Description:
<jats:p>The ultraviolet absorption spectra of a number of alkyl iodides which have been found in the troposphere, CH<jats:sub>3</jats:sub>I, C<jats:sub>2</jats:sub>H<jats:sub>5</jats:sub>I, CH<jats:sub>3</jats:sub>CH<jats:sub>2</jats:sub>CH<jats:sub>2</jats:sub>I, CH<jats:sub>3</jats:sub>CHICH<jats:sub>3</jats:sub>, CH<jats:sub>2</jats:sub>I<jats:sub>2</jats:sub>, and CH<jats:sub>2</jats:sub>ClI, have been measured over the wavelength range 200–380 nm and at temperatures between 298 and 210 K. The absorption spectra of the monoiodides CH<jats:sub>3</jats:sub>I, C<jats:sub>2</jats:sub>H<jats:sub>5</jats:sub>I, CH<jats:sub>3</jats:sub>CH<jats:sub>2</jats:sub>CH<jats:sub>2</jats:sub>I, and CH<jats:sub>3</jats:sub>CHICH<jats:sub>3</jats:sub> are nearly identical in shape and magnitude and consist of single broad bands centered near 260 nm. The addition of a chlorine atom in CH<jats:sub>2</jats:sub>ClI shifts its spectrum to longer wavelengths (σ<jats:sub>max</jats:sub> at 270 nm). The spectrum of CH<jats:sub>2</jats:sub>I<jats:sub>2</jats:sub> is further red‐shifted, reaching a maximum of 3.85×10<jats:sup>−18</jats:sup> cm<jats:sup>2</jats:sup> molecule<jats:sup>−1</jats:sup> at 288 nm and exhibiting strong absorption in the solar actinic region, λ>290 nm. Atmospheric photolysis rate constants, <jats:italic>J</jats:italic> values, have been calculated assuming quantum efficiencies of unity for different solar zenith angles as a function of altitude using the newly measured cross sections. Surface photolysis rate constants, calculated from the absorption cross sections measured at 298 K, range from 3×10<jats:sup>−6</jats:sup> s<jats:sup>−1</jats:sup> for CH<jats:sub>3</jats:sub>I to 5×10<jats:sup>−3</jats:sup> s<jats:sup>−1</jats:sup> for CH<jats:sub>2</jats:sub>I<jats:sub>2</jats:sub> at a solar zenith angle of 40°.</jats:p>