Gcg. Waschewsky et al., EFFECT OF DIMERS ON THE TEMPERATURE-DEPENDENT ABSORPTION CROSS-SECTION OF METHYL-IODIDE, Journal of physical chemistry, 100(28), 1996, pp. 11559-11565
Due to the implication of iodine chemistry in stratospheric ozone depl
etion, an accurate atmospheric lifetime of methyl iodide has recently
become of interest. To calculate this lifetime, a reliable temperature
-dependent UV photodissociation cross section, in the region that over
laps with available solar light, is vital. Unfortunately, measurement
of this cross section is complicated by the fact that, at typical labo
ratory pressures, methyl iodide readily forms dimers whose ultraviolet
absorption differs from that of the monomer and that dimer formation
is also temperature-dependent. We use a combination of theory and expe
riment to separate the changes in the absorption due to the temperatur
e dependence of dimer formation from the narrowing of the absorption b
and that results from rotational and vibrational cooling of isolated m
ethyl iodide molecules. Calculation of the predicted absorption cross
section shows that the valence band absorption spectrum narrows only s
lightly upon cooling from 25 to -73 degrees C (200 K). Absorption spec
tra were also measured experimentally at a range of pressures from 0.1
to 2.4 Torr and a range of temperatures from -22 to 100 degrees C. Th
e temperature dependent cross section measured at 0.1 Torr agrees well
with the calculated temperature dependence. The spectra at higher pre
ssures show strong pressure as well as temperature dependence. This pr
essure dependence allowed us to constrain the temperature-dependent eq
uilibrium constant for dimer formation.