Strong dependence of ir multiphoton absorption and dissociation yield of SF6 molecule on intensity (duration) of exciting laser pulse

The role of played by the intensity of the exciting TEA CO₂ laser pulses in the processes of ir multiphoton absorption (MPA) of SF₆ molecules cooled toT _R?40 K andT _v?160 K in a pulsed supersonic jet and the dissociation of SF₆ in a bulk atT?300 K under essentially collisionless conditions have been investigated. A strong dependence of MPA and the dissociation yield on pulse intensity were observed. The frequency dependences of the intensity effects were studied.     

Gas dynamics of a supersonic radial jet. Part I

The gas dynamics of a supersonic radial jet was studied under conditions close to cold spraying. The jet visualization was performed for exhaustion into submerged space with atmospheric pressure and jet impingement to a target. For the cases of swirled and unswirled supersonic radial jets, the pressure profiles measured by a Pitot tube were taken for different distances from the nozzle outlet and for different widths of supersonic part δ _ex = 0.5?2 mm and for prechamber pressure in the range p ₀ = 1?2.5 MPa.     

Oscillatory modes of interaction of supersonic overexpanded jets with barriers

The self-oscillatory interaction of supersonic jets with barriers has mainly been studied for under-expanded jets. There are only a few experimental studies examining the case of overexpanded jets, with little computational work done in this direction. To fill this gap, we performed numerical simulations of overexpanded supersonic jets with barriers. The calculations were performed by the Godunov method on fine grids using parallel programming techniques. In the course of numerical simulations, the gasdynamic parameters of the jet and the geometric parameter of the barrier were varied. The barrier had the shape of a cylindrical cavity of depth l = (0 − 18)r _a , where r _a is the nozzle exit radius (the case l = 0 corresponds to a flat-end barrier). Based on the results of the numerical simulations, the conclusion on whether the self-oscillation process occurs was drawn and the dependence its characteristics (frequency and amplitude) on the governing gasdynamic and geometric parameters were obtained. Good agreement with experimental data on the fundamental tone frequency was demonstrated. A low-frequency oscillation mode was mostly realized. In this case, the jet experienced periodic suctions into and ejections from the cavity, counter the oncoming jet flow, with the formation of a structure consisting of three discontinuity surfaces (two shock waves and a separating surface contact).     

Polarized fluorescence of jet-cooled molecular iodine

The degree of polarized fluorescence of molecular iodine ¹²⁷I₂ cooled in a supersonic jet under rotationally selective excitation in the electronic transition B ³Π_0u ⁺-X ¹Σ _g ⁺ has been measured and calculated. It was found that the interaction of the angular momentum of the molecule with the nuclear spins of iodine atoms leads to a considerable depolarizing effect. This effect is most pronounced for small rotational quantum numbers of the angular momentum that are comparable with the total nuclear spin of the iodine molecule, which is equal to 5.     

The High-Resolution ν5 Band of Jet-Cooled Si2F6

Vibration-rotation spectra of the parallel ν₅ band of hexafluorodisilane have been measured in a supersonic free jet with 0.001 cm⁻¹ resolution. Three isotopic species, ^28,28Si₂F₆, ^28,29Si₂F₆, and ^28,30Si₂F₆, have been studied. The effect of internal rotation is not observed, indicating that the splitting is smaller than our spectral resolution. A very weak parallel band observed with a slight red shift from the ν₅ fundamental band has been assigned tentatively to the (ν₄ + ν₅)-ν₄ hot band.     

The characteristics of multiple-photon absorption of SF6 molecules cooled in free jet expansion from a pulsed supersonic nozzle

The multiple-photon absorption of pulsed TEA CO₂ laser radiation by SF₆ molecules cooled toT _R40K andT _v160K in the free jet expansion from a pulsed supersonic nozzle has been investigated at energy fluences of 0.1 to 3.0 J·cm^–2.For practically all laser lines which coincide with the linear absorption spectrum of thev ₃ vibrational mode of SF₆ [P(12)...P(28), 10,6 m], the dependence of the absorbed energyE _ab on the exciting energy fluence was found to be steeper than linearE _abⁿ, wheren=(1.1 to 1.8). Considerable increases of the absorption cross sections with increasing energy fluence were observed.The fraction of the molecules interacting with the laser radiation is estimated.     

