Effect of Dimers on the Temperature-Dependent Absorption Cross Section of Methyl Iodide

Due to the implication of iodine chemistry in stratospheric ozone depletion, 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 overlaps with available solar light, is vital. Unfortunately, measurement of this cross section is complicated by the fact that, at typical laboratory 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 experiment to separate the changes in the absorption due to the temperature dependence of dimer formation from the narrowing of the absorption band that results from rotational and vibrational cooling of isolated methyl iodide molecules. Calculation of the predicted absorption cross section shows that the valence band absorption spectrum narrows only slightly upon cooling from 25 to −73 °C (200 K). Absorption spectra 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 °C. The temperature-dependent cross section measured at 0.1 Torr agrees well with the calculated temperature dependence. The spectra at higher pressures show strong pressure as well as temperature dependence. This pressure dependence allowed us to constrain the temperature-dependent equilibrium constant for dimer formation.