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

Abstract

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 H 2 SO 4 . 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.

Gas phase dissociation of H2SO4: A computational study

“…[19]. For reaction (7) our data is in agreement with the published experimental upper limit [10,17] as well as all the other previous computational predictions [13,19,28,36]. Apparently for this reaction the ZPE and BSSE corrections seem to play an important role and cannot be neglected.…”

“…However, many fundamental aspects of the atmospheric cycle and photolysis reaction are still unknown [7][8][9] and experimental and theoretical investigations would be necessary. The reverse reaction, i.e., the oxidation of SO 2 (g) to H 2 SO 4 (g), takes place spontaneously in the upper part of the atmosphere [10] via a number of reactive intermediate gaseous species [11,12].…”

Photolysis of sulfuric acid vapor by visible light as a source of the polar stratospheric CN layer

“…The measurements of Hintze et al [2003] set new upper limits in the 140–195 nm spectrum. A range of ab initio calculations suggest that the electronic cross sections of H 2 SO 4 are significantly less than measured upper bounds for wavelengths longer than 179 nm [ Hintze et al , 2003; Robinson et al , 2003; Wrenn et al , 1999]. Hence an alternative to UV photolysis is needed to explain observations of SO 2 and the CN layer.…”

“…The mechanism involves excitation of OH stretch vibrational overtones of H 2 SO 4 leading to photolysis via SO 3 thus produced photolyzes within hours above 40 km, producing SO 2 [ Burkholder and McKeen , 1997]. Observations of photoluminescence [ Jin et al , 1990] and photo‐oxidation of CO [ Mills and Phillips , 1993; Mills et al , 1996] in sulfuric acid aerosols had previously led to supporting calculations for the energies of excited states of H 2 SO 4 [ Hintze et al , 2003; Wrenn et al , 1999].…”

Calculated two-photon electronic transitions in sulfuric acid and its atmospheric relevance

“…The photodissociation process of H 2 SO 4 was initially thought to occur via electronic excitation in the UV region [6,7]. However, both experiments and ab initio calculations have repudiated this [8][9][10][11][12]. Attempts to measure the absorption spectrum of H 2 SO 4 up to 140 nm (8.6 eV) have failed to identify any electronic transitions [5,8].…”