Supersonic Jets with Periodic Structure due to Arc plasma Expansion into a Low pressure ambient

A supersonic plasma jet was produced by a d.c. arc plasma generator operated at normal pressure and connected to a low-pressure (p∞ = 0.2-50 kPa) chamber via cylindrical nozzle with diameter of 2.5 mm. The argon gas flow rate was G = 0.025 to 0.35 g.s^?1. In some experiments current I_E ≦ 30 A passed coaxially through the initial part of the jet. Photographic records of the jet and pressure measurements are in agreement with theoretical predictions by a simple one-dimensional, gasdynamical model capable of self-consistent calculations throughout the plasma source/jet system. Periodic jet structure is observed over a wide range of experimental conditions, incl. in highly under-expanded flow. The jet expansion angle and Mach disc position vary with p∞, G and I_E, but are nearly constant at different arcing currents.     

Isotope-selective infrared multiphoton dissociation of CF3Br in a supersonic free jet

The carbon-isotope selectivity in the multiphoton dissociation of CF₃Br is studied in the collisional region of supersonic free jet. The isotopic abundance of¹²C and¹³C in C₂F₆ formed by recombination of the dissociation products is measured with a quadrupole mass spectrometer. An enrichmet factor of 9.4 is obtained for¹²C with the 9R(30)CO₂ laser line while the factor of 6.9 is obtained for¹³C with the 9P(16) line.     

The rotational spectrum of methyl trifluoroacetate

ABSTRACT

The pulsed supersonic jet expansion microwave spectra of the parent and all three ¹³C mono-substituted isotopologues of methyl trifluoroacetate have been measured in the 6.5–18 GHz range. All observed transitions are split into two component lines, due to the internal rotation of the methyl group. The corresponding barrier has been determined to be V ₃ = 4.379(3) kJ/mol. Structural information has been obtained from the 12 available rotational constants.     

Reinvestigation of the ν2 + 2ν3 subband in the overtone icosad of 12CH4 using cavity ring-down (CRD) spectroscopy of a supersonic jet expansion

We report a detailed reinvestigation of the ν₂?+?2ν₃ combination band of methane ¹²CH₄ centred at (7510.3378?±?0.003)?cm^?1 ((225.154263?±?0.0001)?THz) within the icosad of the overtone absorption. A new experimental setup is described, allowing us to carry out cw-laser cavity ring-down spectroscopy (cw-CRDS) at instrumental resolution in the MHz range in seeded supersonic jet expansions down to rotational temperature of 7?K compared to previous cw-CRDS measurements in our group achieving about 50?K in expansions of neat CH₄. We provide a careful re-analysis on the basis of our new experimental results for the Q and R branch transitions including data obtained between about 7 and 300?K under various conditions. We resolve previously observed discrepancies of assignments and are able to present a definitive assignment for lines involving angular momentum quantum numbers up to J?=?4. The analysis of relative intensities in spectra taken at rotational and effective translational temperatures between about 50?K and less than 10?K indicate conservation of nuclear spin symmetry upon supersonic jet expansion, in agreement with previous results using other techniques and covering other spectral ranges.     

Infrared Laser Spectroscopy of Jet-Cooled C2F6near 10 μm

The ν₅(A_2u) and ν₇(E_u) C–F stretching fundamentals of hexafluoroethane, C₂F₆, have been recorded in a supersonic jet by diode laser absorption spectroscopy. The parallel 5¹_oband is accompanied by five satellite bands, of which three have been assigned to hot bands. A fourth satellite band arises from¹²CF₃¹³CF₃. Transitions satisfying 0 ≤KΔK≤ 7 of the perpendicular 7¹_oband are unperturbed while those having −10 ≤KΔK≤ 7 can be fitted assuming anR_z-Coriolis interaction with a state lying at ν_p= 1256 cm⁻¹. A second localized perturbation affects lines withKΔK≥ 8. The band origins are 1117.10736 (7) cm⁻¹(5¹_o) and 1252.96950 (17) cm⁻¹(7¹_o, 0 ≤KΔK≤ 7), and the rotational constantB₀= 0.0615759 (27) cm⁻¹.     

