Mh. Harwood et al., KINETICS AND MECHANISM OF THE BRO SELF-REACTION - TEMPERATURE-DEPENDENT AND PRESSURE-DEPENDENT STUDIES, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 102(10), 1998, pp. 1790-1802
The flash photolysis/UV absorption technique has been used to study th
e self-reaction of BrO radicals over the temperature range 222-298 K a
nd the pressure range 100-760 Torr of N-2 or O-2. TWO chemical sources
of BrO radicals were used: photolysis of Br-2 in the presence of exce
ss ozone and photolysis of O-2 in the presence of excess Br-2. The ove
rall rate constant, k(1), for the BrO self-reaction (defined by -d[BrO
]/dt = 2k(1)[BrO]2) was found to be temperature and pressure independe
nt at T greater than or equal to 250 K, with k(1) = (2.88 +/- 0.20) x
10(-12) cm(3) molecule(-1) s(-1). At temperatures below 250 K, k(1) wa
s found to be pressure dependent, due to the emergence of a new termol
ecular channel of the BrO self-reaction 1c, -1c forming the BrO dimer,
Br2O2 (BrO + BrO + M reversible arrow Br2O2 + M) Channel-specific rat
e constants were determined for the two bimolecular channels of the Br
O self-reaction above 250 K, giving for (1a) (BrO + BrO --> 2Br + O-2)
k(1a) = (5.31 +/- 1.17) x 10(-12) exp{(-211 +/- 59)/T} cm(3) molecule
(-1) s(-1) and for (1b) (BrO + BrO --> Br-2 + O-2) k(1b) = (1.13 +/- 0
.47) x 10(-14) exp{(983 +/- 111)/T} cm(3) molecule(-1) s(-1). Below 25
0 K, the overall rate coefficient of the two bimolecular channels is r
educed as the dimer forming channel emerges. At 235 and 222 K, rate co
nstants for the formation (k(1c)) and decomposition (k(-1c)) of Br2O2
Were recorded. Using the values for K-1c, Delta H-r for reaction Ic wa
s-estimated as -58.6 +/- 0.1 kJ mol(-1). A UV absorption spectrum attr
ibuted to Br2O2 was also recorded over the wavelength range 300-390 nm
. The cross section of the smooth Br2O2 spectrum was found to be 1.2 x
10(-17) cm(2) molecule(-1) at 320 nm. These results are rationalized
in terms of a mechanism of the BrO self-reaction that shows competitio
n, at low temperatures, between collisional quenching and unimolecular
,dissociation of an excited BrOOBr intermediate. The rate constant fo
r the reaction of oxygen atoms with molecular bromine was also determi
ned in the course of these experiments [O + Br-2 --> BrO + Br (5)], gi
ving k(5) = (5.12 +/- 1.86) x 10(-13) exp{(989 +/- 91)/T} cm(3) molecu
le(-1) s(-1). All errors are 1 sigma. The atmospheric implications of
these results are discussed.