Collisional behaviour of electronically excited silicon atoms,Si(3p2(D1D2)), by atomic absorption spectroscopy

The collisional behaviour of electronically excited silicon atoms in the optically metastable 3p²(¹D₂) state (0.781 eV) is investigated by time-resolved resonance absorption in the ultraviolet. Si(3 ¹D₂) was generated by the repetitive pulsed irradiation of SiCl₄ at λ > 165 nm in a flow system, and monitored by attenuation of resonance radiation at λ = 288.16 nm (4s(¹P⁰₁) ← ep²(¹D₂)) using signal averaging. Absolute second-order rate constants (k_R, cm³ molecule^?1 s^?1, 300 K) are reported for the gases: H₂[(8.1 ± 1.5) × 10^?11], O₂[(2.3 ± 0.4) × 10^?11], He (? 10^?15) and SiCl₄ [(2.9 ± 0.4) × 10^?10]. These results are compared with the analogous data reported hitherto for Si(3³P_J) and Si(3 ¹S₀). Those for H₂ and O₂ are discussed within the context of symmetry arguments on the nature of the potential surfaces involved using the weak spin orbit coupling approximation. Finally, pulsed stimulated emission operating on the transition Si(3P²)(¹So → ¹D₂) (λ = 1.0995 μ) was not detected in high energy pulse experiments using a confocal cavity, despite the population inversion between Si(3 ¹S₀ and Si(3 ¹D₂) observed by resonance absorption following the photolysis of SiCl₄.     

Experimental study of the D + H2(ν = 1) reaction

The D + H₂(ν = 1) reaction, D + H₂(ν = 1) → K_a HD(ν = 1) + H, → K_n HD(ν = 0) + H, → K_r D + H₂(ν = 0) has been studied. The measurements were made in a flow-tube apparatus at 300 K. Vibrationally excited H₂ was generated in a furnace and D atoms in a microwave discharge. EPR and thermometric techniques were used for the detection of D and H atoms and H₂(ν = 1). The product branching rate constants (in CM³/Molecule s) were found to be K_a = (10.7 ± 4.1) × 10^?13. K_n = (5.4 ± 2.7) × 10^?13, K_r, < 2.7 × 10^?13.     

Reactive and non-reactive quenching of O(1D2) by COF2

Quenching of O(¹D₂) by COF₂ has been investigated by time-resolved resonance fluorescence monitoring of the product O(³P_J) following 248 nm pulsed laser photolysis of O₃. The rate constant for total removal of O(¹D₂) by COF₂ is (7.4 ± 1.2) × 10^?11 cm³ molecule^?1 s^?1. 71 ± 7% of the quenching interactions result in formation of O(³P^J).     

Reactions of HO2 with NO,OH and HO2 studied by EPR/LMR spectroscopy

A combined EPR/LMR spectrometer and fast-flow system has been used to investigate the reactions HO₂ + NO(k₁), HO₂ + OH(k₂), HO₂ + HO₂(k₃) at room temperature. The rate constants have been measured: k₁ = (7.0 ± 0.6) × 10^?12 cm³ s^?1 (P = 7–10 Torr);k₂ = (5.2 ± 1.2) × 10^?11 cm³ s^?1 (P = 8–10 Torr);k₃ = (1.65 ± 0.3) × 10^?12 cm³ s^?1 (P = 2.1–24.9 Torr). The conclusion is drawn from analysis of the literature and the present work that k₂ and k₃ do not depend on pressure up to 1 atm.     