Effective generation of characteristic <Emphasis Type="Italic">K</Emphasis>-rays from large laser-excited SF<Subscript>6</Subscript> clusters in the presence of an Ar carrier gas

The parameters of characteristic sulfur K _α-rays generated from SF₆ clusters that are surrounded by argon atoms and are irradiated by intense laser radiation have been analyzed. It has been found that the formation of large SF₆ clusters under the optimum experimental parameters is accompanied by the high-efficiency generation of the characteristic X rays, and the flux density of the characteristic X-ray photons is 100 photons/(mrad² pulse) at a laser-pulse energy of 5 mJ. It has been shown that the third-harmonic generation process can be used to characterize the spatial sizes of the gas-cluster jet and the region of the cluster plasma.     

Rotationally-Resolved Excitation Spectroscopy of the Methoxy Radical in a Supersonic Jet

Abstract

Laser-induced fluorescence excitation spectra of the methoxy radical have been obtained under sufficiently high resolution in a supersonic jet expansion. The rotational structure associated with its origin band has been identified in the midst of strong overlapping rotational transitions due to the hydroxyl radical in the 31490–31700 cm^?1 spectral region. Rotationally-resolved A ²A₁ - X ²E spectra of the 0₀ ⁰ band of methoxy have been explicitly assigned using the nomenclature for prolate symmetric top transitions in doublet states.     

The structure and helicity of perfluorooctanonitrile, CF3–(CF2)6–CN

New molecular structural data is presented for a cyanide terminated oligomer of polytetrafluoroethene. The target molecule, CF₃–(CF₂)₆–CN, has been seeded within a pulsed supersonic expansion of argon. The result of this action is to cool the species to rotational temperatures below 4 K. Within this state, the pure rotational spectrum of the oligomer has been recorded using two types of Fourier transform microwave spectroscopy. A total of 111 transitions have been identified involving rotational J levels between 6 and 40. Only a- and b-type transitions were observed. The spectrum has been analyzed using a Hamiltonian containing all three rotational constants and one centrifugal distortion constant, D_J. The experimental spectroscopic constants have been used to develop an effective molecular structure by scaling the quantum chemical calculated structure. The data shows that the seven carbon perfluorinated chain for the isolated oligomer twists ≈104°. This compares well to the C₇F₁₃-twist of ≈97° anticipated from the X-ray structure of phase II polytetrafluoroethene.     

Submillimeter-wave spectroscopy of the K = 2–1 subband of the Ne–CO complex

The pure rotational spectrum of the Ne–CO van der Waals complex has been measured in the frequency range of 208–230 GHz by using the Cologne supersonic jet spectrometer for terahertz applications (SuJeSTA). Eleven new transitions were assigned as belonging to two R- and two Q-branches of the K = 2–1 subband detected for the first time in the ground vibrational state of CO (ν_CO = 0). Improved molecular parameters of the Ne–CO complex were obtained which allowed for a sensitive test of the available intermolecular potential energy surfaces of the Ne–CO system.     

Laser Excitation Spectroscopy of the Jet-Cooled Methoxy Radical Amidst Hydroxyl Transitions

Abstract

Laser-induced fluorescence excitation spectra of the methoxy radical have been recorded under high resolution in a supersonic jet expansion. The rotational structure associated with the 3¹ ₀ band of methoxy has been identified in the midst of strong overlapping rotational transitions due to the hydroxyl radical in the 32240 - 32580 cm^?1 spectral region. Rotationally-resolved à ²A₁ – [Xtilde] ²E spectra of the 3¹ ₀ band of methoxy have been explicitly assigned using the nomenclature for prolate symmetric top transitions in doublet states.     

Shock associated noise of supersonic jets from convergent-divergent nozzles

Results of experimental and theoretical studies of the characteristics of shock associated noise from imperfectly expanded supersonic jets over an extensive range of underexpanded and overexpanded operating conditions are described. This kind of broadband noise is believed to be generated by the weak but coherent interaction between the downstream propagating large scale turbulent flow structures in the mixing layer of the jet and the nearly periodic shock cell system. Theoretical reasoning based on this mechanism leads to the scaling formula that the intensity of shock associated noise varies as (M_j² ? M_d²)² where M_j and M_d are the fully expanded jet operating Mach number and nozzle design Mach number, respectively. This formula holds for underexpanded as well as overexpanded jet Mach numbers. In addition, a peak frequency formula is also derived from the same model. The noise intensity, directivity and spectra of supersonic jets from a convergent-divergent nozzle of design Mach number 1·67 were measured in an anechoic facility over the Mach number range of 1·1 to 2·0. The effect of jet temperature was investigated by operating the jet at three temperature conditions. These sets of data provide sufficient information for fully assessing the relative importance and characteristics of shock associated noise of supersonic jets from convergent-divergent nozzles. Comparisons between theoretical results and measurements show very favorable agreement.     

