Rk. Frost et al., ULTRAVIOLET PHOTOCHEMISTRY OF DIACETYLENE - METASTAHLE C4H2-ASTERISK+C2H2 REACTION IN HELIUM AND NITROGEN, Journal of physical chemistry, 99(23), 1995, pp. 9408-9415
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.