Studies on the reaction N + N3 → N2(B 3Πg) + N2(X 1Σ+g)

Strongly enhanced N₂ first positive emission N₂(B ³Π_g → A ³Σ⁺_u) has been observed on addition of N atoms into a flowing mixture of Cl and HN₃. The dependence of the emission intensity on N atom concentration gave a rate constant for the reaction N + N₃ → N₂(B ³Π_g) + N₂(X ¹Σ⁺_g) of _i(1.6 ± 1.1) × 10^?11 cm³ molecule^?1 s^?1. That for the reaction Cl + HN₃ → HCl + N₃ is (8.9 ± 1.0) × 10^?13 cm³ molecule^?1 s^?1 from the decay of the emission. Comparison of the emission intensity in ClHN₃ with that in ClHN₃N gave the rate constant of the reaction N₃ + N₃ → N₂(B ³Π_g) + 2N₂(X ¹Σ⁺_g) as 1.4 × 10^?12 cm³ molecule^?1 s^?1 on the assumption that N + N₃ yields only N₂(B ³Π_g) + N₂(X ¹Σ⁺_g).     

The time-resolved LMR method as used to measure elementary reaction rates of CI atoms and SiH3 radicals in pulse photolysis of S2Cl2 in the presence of SiH4

The time-resolved laser magnetic resonance (LMR) method has been applied to kinetic measurements for the first time. An intracavity spectrometer based on a CO₂ laser with resonant modulation of the magnetic field and with phase-sensitive detection of the signal has been used. Kinetic curves of generation and disappearance of CI atoms and SiH₃ radicals were obtained in the pulse photolysis of a mixture of S₂Cl₂ + SiH₄ under the fourth harmonic of a Nd laser (265 nm, 0.5 mJ, 12.5 Hz) at a total pressure of 520–980 Pa (he as diluent) and a temperature of 326 K. The reagent concentrations were: [S₂Cl₂ = (2.0?10.2)×10¹⁴ cm^?3, [SiH₄ = (2.4?17.4)×10¹³ cm^?3. To remove the transition saturation, 5.3×10¹⁵ cm^?3 CCl₄ was introduced into the reactor. The fraction of dissociated S₂Cl₂ was 1‰ Rate constants of the reactions (I) Cl+S₂Cl₂ → products, (II) Cl+SiH₄ → HCl+SiH₃ and a preliminary rate constant of the reaction (III) SiH₃ + S₂Cl₂ → products were obtained: k₁ ≤ (4.3±1.2)×10^?12 cm³/s, k₂ = (2.3±0.5)×10^?10 cm³/s, k₃ = (2.4±0.5)×10^?11 cm³/s. At a signal-to-noise ratio of 1:1, 1000 pulses and a 12 cm long detection zone the sensitivity to Cl atoms and to SiH₃ radicals was 4×10¹⁰ cm^?3 and = 10¹¹ cm^?3, respectively. The time resolution of the method was 4 μs. The method is shown to be promising for kinetic investigations and experiments on fast processes.     

The production of H(2S) atoms in the energy-transfer reaction of O2(1Δg) with HO2(X̃2A″)

The reaction of O₂(¹Δg) with HO₂(X?) was studied in an isothermal flow reactor in the pressure range 7?p? 10.7 mbar at temperatures between 299?T? 423 K. H-atom production was observed in the reaction O₂(¹Δg) + HO₂(òA′) - H(²S)+ 2O₂ (³Σ_g^?). The rate of this reaction (k₁) is estimated to be k₁ = (1 ± 0.5) × 10¹⁴ CM³ Mol^?1 s^?1. The implications of this reaction to recent determinations of the rate of the reaction H + O₂(¹Δg) are discussed.     

The reactions of S(3PJ) atoms with thiiranes

The reactions of ground-state S(³P_J) atoms with thiirane, methylthiirane, and trans-2,3-dimethylthiirane have been studied by flash photolysis-VUV kinetic absorption spectroscopy. From the analysis of the S(³P_J) decay plots the following rate constants were determined: (1.4 ± 0.2) × 10¹³, (2.7 ± 0.3) × 10¹³ and (4.0 ± 0.2) × 10¹³ (in cm³ mol^?1 s^?1 units) for thiirane, methylthiirane and trans-2,3-dimethylthiirane, respectively, showing an upward trend with increasing methylation.     

