Transversely optically pumped Ar:He laser with a pulsed-periodic discharge.

Optically pumped rare gas lasers have the potential for scaling to high-power cw systems with good beam quality. Metastable atoms of heavier rare gases that are the lasing species are produced in an electric discharge at near atmospheric pressure. The key problem for this class of lasers at present is the development of a suitable discharge system. In this paper, we present the results of optimization of a pulsed discharge system with the goal of minimizing cathode sputtering and peak discharge current. The first demonstration of a transversely pumped system and measurements of the optical pumping threshold for the Ar:He laser are also presented.

Scission of the Five-Membered Ring in 1-H-Inden-1-one C9H6O and Indenyl C9H7 in the Reactions with H and O Atoms.

Quantum chemical G3(MP2,CC)//B3LYP/6-311G(d,p) calculations of the C9H7O potential energy surface were utilized to investigate the mechanism of the 1-H-inden-1-one (C9H6O) + H and indenyl (C9H7) + O reactions and were combined with Rice-Ramsperger-Kassel-Marcus Master Equation (RRKM-ME) calculations to predict temperature- and pressure-dependent reaction rate constants and product branching ratios. The most favorable reaction pathways for C9H6O + H lead to the bimolecular C8H7 + CO products, which are slightly endothermic with respect to the reactants. The reaction begins with H addition to the ortho or meta C atoms in the five-membered ring, C9H6O + H → w2/w3, and then proceeds by isomerization to w1, (w3 →) w2 → w1. From thereon, the w1 → w9 → w8 → p1 and w2 → w8 → p1 pathways lead to ortho-vinyl phenyl + CO, whereas w1 → w10 → w11 → p2 and C9H6O + H → w10 → w11 → p2 produce styrenyl + CO. The results of the RRKM-ME calculations showed that only the well-skipping C9H6O + H → C8H7 (p1/p2) + CO mechanism is relevant under combustion conditions. A comparison with a smaller prototype 2,4-cyclopentadienone + H → C4H5 + CO reaction demonstrated that the H atom is a less efficient destroyer of a cyclopentadienone-like moiety when this moiety is linked to an aromatic or a PAH structure. The C9H7 + O reaction begins with highly exothermic barrierless addition of the oxygen atom to the radical site in the five-membered ring of indenyl producing w1 and then, the reaction mostly proceeds by ß-scission in the five-membered ring, which may be preceded by H migration to w2 or w10, and completes by the CO loss forming the highly exothermic C8H7 radical products. Modified Arrhenius expressions for the rate constants of all reactions pertinent to the formation of C8H7 + CO from C9H6O + H, C9H7 + H, and unimolecular decomposition of benzopyranyl have been generated and suggested for combustion kinetic modeling. It is concluded that the oxidation reactions of a five-membered ring with atomic oxygen remain fast in the presence of attached or surrounding six-membered rings.

The mechanism and rate constants for oxidation of indenyl radical C9H7 with molecular oxygen O2: a theoretical study.

Ab initio G3(MP2,CC)//B3LYP/6-311G(d,p) calculations have been carried out to map out the C9H7O2 potential energy surface in relation to the reaction of the 1-indenyl radical with molecular oxygen. The resulting energetics and molecular parameters of the species involved in the reaction have been then utilized in Rice-Ramsperger-Kassel-Marcus master equation calculations of temperature- and pressure-dependent reaction rate constants and product branching ratios. The results demonstrate that, while the reaction is insignificant at low temperatures, at higher temperatures, above 800 K or higher depending on the pressure, the prevailing reaction channel leads to the formation of the 1-H-inden-1-one + OH products via a 1,3-H shift from C to O in the initial association complex W1 accompanied by OH elimination through a high barrier of 25.6 kcal mol-1. The branching ratio of 1-H-inden-1-one + OH increases from ∼61% to ∼80% with temperature, whereas c-C6H4-CH2CHO + CO (32-12%) and coumarin + H (7-6%) are significant minor products. The total rate constant of the indenyl + O2 reaction leading to the bimolecular products is independent of pressure and exceeds 1.0 × 10-15 cm3 molecule-1 s-1 only at temperatures above 2000 K, reaching 6.7 × 10-15 cm3 molecule-1 s-1 at 2500 K. The indenyl + O2 reaction is concluded to be too slow to play a substantial role in oxidation of the five-member ring in indenyl and the present results corroborate the assertion that molecular oxygen is not an efficient oxidizer of five-member-ring radicals.

Functional Relationships between Kinetic, Flow, and Geometrical Parameters in a High-Temperature Chemical Microreactor.

Computational fluid dynamics (CFD) simulations and isothermal approximation were applied for the interpretation of experimental measurements of the C10H7Br pyrolysis efficiency in the high-temperature microreactor and of the pressure drop in the flow tube of the reactor. Applying isothermal approximation allows the derivation of analytical relationships between the kinetic, gas flow, and geometrical parameters of the microreactor, which, along with CFD simulations, accurately predict the experimental observations. On the basis of the obtained analytical relationships, a clear strategy for measuring rate coefficients of (pseudo) first-order bimolecular and unimolecular reactions using the microreactor was proposed. The pressure- and temperature-dependent rate coefficients for the C10H7Br pyrolysis calculated using variable reaction coordinate transition state theory were invoked to interpret the experimental data on the pyrolysis efficiency.

