Richard A. Copeland

Rotational‐level‐dependent quenching of A 2Σ+ OH and OD

Rate constants kQ for collisional quenching of A 2Σ+, v’=0, OH and OD have been measured for specific rotational levels N’ of the radical and a wide variety of collision partners. Through measurements of the time‐dependent laser‐induced fluorescence in a low pressure discharge flow at room temperature, we observe a decrease in kQ with increasing rotational quantum number for most quenchers. The internal levels of the collision pairs appear unimportant from experiments involving deuterium substitution. A comparison of rotationless rates for different colliders [kQ(N=0)] with calculations based on collision complex formation indicate that attractive forces play a role in the quenching process.

Laser-induced fluorescence determination of temperatures in low pressure flames

Spatially resolved temperatures in a variety of low pressure flames of hydrogen and hydrocarbons burning with oxygen and nitrous oxide are determined from OH, NH, CH, and CN laser-induced fluorescence rotational excitation spectra. Systematic errors arising from spectral bias, time delay, and temporal sampling gate of the fluorescence detector are considered. In addition, we evaluate the errors arising from the influences of the optical depth and the rotational level dependence of the fluorescence quantum yield for each radical. These systematic errors cannot be determined through goodness-of-fit criteria and they are much larger than the statistical precision of the measurement. The severity of these problems is different for each radical; careful attention to the experimental design details for each species is necessary to obtain accurate LIF temperature measurements.

Spectrally resolved absolute fluorescence cross sections for bacillus spores.

Absolute fluorescence cross sections for Bacillus subtilis and B. cereus bacterial spores as both aqueous suspensions and aerosols were measured at a number of excitation wavelengths between 228 and 303 nm. The fluorescence was spectrally resolved at each excitation wavelength. We found that the optimum excitation wavelength for spore fluorescence is between 270 and 280 nm. The fluorescence cross section for aqueous suspensions is four times larger than for dry aerosols when measured under similar conditions. Measurements on wet aerosols showed an increase in fluorescence cross section over dry aerosols, indicating an enhancement of the fluorescence when the bacterial spores are wet. Mie scattering cross sections at 90 degrees to the direction of the incident radiation and extinction cross sections as a function of wavelength for B. subtilis suspensions and fluorescence cross sections for tryptophan are also reported.

Rotational-level-dependent quenching of OH(A2Σ+) at flame temperatures

Abstract The collisional quenching of OH (A 2 Σ + , ν′ = 0) is studied by laser-reduced fluorescence in the burnt gases of low-pressure (7 Torr) stoichiometric H 2 /O 2 /N 2 O flames. The temperature of these flames is adjusted between 1200 and 2300 K by altering the O 2 /N 2 O mixing ratio. The variation of the quenching rate constant with rotational level in the OH(A) for H 2 O collider is substantially less at 2300 K than previously observed at room temperature. The OH(A) quenching rate constant by atomic hydrogen at 1200 K is estimated k = 8 × 10 −10 cm 3 s −1 .

Time-resolved CH (A 2 Δ and B 2 ∑ − ) laser-induced fluorescence in low pressure hydrocarbon flames

Total collisional removal rate constants for the CH A(2)Delta and B(2)Sigma(-) electronic states are obtained in low pressure (<20-Torr) hydrocarbon flames. The B state is consistently removed ~70% faster than the A state. Variations of +/-50% are observed for different rotational levels and positions in the flame. For these flames, A-state emission following excitation of the B state indicates a rapid electronic-to-electronic energy transfer pathway that is insensitive to collision environment. Upper limits to the collider specific cross sections are obtained for H(2)O, N(2), and CO(2). The CH concentration and temperature profiles are measured and parametrized using a unique method.

Rotational level dependence of electronic quenching of OH(A 2Σ+, ν′ = 0)

Abstract Rotational state-specific rate constants are measured for the electronic quenching of the OH(A 2 Σ + , ν′ = 0) state by N 2 , O 2 , H 2 , D 2 , H 2 O and D 2 O using time-resolved laser-induced fluorescence. A significant decrease in the electronic quenching rate constant is observed as the amount of rotational excitation in the OH is increased from N ′ = 0 to N ′ = 7. The decrease is ⩾60% for the diatomics and ⩾2O% for H 2 O and D 2 O.

