Reactions of neutral, ground-state yttrium atoms with formaldehyde, acetaldehyde, and acetone (YϩRRЈCO, where R,RЈϭH,CH 3) were studied in crossed molecular beams. At collision energies greater than 24 kcal/mol, four product channels were observed corresponding to elimination of CO, H 2 , H, and nonreactive scattering. For the dominant CO elimination channel, a large fraction ͑34%-41%͒ of the available energy appeared as kinetic energy of the products. RRKM modeling indicated this was a result of two factors: a large potential energy barrier for RЈ migration leading to (R)(RЈ)YCO and dissociation of this complex prior to complete energy randomization. The CM angular distributions were all forward-backward symmetric, indicating the existence of at least one long-lived reaction intermediate. The angular distributions ranged from being quite forwardbackward peaking for the YϩH 2 CO reaction to isotropic for Yϩ͑CH 3 ͒ 2 CO. A simple equation is derived based on statistical complex theory that relates the shape of the CM angular distributions to the structure of the dissociating complex.

The reaction of ground-state Y (a 2 D) atoms with ketene (H 2 CCO) was studied at two collision energies, 22.7 and 10.4 kcal/mol. At both collision energies, four competing processes were observed corresponding to formation of YCH 2 , YCCO, YCHCO (with elimination of CO, H 2 , and H, respectively), and nonreactive scattering. The endoergicity of the YCHCO + H product channel was 10.5 ( 2.0 kcal/mol, leading to D 0 (Y-CHCO) ) 93.4 ( 2.0 kcal/mol. Product branching ratios measured at both collision energies show formation of YCH 2 + CO to be dominant. The trend in branching ratios as a function of collision energy, combined with center-of-mass distributions obtained through fits to the experimental data and analogies to the Y + H 2 CO system, allows a qualitative description of relevant features of the Y + H 2 CCO potential energy surface.