Collisional deactivation Of Ca(4p3PJ) by barium atoms

Collisional deactivation of Ca(4p³P_J by barium atoms proceeds with a thermal cross section of 0.26±0.02 nm² at 850 K. No evidence for the corresponding deactivation of electronically excited Ca(4p³P_J) by Ca(4s¹S_o) was obtained. The optical lifetime of Ca(4p³P_J) was measured to be 0.33 ± 0.03 ms, in good agreement with previous studies.     

Laser induced photodissociation of C2F5I. Radiative lifetime of the metastable iodine atom (2P12)

Excited iodine atoms I(²P_{$\text{1}\text{2}$}) are formed by laser irradiation of C₂F₅I at 2950 Å. The mean radiative lifetime τ of these metastable atoms and their bimolecular rate constant k₂ for deactivation in collissions with C₂F₅I were measured to be: τ = 108 ± 10 ms; k₂ = (1.8 ± 0.1) × 10^?17 cm³/molec s.     

Absolute rate constants for the reaction of O(3P) aroms with n-butane and NO(M  Ar) over the temperature range 298–439 K

Absolute rate constants for the reaction of O(³P) atoms with n-butane (k₂) and NO(M  Ar)(k₃) have been determined over the temperature range 298–439 K using a flash photolysis-NO₂ chemiluminescence technique. The Arrhenius expressions obtained were k₂ = 2.5 × 10^?11exp[-(4170 ± 300)/RT] cm³ molecule^?1 s^?1, k₃ = 1.46 × 10^?32 exp[940 ± 200)/ RT] cm⁶ molecule^?2 s^?1, with rate constants at room temperature of k₂ = (2.2 ± 0.4) × 10^?14 cm³ molecule^?1 s^?1 and k₃ = (7.04 ± 0.70)×10^?32 cm⁶ molecule^?2 s^?1. These rate constants are compared and discussed with literature values.     

Vibrational relaxation of N2(A3Σu+,υ = 1, 2,3) by CH4 and CF4

N₂(A, υ = 0-3) produced by the Ar(³P_0,2) + N₂ reaction and detected by laser-induced fluorescence undergoes rapid, stepwise vibrational relaxation but slow electronic quenching with added CH₄ or CF₄. Rate constants, k_Q^υ, of 1.5, 3.1, and 5.0 × 10^?12 cm³ s^?1 are measured for Q = CH₄, υ = 1-3, and 0.47, 1.8, and 5.5 × 10^?12 cm³ s^?1 for Q = CF₄, υ = 1-3, with ≈±20% accuracy (1σ). Information is also obtained for the unrelaxed, relative υ populations.     

Molecular beam chemiluminescence. VIII: Pressure dependence and kinetics of Sm + (N2O,O3, F2, Cl2) and Yb + (O3, F2, Cl2) reaction. Dissociation energies of the diatomic reaction products

The CL spectra of the title reactions and their pressure dependences have been studied over the 5 × 10^?6 ? 5 × 10^?3 torr range in a beam-gas experiment. In the Sm + N₂O, O₃ and Yb + O₃ reactions simple bimolecular formation of the short lived (radiative lifetime τ_R < 3 × 10^?6 s) MO* emitters dominates the entire pressure range. In the other systems Sm + (F₂, Cl₂), Yb + (F₂, Cl₂) the CL spectra are strongly pressure dependent, indicating extensive energy transfer from long-lived intermediates. Reaction mechanisms are suggested. The quantum yields Φ, obtained by calibrating relative quantum yields with Dickson and Zare's absolute value for Sm + N₂O [Chem. Phys. 7 (1975) 367], range from Φ = 2.3% (for Sm + F₂, the most efficient reaction) down to Φ = 0.005% for Yb + Cl₂. The following lower limit estimates were obtained for the product dissociation energies from the short wavelength CL cutoffs: D⁰₀(SmF) ? 121.3 ± 2.4 kcal/mole, D⁰₀(SmCl) ? ? 100 ± 3 kcal/mole, D⁰₀(YbO) ? 94.2 ± 1.5 kcal/moie, D⁰₀(YbF) ? 123.7 ± 2.3 kcal/mole.     

