ULTRAVIOLET PHOTOCHEMISTRY OF DIACETYLENE - METASTAHLE C4H2-ASTERISK+C2H2 REACTION IN HELIUM AND NITROGEN

The collisional photochemistry of diacetylene triplet metastable state (C4H2) With acetylene in nitrogen and helium buffers is studied in a reaction tube attached to a pulsed nozzle. The photolysis laser count erpropagates the molecular expansion through the reaction tube excitin g the C4H2 (1) Delta(u) <-- (1) Sigma(g)(+) 2(0)(1),6(0)(1), transitio n at 231.4 nm. Efficient intersystem crossing forms the triplet state from which reaction occurs. The finite 8 mm len,oth of the tube serves to quench the reaction after 10-30 mu s so that primary products and not polymer are formed. Upon exiting the reaction tube, the photochemi cal products are soft ionized with 118 nm vacuum-ultraviolet light and mass analyzed in a Linear time-of-flight mass spectrometer. The prima ry photochemical processes are C4H2 + C2H2 --> C6H2 + 2H (H-2) and C4 H2 + C2H2 --> C C6H3 + H. Under all conditions examined, C6H2 dominat es the observed products, with C6H3 present above the (CC5H2)-C-13 bac kground only at high nitrogen concentrations which deactivate diacetyl ene within its metastable vibrational manifold more efficiently than d oes helium. C6DH is the major product in the C4H2 + C2D2 reaction, in dicating that both C4H2 and C2H2 contribute one hydrogen to the C6H2 product. C8H4 is observed as a new minor product of the reaction of ex cited diacetylene with C4H2, but only at long reaction times. Formatio n of C3H4 is significantly enhanced by the presence of C2H2. A simple mechanism for the reaction of diacetylene and acetylene with metastabl e excited diacetylene is formulated. Assuming equal photoionization cr oss sections for the products, concentration data are used to determin e that k(C4H2)/k(C2H2) = 11 +/- 2 in helium buffer and 7 +/- 1 in nitr ogen buffer, where k(i) is the total rate constant for the reaction of C4H2 with species i.