Reactions of hyperthermal C(³P) generated by laser ablation with H₂, HCl, HBr, and CH₃OH

The reactions of C(³P) with H₂, HCl, HBr, and CH₃OH(D) were investigated in a crossed-beam configuration using laser ablation of graphite as the source of C(³P). Upon pulsed irradation of graphite with focused laser output at 266 and 355 nm, hyperthermal C(³P) is produced and expands freely into the vacuum. In this "free ablation" mode, directional beams of monomeric carbon are produced with a peak velocity of ∼8000 m s^-1 and a broad velocity distribution that can be described by temperatures of ∼21 000 and ∼9500 K when using 266 and 355 nm ablation wavelengths, respectively. Using 266 nm ablation, the endothermic reactions of C(³P) with the title molecules were investigated by probing the CH product. CH is produced predominantly in v = 0 with rotational distributions that are well described by temperatures in the range 1500-2200 K, depending on the molecular reactants. The spin-orbit and Λ-doublet sublevels are equally populated. In reactions with CH₃OD, both CH and CD are detected, identifying both the methyl and the hydroxyl groups as reactive sites. Comparisons with the CH internal energy distributions obtained in the reaction of C(¹D) with H₂ show remarkable similarities. On the basis of theoretical investigations and the known electronic states of the methylene intermediate, it is suggested that the reactions of both C(³P) and C(¹D) proceed via insertion involving carbene intermediates. The participation of several low-lying states of the carbenes may lead both to lowering of the activation barrier for insertion and to CH products with similar populations of the two Λ-doublet components.