Nancy L. Garland

Energy transfer processes in CH A 2 Δ and B 2 Σ − in an atmospheric pressure flame

Laser-induced fluorescence studies have been made of the CH radical in atmospheric pressure flames of methane, oxygen, and nitrogen. Individual rotational levels within the υ′ = 0 and 1 vibrational levels of the A2Δ and B2Σ− states are excited by a dye laser operating in the 380–440-nm region. Studies were made of collisional energy transfer pathways within and between the states, using rotationally resolved fluorescence scans. Rotational transfer occurred at a rate, dependent on rotational level, 2–5 times that of quenching. Vibrational transfer was slow but transfer between A(υ′ = 1) and B(υ′ = 0) took place at rates ~0.2 that of quenching.

On the collisional quenching of electronically excited OH, NH and CH in flames

Relating emission intensities to molecular concentrations in laser-induced fluorescence measurements requires knowledge of collisional quenching rates in flame environments. We survey the current knowledge of quenching of electronically excited states of the radicals OH, NH and CH, as pertains to combustion studies. Considered are the dependence of the cross sections on collision partner, rotational level, and temperature. Using independently determined cross sections for OH, total quenching rates are estimated for hydrocarbon and ammonia flames described in the literature. We conclude that at present OH quenching can be estimated to within 30–50% in many cases, but only to within a factor of three for NH and CH.

Rotational level dependent quenching of the A 3Πi, v’=0 state of NH

Quenching cross sections σQ have been measured for several rotational levels N’ in the A 3∏i, v’=0 state of NH, for a variety of collision partners. Ground state NH was generated in a room temperature discharge flow and excited with a pulsed laser, and the time decay of fluorescence was measured. The radiative lifetime for the levels N’=1 to 5 is 418 ± 8 ns. The σQ’s are generally large, up to 90 A2, and decrease with increasing N’. This indicates the influence of an anisotropic, attractive interaction in most but not all cases. The present values of σQ are compared with those of other experiments; because σQ depends both on N’ and collision energy, experimental conditions must be carefully specified to yield results which are readily comparable.

NH A 3Πi quenching at 1400 K

We have measured electronic quenching rate constants, kQ, in the NH A 3Πi, v’=0 state for several quencher molecules at 1400 K. The NH radicals were produced in a laser pyrolysis apparatus when a mixture of SF6, CF4, NH3, and added quencher was irradiated by a CO2 laser. The ground state NH radicals were then excited to the A state by a pulsed tunable dye laser. The fluorescence decay rates as a function of added quencher pressure yielded the rate constants. The thermally averaged cross sections, σQ=kQ/〈v〉, for NH at 1400 K are all smaller than those observed for OH at 1200 K in this laboratory. In most cases, our high temperature σQ’s are the same as those observed elsewhere at room temperature. These experimental σQ’s are all less than those calculated with an attractive forces ‘‘collision complex’’ model.

Relative transition probability measurements in the A-X and B-X systems of CH

Abstract Relative Einstein emission coefficients have been measured for transitions from the ν′ = 0 and 1 levels in the A2Δ−X2Π and B2Σ-−X2Π systems of the CH radical. The measurements were made in an atmospheric pressure methane-air flame, and the results are compared with theoretical calculations.