Population cycling in saturated laser-induced fluorescence detection of nitric oxide
We describe experiments designed to study population cycling in laser-induced fluorescence (LIF) detection of nitric oxide. Population cycling refers to the process by which a molecule is excited multiple times by the same laser pulse. This cycling occurs when collisions rapidly quench laser-excited molecules to the ground state and subsequent rapid rotational energy transfer in the ground state refills the laser-pumped level. This can be a significant contribution to the overall LIF signal during a saturating laser pulse. Owing to extremely slow ground-state vibrational energy transfer in NO, population cycling is only significant when there is rapid and direct electronicquenching to NO X 2P(v??=0). This potential quenching channel was investigated for several collision partners by measuring recovery of the ground-state population following intense laser excitation. Experiments were conducted in a room-temperature flow cell containing dilute mixtures of NO in quenching gases. An intense nanosecond laser pulse, tuned to the NO A 2S+–X 2P(0,0) Q1+Q21 bandhead at 226.3 nm, depopulated more than 20% of the equilibrium population in the X 2P(v??=0) manifold. A weak, time-delayed, picosecond laser pulse, tuned to the A–X(1,0) Q1+Q21 bandhead at 214.9 nm, measured recovery of population in X(v??=0) under different quenching conditions. Results are presented for quenching gases: N2, CO2, O2, and H2O. The results are an important step toward providing a detailed understanding of the physics of saturated LIF, which is necessary to properly model detection strategies based on high laser fluences.
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