Branching ratios for quenching of nitric oxide A 2Σ+(ν′ = 0) to X 2Π(ν″ = 0)

Abstract

We describe experiments designed to measure the fraction of nitric oxide molecules that undergo quenching from A ²Σ⁺(ν′ = 0) directly to X ²Π(ν″ = 0). This quenching channel was investigated for room temperature collisions with O₂, CO, CO₂, and H₂O 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, N₂, and the quenching gases. An intense nanosecond laser pulse, tuned to the NO A ²Σ⁺–X ²Π(0,0) Q₁₁ + ^pQ₂₁ bandhead at 226.3 nm, depopulated more than 20% of the equilibrium population in the X ²Π(ν″ = 0) manifold. A weak, time-delayed, picosecond laser pulse, tuned to the A ²Σ⁺–²Π(1,0) Q₁₁ + ^pQ₂₁ bandhead at 214.9 nm, probed recovery of population in X(ν″ = 0) via subsequent LIF for each of the investigated quenchers. Remarkably large branching ratios were observed for direct quenching to X ²Π(ν″ = 0). Water, carbon monoxide, and oxygen quench NO A ²Σ⁺(ν′ = 0) to X ²Π(ν″ = 0) with branching ratios that are approximately 0.3. The significantly higher branching ratio for quenching by carbon dioxide is 0.6. The results provide insight on the NO quenching process and represent an important step toward a detailed understanding of the effects of collisional energy transfer on saturated laser-induced fluorescence, which is necessary to properly model detection strategies based on high laser fluences.