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Scheme for excitation and detection of electronically excited NO molecules. The rotational energy level spacings are greatly exaggerated for visual clarity.
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Experimental images (a–c), Newton diagrams (d–f), and differential cross sections (g–i) for rotationally inelastic collisions between NO(A) molecules and argon atoms. (a), (d), (g): N = 0 → N ′ = 2; (b), (e), (h): N = 0 → N ′ = 7; (c), (f), (i): N = 0 → N ′ = 14.
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We report direct doubly differential (quantum state and angle-resolved) scattering measurements involving short-lived electronically excited molecules using crossed molecular beams. In our experiment, supersonic beams of nitric oxide and argon atoms collide at 90°. In the crossing region, NO molecules are excited to the A2Σ+state by a pulsed nanosecond laser, undergo rotationally inelastic collisions with Ar atoms, and are then detected 400 ns later (approximately twice the radiative lifetime of the A2Σ+state) by 1 + 1′ multiphoton ionization via the E2Σ+ state. The velocity distributions of the scattered molecules are recorded using velocity-mapped ion imaging. The resulting images provide a direct measurement of the state-to-state differential scattering cross sections. These results demonstrate that sufficient scattering events occur during the short lifetimes typical of molecular excited states (∼200 ns, in this case) to allow spectroscopically detected quantum-state-resolved measurements of products of excited-state collisions.
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