8378
J. Phys. Chem. A 2002, 106, 8378-8385
Bromine Nitrate Photochemistry: Quantum Yields for O, Br, and BrO Over the Wavelength Range 248-355 nm† R. Soller,‡,§ J. M. Nicovich,| and P. H. Wine*,‡,| School of Earth and Atmospheric Sciences and School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332 ReceiVed: January 9, 2002; In Final Form: March 27, 2002
A laser flash photolysis-resonance fluorescence technique has been employed to investigate the production of Br, O, and BrO from photodissociation of bromine nitrate (BrONO2) at wavelengths in the range 248355 nm. The values obtained for the Br atom quantum yields are 0.35 ( 0.08, 0.65 ( 0.14, >0.62 ( 0.11, and 0.77 ( 0.19 at 248, 266, 308 and 355 nm, respectively. The values obtained for the O atom quantum yields are 0.66 ( 0.15, 0.18 ( 0.04, 0.60 >0.62 (av) 1.09-8.67 0.81-3.77 0.79
a
(VI)
[Br]0 ) [Br]0BN + [Br]0Br2O
no. of expts λa,b
Φ(Br)
Units are T (K); P (Torr); concentrations λ ≡ photolysis laser wavelength.
(1014
per
no. of expts.
λa,b
4 4 3 5
248 248 248 248
4 4 4 4 4 4
266 266 266 266 266 266
3 3
308 308
2
355
cm3);
λ (nm).
the production of BrO molecules. Equation IX defines the parameter Z, a quantity obtained from the BrO conversion technique. Equation X states (as discussed previously) that Br and BrO are generated simultaneously from the photolysis of Br2O. Results and Discussion All quantum yields for bromine nitrate were found to be independent of pressure. Temperature variations were performed at 266 nm for Φ(Br) and Φ(Br)/Φ(BrO), and no dependence on temperature was observed. Experimental conditions are summarized in Tables 1-3, and results are given in Table 4. All values for Φ(Br) and Φ(O) at 248, 266, and 308 nm were determined experimentally using the RF calibration technique. The reported value for Φ(Br)/Φ(BrO) at 266 nm, and thus Φ(BrO), was determined using the BrO conversion technique. Φ(Br) and Φ(BrO) were obtained at 355 nm by using both techniques and then systematically solving the set of five linear equations (see above). Quantum yields for NO3 have been reported by Harwood et al.,9 and their results are quoted in Table 4 for comparison. On the basis of examination of all possible photolytic pathways (1a)-(1f), Φ(Br) should be equal to or
Bromine Nitrate Photochemistry
J. Phys. Chem. A, Vol. 106, No. 36, 2002 8383
TABLE 3: Summary of Data for Φ(Br)/Φ(BrO)
b
Ta
Pa
[BrONO2]a
[NO]a
298 298 298 298 298 298 298 245 323
20 49 50 50 50 52 200 50 50
1.50-2.31 1.21-3.99 1.83-3.60 2.68-2.73 1.32-5.27 0.86-4.47 1.37-3.64 2.39-4.04 1.59-3.57
1.29-1.94 2.84-4.45 1.16-1.48 0.82-0.91 0.48-1.48 1.14-28.7 1.45-3.05 1.10-1.32 0.89-1.01
298
50
0.88-4.45
0.83-4.78
Φ(Br)/ Φ(BrO)
no. of expts
λa,b
1.76 1.61 1.71 1.78 1.76 1.55 2.05 1.59 1.52 1.74 (av) 3.2
4 5 5 3 6 3 3 4 3
266 266 266 266 266 266 266 266 266
6
355
a Units are T (K); P (Torr); concentrations (1014 per cm3); λ (nm). λ ≡ Photolysis laser wavelength.
TABLE 4: Summary of Reported Bromine Nitrate Quantum Yields λa
Φ(Br)b,c
Φ(O)b,c
248 0.35 ( 0.08 0.66 ( 0.15 266 0.65 ( 0.14 0.18 ( 0.04 308 >0.62 ( 0.11
