NEAR-THRESHOLD PHOTODISSOCIATION OF C2H2, C2HD, AND C2D2 STUDIED BY H(D) ATOM PHOTOFRAGMENT TRANSLATIONAL SPECTROSCOPY

The photodissociation of jet-cooled C2H2, C2HD, and C2D2 molecules fol lowing excitation within their respective predissociated Angstrom-X ab sorption band systems has been investigated using the technique of H(D ) Rydberg-atom photofragment translational spectroscopy. Analyses of t he rotational structure apparent in the resulting total kinetic-energy release spectra yields precise values for the DCC-H, HCC-D, and DCC-D bond dissociation energies: D-0(0)(DCC-H)=46144+/-50 cm(-1), D-0(0)(H CC-D)=46687+/-50 cm(-1), and D-0(0)(DCC-D)=46754+/-50 cm(-1) respectiv ely. The average internal excitation of bending vibration in the C2H(C 2D) fragments formed from these photolyses of all three parent isotopo mers is shown to scale linearly with the excess energy over and above that required for bond fission by ca. 540 cm(-1).