Kinetics of the association of H+3 with H2 at low temperatures

The rate constant for the formation of H⁺₅ (D⁺₅) at (86 ± 3) °K by the three-body process has been determined (k₃(H) = (2.16 ± 0.10) × 10^?28 × 10^?28 cm⁶/molecule² sec and k₃(D) = (1.47 ± 0.20) × 10^?28 cm⁶/molecule² sec) in a high pressure mass spectrometer. Comparison of this result with published rate data at 300 °K indicates the reaction has an apparent activation energy of ?1.5 kcal/mole.     

The collisional quenching of O2*(1Δg) by NO and CO2

Rate coefficients for the collisional quenching of O₂^*(¹Δ_g) by NO and CO₂ at 2–8 torr and 300 K have been determined. k_NO = (2.48 ± 0.23) × 10^?17 cm³ molecule^?1 s^?1 and

= (2.56 ± 0.12) × 10^?18 cm³ molecule^?1 s^?1.     

Temperature dependence of the reaction rate constants for O(3P) atoms with C2H4, C3H6 and NO(M = N2O), determined by a modulation technique

Rate constants for the reaction of O(³P) atoms with C₃H₄, C₃H₆ and NO(M = N₂O) have been measured over the temperature range 300–392°K using a modulation-phase shift technique. The Arrhenius expressions obtained are:C₂H₄, k₂ = 3.37 × 10⁹ exp[?(1270 ± 200)/RT]liter mole^?1 sec^?1,C₃H₆, k₂ = 2.08 × 10⁹ exp[?(0 ± 300)/RT]liter mole^?1 sec^?1,NO(M = N₂O), k₁ = 9.6 × 10⁹ exp[(900 ± 200/RT]liter² mole^?2 sec^?1.These temperature dependencies of k₂ are in good agreement with recent flash photolysis-resonance flourescence measurements, although lower than previous literature values.     

Deuterium isotope effect in the T1 → S0 radiationless transition of propynal in the gas phase

The phosphorescence lifetimes of propynal-h₁ and propynal-d₁ have been measured at room temperature in the 40 mTorr-1 Torr pressure range The reciprocal of the zero-pressure lifetime (k₀) is (3.10 ± 0.05) × 10³ and (1.70 ± 0.04) × 10³ s^?1 for propynal-h₁ and propynal-d¹ For both compounds the rate constant for self-quenching between triplet and ground-state molecules is k_SQ = (1 2±007) × 10³ Torr^?1 s^?1 The deuterium isotope effect is attributed to the T₁ → S₀ radiationlcss decay, for which k^H_ISC/k^D_ISC = 2 4     

Electronic energy transfer from the S2 state of rhodamine 6G

In this paper we present the results of an experimental study of intermolecular electronic energy transfer (EET) from the short-lived Second excited singlet state of rhodamine 6G (R6G) to the ground state of 2,5-bis [5′-tert-butyl-2-benzoxazolyl] thiophene (BBOT). The S₂ state of the donor was excited by sequential, time-delayed, two-photon excitation (STDTPE) utilizing the second harmonic and the first harmonic of a mode-locked Nd³⁺: glass laser, while the EET process was interrogated by monitoring the enhancement of the S₁ → S₀ fluorescence of BBOT. The enhancement of the fluorescence intensity of BBOT was found to be linear in the energies of the two exciting pulses, and linear in the concentration of the energy acceptor (over the BBOT concentration range of (0.3–7) × 10^?5 M), which is in accord with the predictions of the Forster—Dexter mechanism for resonant EET from an ultrashort-lived donor state at low acceptor concentrations. Quantitative measurements of the S₂ → S₀ fluorescence yield in R6G solution directly excited by STDTPE and of the S₁ → S₀ fluorescence of BBOT from R6G + BBOT solutions resulting from EET led to the values of Y_D(S₂ → S₀) = (2.1 ± 0.5) × 10^?6 for the emission quantum yield of the S₂ state of R6G and τr_D(S₂) ≈ 3 × 10^?14 s for the lifetime of the metastable S₂ state of this molecule.     

