Rate constants for the quenching of excited I(5 2P1/2) atoms by I2, O2, ICl, and Cl2 have been measured using time resolved I(2P1/2→2P3/2) 1.315 μm fluorescence from iodine atoms produced by the UV laser induced photodissociation of either CF3I or i-C3F7I. Using the pulsed photodissociation of CF3I at 248 nm as an I(5 2P1/2) source the measured rates (cm3/molecule s) were (3.0±0.1)×10−11, (2.1±0.1)×10−11, (3.3±0.4)×10−11, and (2.0±0.1)×10−14 for I2, O2, ICl, and Cl2, respectively. Using the pulsed photodissociation of i-C3F7I at 308 nm as an I(5 2P1/2) source, similar rates were measured for I* quenching by I2 and ICl, but the I(5 2P1/2)+Cl2 quenching rate increased strongly with increasing amounts of Cl2 photodissociation. For approximately 1.8% Cl2 dissociation at 308 nm the measured rate constant for I(5 2P1/2)+Cl2 quenching was (4.3±0.1)×10−13 cm3/molecule s. This increase in the I*+Cl2 rate is attributed to the two body quenching reaction I*+Cl→I+Cl with a rate constant on the order of 1.5×10−11 cm3/molecule s.

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Rate constants for total and state-specific rotational energy transfer (RET) of OH A(2) ?(+) (v? = 2) have been measured directly in atmospheric methane-air and methane-oxygen flames for the first time to our knowledge. We used a picosecond Raman-excimer laser (tau(l) = 300 ps, lambda = 268 nm) to excite the P(11) (12.5) and Q(11) (16.5) A -X transitions in the (2, 0) band of OH molecules. We analyzed the resultant fluorescence with a high-resolution spectrometer in combination with a fast-gated, intensified CCD camera (tau(g) = 400 ps). We recorded the temporal evolution of the emission spectrum by shifting the detection time with respect to the laser pulse. Measured emission spectra were inverted to yield the time-dependent population of rotational levels in the excited state. We calculated rate constants for RET from the results of the fit. The total RET in v? = 2 is similar to v? = 0, 1. The state-specific rates are represented well by a simple energy-gap law.

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A picosecond, excimer-Raman laser (268 nm, 400 ps FWHM) was used for laser sheet excitation of OH in the (2, 0) band. The fluorescence was detected with a fast-gated, intensified camera (400-ps gate width). The effective collisional lifetime of the spectrally integrated fluorescence was measured in two dimensions by shifting the intensifier gate across the decay curve. The average lifetime is ~2.0 ns for a stoichiometric methane -air flame with spatial variations of +/-10 %. Shorter collisional lifetimes were measured for rich flame conditions that are due to a higher number density of the quenchers. Vibrational energy transfer (VET) was observed in premixed methane -air and methane -oxygen flames by putting the fast-gated camera behind a spectrometer. The spectrum of the methane -air flame shows strong VET in contrast with the methane -oxygen flame. This is because N2 is a weak electronic quencher but a strong VET agent. By fitting the measured time dependence of the different vibrational populations ( ' = 2, 1, 0) to a four-level model, rate constants for quenching and VET were determined. For the lower states ( ' = 0, 1) our results are in good agreement with literature values. For a prediction of a spectrally integrated, collisional lifetime in a known collisional environment it is important to consider not only the quenching but also the amount of energy transfer in the excited state as well as the spectral detection sensitivity.