Temperature dependent electronic quenching of OH(A 2Σ+, v’=0) between 230 and 310 K

Measurements were made of the temperature dependence of the rate constant for collisional quenching of the v’=0 level of the A 2Σ+ electronically excited state of the OH radical. For all quenchers examined (CO2, H2, O2, and N2), we observed that the thermally averaged cross section σQ increased as the temperature is decreased. For N2, the slowest quencher, σQ shows a dramatic increase from 4.0±0.7 A2 at 311±10 K to 7.0±0.6 A2 at 232±5 K. The results of this work and other high temperature (900–1300 K) studies from this laboratory are compared with a description of the collisional encounter involving attractive forces and complex formation.

Experimental Investigation of Surface Reactions in Carbon Monoxide and Oxygen Mixtures

During hypersonic entry into the CO 2 atmosphere of Mars, competing exothermic chemical reactions may oceur on a spacecraft heatshield surface. Two possible surface reactions are O+O → O 2 and CO+O → CO 2 . The relative importance of these reactions on quartz is investigated using a diffusion tube side-arm reactor together with two-photon laser-induced fluorescence for both O and CO species detection. The experiments show 1) that the presence of CO in the gas phase does not -significantly affect the oxygen recombination reaction on quartz and 2) that the gas-phase CO concentration is not significantly altered by the presence of atomic oxygen. These results indicate that for our experimental conditions the dominant surface reaction on quartz in oxygen-carbon monoxide mixtures is O+O → O 2 . Current heating computations for Martian entries assume CO oxidation to be fully catalytic. The resulting entry heating values are significantly higher than those computed using the assumption of fully catalytic oxygen recombination. The data presented here indicate that the assumption of fully catalytic CO oxidation may be overly conservative for heatshleld sizing purposes

Collisional removal of O2(b 1Σg+,υ=2,3)

The temperature dependence of the collisional removal of O2 in the υ=3 level of the b 1Σg+ state by N2, O2, and CO2 was investigated at room temperature and below. Measurements on the υ=2 level with the colliders, O2, N2O, Ar, and He are also reported. For υ=3 removal by O2, the trend of sharply decreasing loss rate coefficients with increasing υ is seen to continue. For example, at 200 K the rate coefficient for collisional removal of O2 from the υ=3 level by O2 is some three orders of magnitude smaller than that for the υ=1 level. We argue that the mechanism of the deactivation is electronic–electronic (E–E) energy transfer. Observation of emission from a broad range of O2(b 1Σg+) vibrational levels in the terrestrial nightglow, recently discovered in astronomical sky spectra, show less than an order of magnitude difference in population between the more highly populated υ=3 level and the less populated υ=1 level. The implications of these two observations on the vibrational-level-specific atmospheric s...The temperature dependence of the collisional removal of O 2 in the υ=3 level of the b 1 Σ g + state by N 2 , O 2 , and CO 2 was investigated at room temperature and below. Measurements on the υ=2 level with the colliders, O 2 , N 2 O , Ar, and He are also reported. For υ=3 removal by O 2 , the trend of sharply decreasing loss rate coefficients with increasing υ is seen to continue. For example, at 200 K the rate coefficient for collisional removal of O 2 from the υ=3 level by O 2 is some three orders of magnitude smaller than that for the υ=1 level. We argue that the mechanism of the deactivation is electronic–electronic (E–E) energy transfer. Observation of emission from a broad range of O 2 (b 1 Σ g + ) vibrational levels in the terrestrial nightglow, recently discovered in astronomical sky spectra, show less than an order of magnitude difference in population between the more highly populated υ=3 level and the less populated υ=1 level. The implications of these two observations on the vibrational-level-specific atmospheric sources of vibrationally excited molecules in the b 1 Σ g + electronic state are explored.

Collisional removal of O2(b 1Σg+,v=1,2) by O2,N2, and CO2

A state-specific two-laser technique is used to investigate the collisional removal of O2 molecules in the b 1Σg+(v=1,2) levels, where we directly excite O2 and then probe the populations by resonance-enhanced multiphoton ionization. We find general agreement with earlier 300 K values for v=1 removal by O2, and show that v=2 removal is slower by a factor of 5.6±0.6 than v=1 removal. Only upper limits are obtained for N2 as a collider. For removal of v=1 in the atmosphere, N2 is unimportant compared to O2, but it might be competitive for v=2. For CO2 as a collider, addressing O2(b 1Σg+) removal in the atmospheres of Mars and Venus, the removal rate coefficients of the vibrationally-excited levels are similar to that for v=0. The significance of the large difference in the v=1 and v=2 rate coefficients for O2 collisions will be discussed as it relates to the modeling of recent earth nightglow observations.