Calculated electronic transitions in sulfuric acid and implications for its photodissociation in the atmosphere.

We have calculated electronic transitions for sulfuric acid in the ultraviolet region using a hierarchy of coupled cluster response functions and correlation consistent basis sets. Our calculations indicate that the lowest energy singlet transition occurs at 8.42 eV with an oscillator strength of 0.01. The lowest energy triplet state occurs at 8.24 eV. Thus, the cross section of sulfuric acid in the actinic region is likely to be very small and smaller than the upper limit put on this cross section by previous experimental investigations. We estimate the cross section of sulfuric acid in the atmospherically relevant Lyman-alpha region ( approximately 10.2 eV) to be approximately 6 x 10 (-17) cm (2) molecule (-1), a value approximately 30 times larger than the speculative value used in previous atmospheric simulations. We have calculated the J values for photodissociation of sulfuric acid with absorption of visible, UV, and Lyman-alpha radiation, at altitudes between 30 and 100 km. We find that the dominant photodissociation mechanism of sulfuric acid below 70 km is absorption in the visible region by OH stretching overtone transitions, whereas above 70 km, absorption of Lyman-alpha radiation by high energy Rydberg excited states is the favored mechanism. The low lying electronic transitions of sulfuric acid in the UV region do not contribute significantly to its dissociation at any altitude.