Electron-induced chemistry in microhydrated sulfuric acid clusters

Abstract. We investigate the mixed sulfuric acid–water clusters in a molecular beam experiment with electron attachment and negative ion mass spectrometry and complement the experiment by density functional theory (DFT) calculations. The microhydration of (H₂SO₄)_m(H₂O)_n clusters is controlled by the expansion conditions, and the electron attachment yields the main cluster ion series (H₂SO₄)_m(H₂O)_nHSO₄⁻ and (H₂O)_nH₂SO₄⁻. The mass spectra provide an experimental evidence for the onset of the ionic dissociation of sulfuric acid and ion-pair (HSO₄⁻  ⋅  ⋅  ⋅  H₃O⁺) formation in the neutral H₂SO₄(H₂O)_n clusters with n ≥ 5 water molecules, in excellent agreement with the theoretical predictions. In the clusters with two sulfuric acid molecules (H₂SO₄)₂(H₂O)_n this process starts as early as n ≥ 2 water molecules. The (H₂SO₄)_m(H₂O)_nHSO₄⁻ clusters are formed after the dissociative electron attachment to the clusters containing the (HSO₄⁻  ⋅  ⋅  ⋅  H₃O⁺) ion-pair structure, which leads to the electron recombination with the H₃O⁺ moiety generating H₂O molecule and the H-atom dissociation from the cluster. The (H₂O)_nH₂SO₄⁻ cluster ions point to an efficient caging of the H atom by the surrounding water molecules. The electron-energy dependencies exhibit an efficient electron attachment at low electron energies below 3 eV, and no resonances above this energy, for all the measured mass peaks. This shows that in the atmospheric chemistry only the low-energy electrons can be efficiently captured by the sulfuric acid–water clusters and converted into the negative ions. Possible atmospheric consequences of the acidic dissociation in the clusters and the electron attachment to the sulfuric acid–water aerosols are discussed.