The necessity for multiphoton processes in the 193-nm photochemistry of sulphuric acid aerosols

The observation, that absorption of light by concentrated sulphuric acid at wavelengths near 193 nm apparently tends to zero as the acid is progressively purified, is supported by ab initio calculations of the energies of the three lowest excited states of monomeric H2SO4. The electronic-structure calculations indicate that the lowest excited state is not accessible by single-photon absorption for light of wavelengths >150 nm. This limit includes a generous allowance for error; the calculated energy of the transition corresponds to a wavelength <120 nm. Previous observations of photoluminescence and photo-oxidation of CO in sulphuric acid aerosols are attributed to multiphoton processes, caused by internal focusing of the incident light by the aerosol droplets. This finding does not rule out the occurrence of significant photochemistry of sulphuric acid aerosols in the stratospheres of the Earth and Venus, because of the likely presence of transition-metal ions as impurities in the atmospheric aerosols. (C) 1999 Elsevier Science S.A. All rights reserved.