Determination of the Splitting of the 6a0 and 6b0 Bands of C-Hexadeuterobenzene in a Supersonic Jet

By using resonance-enhanced two-photon ionization, rotationally resolved spectra of the 6¹₀ band of ¹²C₆D₆ and (¹³C¹²C₅D₆ molecules have been obtained for the first time at a rotational temperature of 0.7 K in a pulsed supersonic beam. From the former, the values of B″ = 0.1573 ± 0.0008 cm⁻¹, B′ = 0.1508 ± 0.0008 cm⁻¹, and ξ′ = −0.412 ± 0.050 have been derived for rotational and Coriolis constants in the lower and upper levels of ¹²C₆D₆. Also, the spectra corresponding to ¹²C₆H₆ and ¹³C¹²C₅H₆ have been measured and the values B″ = 0.1892 ± 0.0008 cm⁻¹, B′ = 0.1815 ± 0.0008 cm⁻¹, and ξ′ = −0.586 ± 0.050 have been obtained for ¹²C₆H₆, in agreement with previous results. Rotational constants of ¹³C labeled benzene molecules have been geometrically deduced from the constants obtained. Experimental isotopic shifts of the vibronic origins of the 6a¹₀ and 6b¹₀ bands have been determined. There is agreement with previous ¹³C-benzene-h₆ data. The present results are −0.91 ± 0.05 and 3.09 ± 0.05 cm⁻¹ for ¹³C¹²C₅D₆ and −1.64 ± 0.05 and 2.64 ± 0.05 cm⁻¹ for ¹³C¹²C₅H₆. The splittings of vibrational modes 6b and 6a in the ¹B_2u state are 4.00 ± 0.10 cm⁻¹ for ¹³C¹²C₅D₆ and 4.28 ± 0.10 cm⁻¹ for ¹³C¹²C₅H₆.     

Quantitative measurement of naphthalene in low-pressure flames by jet-cooled laser-induced fluorescence

We have recently developed a new laser based set-up (Jet-Cooled Laser-Induced Fluorescence) for the analysis of aromatic compounds generated in flames. This method relies on the extraction of the species from the flame via a thin microprobe and their direct analysis inside a supersonic free jet by Laser-Induced Fluorescence (LIF). Under the supersonic conditions of the jet, the vibronic spectra of the molecules become structured as the possibility of electronic transitions is reduced, allowing their selective detection by LIF. In addition, due to the very low quenching efficiency inside the jet, LIF signals can be directly related to the population of the probed species and easily calibrated into absolute concentrations. All of the work presented here has been carried out for naphthalene, which is an important PAH involved in soot formation mechanisms. The calibration procedure is described in detail. We also report a detailed study of the quantitative features of the technique, in particular cooling efficiencies and collision rates as well as some additional potential factors that could bias the quantitative aspect of the method. Finally, the possibilities of the technique for the measurement of PAH within flames in the presence of soot particles along with its accuracy and reproducibility are demonstrated by recording naphthalene mole fractions profiles in several rich CH₄/O₂/N₂ flames. A detection limit of the order of a ppb is demonstrated under flame conditions with and without the presence of soot particles.     

Multiphoton mass spectra of Xe2 molecules in the range of excited Xe*(6p, 5d) atoms

The (2 + 1) photoionization mass spectra of Xe₂ molecules are studied in a supersonic jet upon excitation by laser radiation in the energy range 80321.3–77821 cm^?1, corresponding to the dissociation of the Xe₂ molecule into atoms Xe(¹ S ₀) + Xe*(6p, 5d). Several vibrational progressions are observed, which are attributed to two-photon transitions of Xe₂ from the ground state to the excited states of the O ⁺ _g, 1_g, and 2_g symmetries. Based on the analysis of these progressions, the molecular constants of a number of excited states of Xe₂ are estimated.