THE ULTRAVIOLET PHOTOCHEMISTRY OF DIACETYLENE - DIRECT DETECTION OF PRIMARY PRODUCTS OF THE METASTABLE C4H-ASTERISK-2 + C4H2 REACTION

The products of diacetylene's ultraviolet photochemistry over the 245-220 nm region are directly determined for the first time. At these wavelengths, the photochemistry is thought to proceed from a metastable excited state of C4H2 rather than by direct photolysis. The experimental method employs a small reaction tube attached to a pulsed nozzle. C4H2 is excited within the reaction tube where collisions of C4H2* with C4H2 form products which are subsequently ionized by vacuum ultraviolet radiation (118 nm) in the ion source of a time-of-flight mass spectrometer. The C4H2* + C4H2 reaction produces C6H2 (+C2H2), C8H2 (+2H,H2), and C8H3 (+H), all in good yield. An extensive set of experiments is carried out to ensure that the products observed are initial products formed by single-photon excitation of gas phase C4H2. Under certain conditions, secondary products formed by subsequent reaction of the initially formed products with C4H2 are also observed. These are dominated by C10H3 and C12H3. Thermochemical arguments are made which point to C8H3+C4H2 as the source of these secondary products. Collisional deactivation of C4H2* from its initially excited energy (approximately 120 kcal/mol above the ground state) to the lower levels of the metastable state (approximately 74 kcal/mol) is important in determining the relative amounts Of C8H2 and C8H3 products. In cases where C4H2* undergoes extensive deactivation prior to reaction, C8H3+H products dominate. When collisional deactivation is minimized, much of the C8H3 products are formed with enough energy to subsequently dissociate further to form C8H2+2H. Mechanisms are postulated for the observed reactions and some suggestions for further work to assess the importance of these reactions in planetary atmospheres are given.