Electron-induced chemistry in microhydrated sulfuric acid clusters.

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)nHSO-4 and (H₂O)nH₂SO-4. The mass spectra provide an experimental evidence for the onset of the ionic dissociation of sulfuric acid and ion-pair (HSO_4- ...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)nHSO-4 clusters are formed after the dissociative electron attachment to the clusters containing the (HSO-4...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)nH₂SO-4 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. [ABSTRACT FROM AUTHOR]