+ 
and the + Reactions
Hydrocarbon Chemistry in the Atmospheres of Titan and the Outer Planets: a Product Study of the + and the + Reactions
L. J. Stief, R. P. Thorn, Jr., W. A. Payne (NASA/GSFC), X. F. Chillier (NASA/AMES), F. L. Nesbitt (Coppin State College)
Reactions of the radical play a prominent role in the
hydrocarbon chemistry of the atmospheres of Titan and all the outer
planets. An important reaction of in these atmospheric
systems is + (1). The self reaction
+ (2), while of minor importance in
atmospheric models, is a complicating secondary reaction in the study
of + . We have determined the product branching
ratios at low pressure (1.3 mbar He) and at T = 298 and 200 K for the
+ and the +
reactions. The only data available for both of these reactions is at
T = 298 K and at rather high pressure (130 mbar). The measurements were
performed in a discharge flow system coupled with collision-free
sampling to a mass spectrometer operated at low electron energies. For
the vinyl self reaction we measured the following product channel
yields: + = 
+
, 
= 1.00; + =
(1,3-butadiene), 
< 0.01 at both T = 298 K
and 200 K. For the methyl + vinyl reaction we measure : +
= + 
, 
= 0.90 at
T= 298 K and 0.68 at T = 200 K; + =
(propylene), = 0.10 at T = 298 K and 0.32
at T = 200 K. Future photochemical models of the outer planets and
satellites should use the more appropriate low temperature/low pressure
data. The disproportionation channel ( 
)
reforms while the combination channel
( 
) leads to the larger hydrocarbon species ,
and . The competition between the
disproportionation and combination channels as a function of temperature
and pressure has a pronounced effect on the chemical composition of
planetary atmospheres as a function of altitude.