Louise Pasternack

Reactions of C2(a 3πu) with CH4, C2H2, C2H4, C2H6, and O2 using multiphoton UV excimer laser photolysis

Abstract C 2 (a 3 π u ) disappearance rate constants of 1.44, 0.96, 0.0296, 0.0130 and −6 (x10 −10 cm 3 s −1 ) are reported for reactions with C 2 H 4 , C 2 H 2 , O 2 , C 2 H 6 , and CH 4 , respectively at 298 K. C 2 (a 3 π u ) fragments are generated by multiphoton ArF excimer laser photodissociation at C 2 H 2 , and monitored by dye laser induced fluorescence. Arguments are presented which favor chemical reactions over the C 2 (a 3 π u ) → (X 1 σ + g ) quenching channel. C 2 + C 2 H 2 represents the one possible exception to the reactive channel.

Temperature dependence of the C2(X1Σg+) reaction with H2 and CH4 and C2(X1Σg+ and a 3Πu equilibrated states) with O2

Abstract Temperature dependence measurements of the rates of reaction of C 2 (X 1 Σ g + ) with H 2 and CH 4 are reported along with similar measurements for an equilibrated mixture of C 2 (X 1 Σ g + and a 3 Π u spin states with O 2 over the 300–600 K range. All the reactions are characterized by low activation energies. Singlet C 2 likely reacts with hydrocarbons by an insertion mechanism followed by fragmentation.

Reactions of C2(X 1Σ+g) produced by multiphoton uv excimer laser photolysis

Abstract C 2 (X 1 Σ + g ) disappearance rate constants at 298 K have been determined for reactions with H 2 : (1.38±0.06) × 10 −12 cm 3 s −1 molecule −1 ; CH 4 (1.87±0.05) × 10 −11 cm 3 s −1 molecule −1 ; C 2 H 6 : (1.59±0.05) × 10 −10 cm 3 s −1 molecule −1 : C 2 H 4 : (3.26±0.05) × 10 −10 cm 3 s −1 molecule −1 : C 2 F 4 : (5.99±0.14) × 10 −11 cm 3 s −1 molecule −1 : O 2 : (2.82±0.09) × 10 −12 cm 3 s −1 molecule −1 : and CO 2 : no apparent reaction. The C 2 (X 1 Σ − g ) was produced by multiphoton UV excimer laser photodissociation of hexafluorobutyne-2. These results are compared to the reaction rates of C 2 (a 3 Π u ). The rates for O 2 reactions are the same for both C 2 states and no reaction is observed for CO 2 with either C 2 state. However, C 2 (X 1 Σ − g ) reacts considerable faster than C 2 (a 3 Π g ) with hydrogen and hydrocarbons. The presence of an allowed hydrogen atom exchange reaction for ground state C 2 to form ground state C 2 H which is forbidden for metastable triplet C 2 is postulated to account for the disparities in reaction rates. Electronic orbital correlation effects are proposed as an explanation.

The effect of nitric oxide on premixed flames of CH4, C2H6, C2H4, and C2H2

Abstract Nitric oxide was doped into premixed, stoichiometric, 10 torr flames of CH4/O2/N2, C2H6/O2/N2, C2H4/O2/N2, and C2H2/O2/N2. Spatial profiles of electronic ground state CH, OH, NO, CN, NCO, NH and metastable state 3C2 were obtained by laser-induced fluorescence. The peak temperatures of the flames were maintained at 1800 K by varying the N2 buffer gas mole fraction. Comparisons of the profiles among the different fuels illustrate differences in the hydrocarbon flame structure that lead to different NO reactivities in these flame systems. Concentrations of the nitrogen-containing intermediates are similar in the methane, ethane, and ethylene flames, but are considerably larger in the acetylene flame, indicating a much greater NO reactivity in this flame. The species profiles are modeled using three different kinetic mechanisms; all of these have significant shortcomings.

Excited state dynamics of NO3

The fluorescence decay of NO3 excited to vibronic levels between the origin at 661.9 and ∼606 nm is measured and found to be nonexponential. An exponential fit to the long‐lived portion of the decay gives an apparent, collision‐free lifetime of 340±20 μs. Electronic quenching rates have been measured for a variety of collision partners including He, N2, O2, C3H8, and HNO3. Our results are interpreted in terms of the Douglas effect, i.e., an extensive coupling of excited electronic levels with nonemissive vibronic levels of the ground electronic state. This coupling is likely responsible for the anomalously long lifetime and the apparent diffuseness of the absorption spectrum.

