies, and that the AM vibrational energies are better than other theoretical data. __________ Translated from Chinese Journal of Atomic and Molecular Physics, 2005, 22 (4) (in Chinese)     

Temperature dependence of self-broadening in molecular-oxygen spectrum

Summary The ^p P(k=9) magnetic-dipole transition of the oxygen 0-0 vibrational band of the red systemb ¹Σ _g ⁺ ←X ³Σ _g ⁻ was observed with absorption spectroscopy using a tunable CW diode laser. A systematic investigation of the self-collisional broadening was performed for different gas pressures and temperatures. The temperature dependence of the self-broadening coefficient was determined to beT ^{−(1.02±0.05)}. Our results are of interest for collisional theory and as reference data in atmospheric monitoring applications. The authors of this paper have agreed to not receive the proofs for correction.     

Electron scattering with open-shell molecules: R-matrix method

Many molecules with an even number of electrons belong to open-shell systems due to π ² ground state electronic configuration. This configuration gives rise to three low-lying states X ³ Σ ⁻, a Δ ¹ and b ¹ Σ ⁺. The inclusion of these target states in a trial wave function of the entire scattering system have important implications in the resonances that may be detected in these open-shell molecules. Various molecules like O₂, PX (X = H and halogens), SO, Si₂, BF have π ² ground state configuration. The R-matrix method is a well established ab initio formalism to calculate differential, integral and momentum-transfer cross sections for the elastic scattering of electrons by molecules. We have calculated these cross sections for PH and SO molecules in the incident electron energy range 0–10 eV. The results are obtained by using the R-matrix method in which the closecoupling expansion of the wave function of the scattering system includes only the ground state. This target state is described by configuration interaction wave function that includes correlation effects. The cross section for electron impact on PH and SO are presented.     

Spectroscopy study of air plasma induced by IR CO2 laser pulses

A spectroscopic study of ambient air plasma, initially at room temperature and pressures ranging from 32 to 101 kPa, produced by high-power transverse excitation atmospheric (TEA) CO₂ laser (λ=9.621 and 10.591 μm; τ _FWHM≈64 ns; power densities ranging from 0.29 to 6.31 GW cm⁻²) has been carried out in an attempt to clarify the processes involved in laser-induced breakdown (LIB) air plasma. The strong emission observed in the plasma region is mainly due to electronic relaxation of excited N, O and ionic fragments N⁺. The medium-weak emission is due to excited species O⁺, N²⁺, O²⁺, C, C⁺, C²⁺, H, Ar and molecular band systems of N ₂⁺(_{2}^{+}( B ²\varSigma _u⁺^{2}\varSigma _{\mathrm{u}}^{+} –X ²\varSigma _g⁺)^{2}\varSigma _{\mathrm{g}}^{+}) , N₂(C³ Π _u–B³ Π _g), N ₂⁺(_{2}^{+}( D² Π _g–A² Π _u) and OH(A² Σ ⁺–X² Π). Excitation temperatures of 23400±700 K and 26600±1400 K were estimated by means of N⁺ and O⁺ ionic lines, respectively. Electron number densities of the order of (0.5–2.4)×10¹⁷ cm⁻³ and (0.6–7.5)×10¹⁷ cm⁻³ were deduced from the Stark broadening of several ionic N⁺ and O⁺ lines, respectively. Estimates of vibrational and rotational temperatures of N ₂⁺_{2}^{+} electronically excited species are reported. The characteristics of the spectral emission intensities from different species have been investigated as functions of the air pressure and laser irradiance. Optical breakdown threshold intensities in air at 10.591 μm have been measured.     

Conservation laws for classical and relativistic collisions. I

On the basis of simple kinematic arguments it is shown that any quantity, depending only on the nature and velocity of a particle, that is conserved in a collision must, in classical mechanics, be of the form λ+Σ_iμ_iυ_i+1/2vυ ² or in relativistic mechanics of the form λ+Σ_iμ_iυ_i[1−(υ ²/c ²)]^−1/2+νc [1−(υ ²/c ²)]^−1/2 where λ,μ _i, andν are particle parameters.     

Determination of absorption cross sections for oxygen molecules in the Schumann-Runge band region at high temperatures

We have measured the absorption cross sections of oxygen molecules in oxygen and in an oxygen-argon mixture heated by a shock wave, in the wavelength range 190–250 nm at temperatures of 1500–7000 K, for thermal equilibrium conditions behind the shock wave front. Analysis of the absorption cross sections obtained allowed us to select a data set that adequately describes the absorption characteristics of the electronic transition X³Σ _g ⁻ → B³Σ _u ⁻ for the oxygen molecule. In order to approximate the temperature dependence of these cross sections at a temperature of 1500–4500 K, we chose the function σ(λ, T) = σ₀(λ)(1 − exp (−θ/T)) exp (− n*θ/T) where θ₀ = 1.4·10⁻¹⁷, 1.4·10⁻¹⁷, 1.2·10^{− 17}, and 1.3·10⁻¹⁷ cm², n* = 3.1, 4.1, 5.6, and 7.47 for wavelengths 190, 210, 230, and 250 nm, respectively; θ = 2240 K is the characteristic temperature of the O₂ molecules. The approximation error was 19–25% and did not exceed the experimental error. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 13–17, January–February, 2006.     

Laser excited fluorescence of Na2

Fluorescence was excited in theB ¹Γ _u −X ¹Σ _g ⁺ system of Na₂, using the 488 and 514.5 nm radiations from a cw Ar⁺ laser. The fluorescence was monitored through a double monochromator employing photoelectronic detection. In the spectrum resulting from the 488 nm excitation, apart from a large number of fluorescence series, a previously unreported series could be identified. On either side of all the members of the strongest series a large number of collisional satellites were observed and assigned.     

Fluorescence studies of CS2 and SO2 at 121.6, 73.6–74.4 and 58.4 nm

The fluorescence spectra of CS₂ and SO₂ have been studied at three incident photon wavelengths of 121.6, 73.6–74.4 and 58.4 nm and relative production cross sections for different product states have been measured. The CS(A ¹Π→X ¹Σ⁺) system between 240 and 290nm has been obtained when CS₂ is photoexcited at 121.6nm whereas CS ₂ ⁺ (B ²Σ _u ⁺ →X ²Π _g ) and CS ₂ ⁺ (A ²Π _u →X ²Π _g ) systems have been produced between 276 and 295 and 437 and 555nm respectively when excited by both the incident photon wavelengths of 73.6–74.4 and 58.4nm. The fluorescence spectra of SO₂ obtained at 121.6 and 73.6–74.4nm include the vibrational bands of SO(A ³Π→X ³Σ⁻) and SO(B ²Π⁻→X ³Σ⁻) systems from 240 to 268 and 268 to 442nm respectively whereas the emission spectrum at 58.4nm, has contributions from the two SO systems and SO⁺(A ²Π→X ²Π) system. In all these emission spectra, the fluorescence bands of different systems have been analyzed and their relative production cross sections have been measured. The results obtained in the present investigations have been compared with a few recent reliable measurements reported in literature.