Ov. Rattigan et al., UV ABSORPTION CROSS-SECTIONS AND ATMOSPHERIC PHOTOLYSIS RATES OF CF3I, CH3I, C2H5I AND CH2ICL, Journal of the Chemical Society. Faraday transactions, 93(16), 1997, pp. 2839-2846
Citations number
46
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
The absorption cross-sections of trifluoromethyl iodide (CF3I), methyl
iodide (CH3I), ethyl iodide (C2H5I) and chloroiodomethane (CH2ICI) ha
ve been determined over the wavelength range 235-400 nm, with a spectr
al resolution of 0.6 nm (FWHM), using a diode array spectrometer. The
spectra consist of a broad continuous absorption band with maximum cro
ss-sections of (6.0 +/- 0.1) x 10(-19) cm(2) molecule(-1) for CF3I at
267 nm, (1.09 +/- 0.02) x 10(-18) cm(2) molecule(-1) for CH3I at 258 n
m, (1.18 +/- 0.04) x 10(-18) cm(2) molecule(-1) for C2H5I at 258 nm an
d (1.21 +/- 0.07) x 10(-18) for CH2ICI at 270 nm. The temperature depe
ndence of the cross-section was investigated over the range 333-243 K.
A decline in the cross-section with decreasing temperature at wavelen
gths longer than ca. 280 nm was observed in all cases, the decrease be
ing most pronounced in the long wavelength tail of the absorption band
. At wavelengths shorter than ca. 270 nm the cross-section increased w
ith decreasing temperatures; the effect, however, was significantly sm
aller (5-10%), being most pronounced around the band maximum. The temp
erature dependence was parametrised in order to calculate the atmosphe
ric photolysis rate as a function of altitude, latitude and season. Mo
del calculations show that during sunlit hours the iodides will be rap
idly photolysed with tropospheric photodissociation lifetimes of ca. 1
day for CF3I, several days for CH3I and C2H5I and several hours for C
H2ICI.