O2(b1Σg+) Removal by H2, CO, N2O, CH4, and C2H4 in the 300-800 K Temperature Range.

Rate constants for the removal of O2 b1Σg+ by collisions with species relevant to combustion, H2, CO, N2O, CH4 and C2H4 have been measured in the temperature range 297-800 K. O2(b1Σg+) was produced from ground-state molecular oxygen by photoexcitation 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 H2, CO, N2O, CH4, and C2H4 could be represented by the modified Arrhenius expressions kH2 = (1.44 ± 0.02) × 10-16 T1.5 exp[(0 ± 10)/ T], kCO = (6.9 ± 0.4) × 10-24 T3 exp[(939 ± 33)/ T], kN2O = (2.63 ± 0.14) × 10-18 T1.5 exp[(590 ± 26)/ T], kCH4 = (3.5 ± 0.2) × 10-17 T1.5 exp[(-220 ± 24)/ T], and kC2H4 = (2.34 ± 0.10) × 10-20 T2.5 exp[(680 ± 16)/ T] cm3 molecule-1 s-1, respectively. All of the rate constants measured at room temperature were found to be in good agreement with previously reported values, whereas the values at elevated temperatures up to 800 K were systematically measured for the first time.

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.

Removal of Rb(6(2)P) by H(2), CH(4), and C(2)H(6).

The saturated hydrocarbons methane and ethane are often used as collisional energy transfer agents in diode-pumped alkali vapor lasers (DPALs). Problems are encountered because the hydrocarbons eventually react with the optically pumped alkali atoms, resulting in the contamination of the gas lasing medium and damage of the gas cell windows. The reactions require excitation of the more highly excited states of the alkali atoms, which can be generated in DPAL systems by energy pooling processes. Knowledge of the production and loss rates for the higher excited states is needed for a quantitative understanding of the photochemistry. In the present study, we have used experimental and theoretical techniques to characterize the removal of Rb(6P2) by hydrogen, methane, and ethane.

Luminescence of the (O2(a(1)Δ(g)))2 collisional complex in the temperature range of 90-315 K: Experiment and theory.

Experimental and theoretical studies of collision induced emission of singlet oxygen molecules O2(a(1)Δg) in the visible range have been performed. The rate constants, half-widths, and position of peaks for the emission bands of the (O2(a(1)Δg))2 collisional complex centered around 634 nm (2) and 703 nm (3) have been measured in the temperature range of 90-315 K using a flow-tube apparatus that utilized a gas-liquid chemical singlet oxygen generator. The absolute values of the spontaneous emission rate constants k2 and k3 are found to be similar, with the k3/k2 ratio monotonically decreasing from 1.1 at 300 K to 0.96 at 90 K. k2 slowly decreases with decreasing temperature but a sharp increase in its values is measured below 100 K. The experimental results were rationalized in terms of ab initio calculations of the ground and excited potential energy and transition dipole moment surfaces of singlet electronic states of the (O2)2 dimole, which were utilized to compute rate constants k2 and k3 within a statistical model. The best theoretical results reproduced experimental rate constants with the accuracy of under 40% and correctly described the observed temperature dependence. The main contribution to emission process (2), which does not involve vibrational excitation of O2 molecules at the ground electronic level, comes from the spin- and symmetry-allowed 1(1)Ag←(1)B3u transition in the rectangular H configuration of the dimole. Alternatively, emission process (3), in which one of the monomers becomes vibrationally excited in the ground electronic state, is found to be predominantly due to the vibronically allowed 1(1)Ag←2(1)Ag transition induced by the asymmetric O-O stretch vibration in the collisional complex. The strong vibronic coupling between nearly degenerate excited singlet states of the dimole makes the intensities of vibronically and symmetry-allowed transitions comparable and hence the rate constants k2 and k3 close to one another.

On the dissociation of I2 by O2(a1Delta): Pathways involving the excited species I2(A'3Pi2u,A3Pi(1u)), I2(X1sigma,upsilon), and O2(a1Delta,upsilon).

Kinetic studies were carried out to explore the role of the excited species I(2)(A(') (3)Pi(2u),A (3)Pi(1u)), I(2)(X (1) summation operator,upsilon), and O(2)(a (1)Delta,upsilon) in the dissociation of I(2) by singlet oxygen. A flow tube apparatus that utilized a chemical singlet oxygen generator was used to measure the I(2) dissociation rate in O(2)(a (1)Delta)/I(2) mixtures. Vibrationally excited I(2)(X) is thought to be a significant intermediate in the dissociation process. Excitation probabilities (gamma(upsilon)) for population of the upsilonth I(2)(X) vibrational level in the reaction I(2)(X)+I((2)P(1/2))-->I(2)(X,upsilon>10)+I((2)P(3/2)) were estimated based on a comparison of calculated populations with experimentally determined values. Satisfactory agreement with the experimental data [Barnault et al., J. Phys. IV 1, C7/647 (1991)] was achieved for total excitation probabilities partitioned in two ranges, such that Gamma(25/=1%) the excited states are populated in the chain stage by collisions of I(2)(X,15