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Natural radiative lifetimes have been measured in the odd-parity Rydberg series 6 pns (nϭ7 -13) and 6 pnd (nϭ6 -13) of neutral lead using time-resolved UV laser-induced fluorescence from a laser-produced plasma. A relativistic Hartree-Fock calculation taking configuration interaction into account in a detailed way has also been performed for odd-as well as even-parity states for testing the ability of this approach to correctly predict radiative properties of heavy atoms. A generally good overall agreement between experimental and theoretical lifetimes has been achieved except for a few levels. ͓S1050-2947͑98͒05605-4͔

We demonstrate non-intrusive, in situ detection of ammonia (NH 3 ) in reactive hot gas flows at atmospheric pressure using midinfrared degenerate four-wave mixing (IR-DFWM). IR-DFWM excitation scans were performed in the v 2 + v 3 and v 1 + v 2 vibrational bands of NH 3 around 2.3 μm for gas flow temperatures of 296, 550 and 820 K. Simulations based on spectroscopic parameters from the HITRAN database have been compared with the measurements in order to identify the spectral lines, and an absorption spectrum at 296 K has also been measured to compare with the IR-DFWM spectra. The signal-to-noise ratio of the IR-DFWM measurement was found to be higher than that of the absorption measurement. Some spectral lines in the measured IR-DFWM and absorption spectra had no matching lines in the HITRAN simulation. The detection limit of NH 3 diluted in N 2 with IR-DFWM in this spectral range was estimated at 296, 550 and 820 K to be 1.36, 4.87 and 7.06 × 10 16 molecules/cm 3 . The dependence of the NH 3 IR-DFWM signal on the quenching properties of the buffer gas flow was investigated by comparing the signals for gas flows of N 2 , Ar and CO 2 with small admixtures of NH 3 . It was found that the signal dependence on buffer gas was large at room temperature but decreased at elevated temperatures. These results show the potential of IR-DFWM for detection of NH 3 in combustion environments.

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In a piloted jet flame, the pilot flame has an effect of stabilizing the main flame. Detailed mechanisms of pilot flame/main flame interaction are however not well studied. It is expected that the pilot flame affects the main flame through the following mechanisms: (a) the pilot flame provides the heat and radicals to the reaction zone of the main flame, (b) the pilot flame prevents the cold ambient air from being entrained into the main flame, and (c) the pilot flame modifies the stretch rate of the main flame. In this paper, detailed numerical simulations of piloted laminar methane/air jet flames are carried out to elucidate the effect of pilot flame on the structure and burning velocity of the main jet flame. One-dimensional (1D) freely propagating flame is also simulated to investigate the effect of hot gas mixing with the unburned fuel/air mixture, and 1D counter-flow flame is simulated to study the diffusion of the hot gas from the pilot flame to the reaction zone of the main flame and the effect of flame stretch. The results showed that heat transfer from the pilot flame to the main flame has a more significant effect on the structures and propagation of the main flame than the mass transfer from the pilot flame to the main flame. The heat and mass transfer from the pilot flame affects the local equivalence ratio and temperature of the unburned mixture, which gives rise to a significant enhancement of burning velocity. When the hot gas from the pilot flame is at sufficiently high temperatures, an ultra-lean fuel/air mixture can burn at equivalence ratio below the flammability limit. The reaction rate and burning velocity of ultra fuel-lean flames are enhanced by the strain rate, whereas for a main flame with the equivalence ratio closer to that of pilot flame, the reactivity and burning velocity of the main flame decrease with increasing strain rate.