Particle formation and growth from ozonolysis of α‐pinene

Observations of particle nucleation and growth during ozonolysis of α-pinene were carried out in Calspans 600 m3 environmental chamber utilizing relatively low concentrations of α-pinene (15 ppb) and ozone (100 ppb). Model simulations with a comprehensive sectional aerosol model which incorporated the relevant gas-phase chemistry show that the observed evolution of the size distribution could be simulated within the accuracy of the experiment by assuming only one condensable product produced with a molar yield of 5% to 6% and a saturation vapor pressure (SVP) of about 0.01 ppb or less. While only one component was required to simulate the data, more than one product may have been involved, in which case the one component must be viewed as a surrogate having an effective SVP of 0.01 ppb or less. Adding trace amounts of SO2 greatly increased the nucleation rate while having negligible effect on the overall aerosol yield. We are unable to explain the observed nucleation in the α-pinene/ozone system in terms of classical nucleation theory. The nucleation rate and, more importantly, the slope of the nucleation rate versus the vapor pressure of the nucleating species would suggest that the nucleation rate in the α-pinene/ozone system may be limited by the initial nucleation steps (i.e., dimer, trimer, or adduct formation).

Temperature dependence of reactions and intersystem crossing of C2a3Πu with hydrogen and small hydrocarbons from 300–600 K

Abstract C 2 a 3 Π u is produced by multiphoton UV excimer laser photodissociation of hexafluorobutyne-2 or benzene. The rate of C 2 depletion is monitored using laser induced fluorescence. The disappearance rates of C 2 a 3 Π u are reported over the temperature range of 300–600 K for reactions with H 2 , D 2 , C 2 H 6 , C 3 H 8 , n -C 4 H 10 , and C 2 H 4 and intersystem crossing due to collisions with Xe. The rates are (1σ confidence limits): Reactant k ( T ) cm 3 molecule −1 s −1 H 2 (1.55 ± 0.10) × 10 −11 exp [(−3012 ± 31)/ T ] D 2 (1.80 ± 0.22) × 10 −11 exp [(−3710 ± 72)/ T ] C 2 H 6 (2.42 ± 0.10) × 10 −11 exp [(−919 ± 15)/ T ] C 3 H 8 (1.84 ± 17) × 10 −11 exp [(−97 ± 36)/ T ] n -C 4 H 10 (4.9 ± 0.5) × 10 −11 exp [(−71 ±41)/ T ] C 2 H 4 (1.20 ± 16) × 10 −10 exp [(5 ± 46)/ T ] Xe (5.54 ± 0.14) × 10 −12 exp [(24 +- 30)/ T ] Probable mechanisms for these reactions are discussed.

Application of saturation spectroscopy for measurement of atomic Na and MgO in acetylene flames

The technique of laser induced fluorescence saturation spectroscopy is evaluated for determination of absolute atomic and molecular concentrations in flame sources. The combustor is an aspirating slot burner using premixed acetylene and air to create a stable homogeneous flame source. Concentration measurements of atomic sodium and MgO (X 1Σ+ and A 1Π) are made by saturation spectroscopy and laser induced fluorescence techniques. Independent atomic and molecular absorption measurements are made for Na and MgO (X 1Σ+) which are in agreement with saturation spectroscopy concentration determinations. The effect of using various laser beam profiles (rectangular, Gaussian, and truncated Gaussian) are evaluated. It is found that MgO exists in the flame in a significant inversion in the (A 1π) state and possible production mechanisms for MgO in the flame are considered.

Experimental and modeling studies of secondary organic aerosol formation and some applications to the marine boundary layer

A series of controlled experiments were carried out in the Calspan Corporations 600 m3 environmental chamber to study some secondary organic aerosol formation processes. Three precursor-ozone systems were studied: cyclopentene-ozone, cyclohexene-ozone, and α-pineneozone. Additionally, SO2 was added to the initial gas mixture in several instances and was likely present at trace levels in the ostensibly organic-only experiments. It was found that all three systems readily formed new submicron aerosols at very low reactant levels. The chemical composition of formed aerosols was consistent with some previous studies, but the yields of organic products were found to be lower in the Calspan experiments. A three-step procedure is proposed to explain the observed particle nucleation behavior: HO · production → H2SO4 formation → H2SO4-H2O (perhaps together with NH3) homogeneous nucleation. It is also proposed that some soluble organic products would partition into the newly formed H2SO4-H2O nuclei, enhance water condensation, and quickly grow these nuclei into a larger size range. While the observations in the two cycloolefin-ozone systems could be well explained by these proposed mechanisms, the exact nature of the nucleation process in the α-pinene-ozone system remains rather opaque and could be the result of nucleation involving certain organics. The results from three simple modeling studies further support these proposals. Their applicability to the marine boundary layer (MBL) is also discussed in some detail. Particularly, such a particle nucleation and growth process could play an important role in secondary aerosol formation and, quite likely, CCN formation as well in certain MBL regions.

Temperature dependence of the C2 (a 3Πu)+CH4 reaction from 337 to 605 K

The reaction rate of C2(a3Πu)+CH4 is reported for temperatures of 337 to 605 K. The C2(a3Πu) is produced by multiphoton 193 nm laser photolysis of hexafluorobutyne‐2 and monitored with a tunable dye laser. The data are fit to an Arrhenius equation of the form kII(T)?0.20)×10−11 exp {[−5.57±0.11 kcal/mol]/RT} cm3 molecule−1 s−1.