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The HRTOF technique (2327) was used to obtain the H + CO product center-of-mass (CM) translational energy and angular distributions. A pulsed HCO beam was generated by photolyzing a ~2% mixture of acrolein in Ar with the 193-nm radiation of an ArF excimer laser focused in front of the pulse nozzle. The HCO radicals produced were entrained in the molecular beam and subsequently cooled (to ~20 K) by supersonic expansion. The radical beam was collimated and then crossed with the visible photolysis radiation from a Nd:YAG pumped dye laser (defined here as the photolysis laser). This photolysis laser radiation was slightly focused and properly delayed with respect to the 193-nm radical production laser but preceded the H-atom product probe lasers; its polarization can be rotated by a Fresnel-Rhomb achromatic λ/2 plate for product angular distribution studies. Hydrogen atoms produced from photodissociation were first excited to the 22P level by a 121.6-nm Lyman-α (L-α) radiation generated by tripling a 364.7-nm dye-laser output in Kr (values given are vacuum wavelengths). The H atoms were further excited to a high-n Rydberg level by the 366.2-nm output of another dye laser (Rydberg probe laser). A small fraction of the radiatively metastable Rydberg atoms drifted with their nascent velocities to a microchannel plate detector that was perpendicular to the molecular beam, and were detected as ions after being field ionized in front of the detector. The nominal flight length was 37.1 cm. The accumulated H-atom TOF spectra represented 50,000 to 100,000 laser firings. Because the HRTOF tagging technique gives high resolution and uniform detection sensitivity for the kinetic energy of the H fragment and was rid of the CO background interference, the observed CO rotational-vibrational state distributions were well resolved and extended to the low-j states and can be more reliably interpreted than those reported previously (1518). It is clear from these experimental data that the CO rotational state distributions exhibit pronounced and reproducible oscillations in nascent product states, extending through the low-j excitation range (see Fig. 3). More details of the experiments and results are provided in the Supplementary Materials.

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