Vaporization chemistry and thermodynamics in the CdGa2S4-Ga2S3 system

The chemistry and thermodynamics of vaporization of CdGa₂S₄(s), CdGa₈S₁₃(s), and Ga₂S₃(s) were studied by computer-automated, simultaneous Knudsen-effusion and torsion-effusion, vapor pressure measurements in the temperature range 967–1280 K. The vaporization was incongruent with loss of Cd(g) + 1/2 S₂(g) and production of CdGa₈S₁₃(s), a previously unknown compound, in equilibrium with CdGa₂S₄(s), until the solid became CdGa₈S₁₃ only. Then, incongruent vaporization continued with production of Ga₂S₃(s) until the solid was Ga₂S₃ only. The latter vaporized congruently. The ΔH°(298 K) of combination of one mole of CdS(s) with one mole of Ga₂S₃(s) to give CdGa₂S₄(s) was ?22.6 ± 0.9 kJ mole^?1. The 2H2(298 K) of combination of one mole of CdS(s) with four moles of Ga₂S₃(s) to give CdGa₈S₁₃(s) was ?25.5 ± 1.1 kJ mole^?1. The 2H2(298K) of CdGa₈S₁₃(s) with respect to disproportionation into CdGa₂S₄(s) and 3 Ga₂S₃(s) was ?2.8 ± 0.6 kJ mole^?1. CdGa₈S₁₃(s) was not observed at room temperature. The 2H2(298 K) of vaporization of the residual Ga₂S₃(s) was 663.4 ± 0.8 kJ mole^?1, which compared well with a value of 661.4 ± 0.3 kJ mole^?1 already available from the literature. Implications of small variations in stoichiometry of compounds in this study were observed and are discussed.     

Reactivity of selected volatile organic compounds (VOCs) toward the sulfate radical (SO4−)

Rate constants for a series of alcohols, ethers, and esters toward the sulfate radical (SO₄^?) have been directly determined using a laser photolysis set‐up in which the radical was produced by the photodissociation of peroxodisulfate anions. The sulfate radical concentration was monitored by following its optical absorption by means of time resolved spectroscopy techniques. At room temperature the following rate constants were derived: methanol ((1.6 ± 0.2) × 10⁷ M^?1 s^?1); ethanol ((7.8 ± 1.2) × 10⁷ M^?1 s^?1); tert‐butanol ((8.9 ± 0.3) × 10⁵ M^?1 s^?1); diethyl ether ((1.8 ± 0.1) × 10⁸ M^?1 s^?1); MTBE ((3.13 ± 0.02) × 10⁷ M^?1 s^?1); tetrahydrofuran (THF) ((2.3 ± 0.2) × 10⁸ M^?1 s^?1); hydrated formaldehyde ((1.4 ± 0.2) × 10⁷ M^?1 s^?1); hydrated glyoxal ((2.4 ± 0.2) × 10⁷ M^?1 s^?1); dimethyl malonate (CH₃OC(O)CH₂C(O)OCH₃) ((1.28 ± 0.02) × 10⁶ M^?1 s^?1); and dimethyl succinate (CH₃OC(O)CH₂CH₂C(O)OCH₃) ((1.37 ± 0.08) × 10⁶ M^?1 s^?1) where the errors represent 2σ. For the two latter species, we also measured the temperature dependence of the corresponding rate constants. A correlation of these kinetics with the bond dissociation energy is also presented and discussed. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 539–547, 2001