As a key precursor in the formation of new particles, sulfuric acid (H₂SO₄) plays an important role in the formation of aerosols and clouds in the atmosphere. Gas-phase sulfuric acid molecules can easily form molecular clusters at the beginning of nucleation through hydrogen bonding and other interactions. Therefore, study of the structure and spectroscopy of H₂SO₄ and tsulfuric acid–water clusters is of great significance for revealing the nucleation mechanism of new particles.

A team of researchers led by Prof. ZHANG Weijun from the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences (CAS), in collaboration with scientists from Synchrotron SOLEIL, France, has recently obtained the high-resolution slow photoelectron spectrum (SPES) of H₂SO₄ and observed three electronic states of H₂SO₄+, X²A, A²A and B²A, combined with high-level theoretical results.

For the first time, the "pure" high-resolution SPES of H₂SO₄ was found, without contaminations from other species in the continuous molecular beam.

The researchers investigated the vacuum ultraviolet photoionization of the sulfuric acid, using the state-of-the-art method of double imaging photoelectron photoion coincidence complemented with theoretical calculations.

The adiabatic ionization energy of H₂SO₄ towards X²A cationic ground state was in good agreement with high-accuracy theoretical data in the literature, according to the researchers.

When increasing photon energies, the H₂SO₄+ cation dissociated into HSO₃+ and OH fragments. In addition, the enthalpies of formation for the species were determined by thermochemical cycle.

This work provides fundamental data and measurement method for in-depth understanding of atmospheric nucleation mechanism.

The findings were published in Physical Chemistry Chemical Physics.

The study was financially supported by the National Natural Science Foundation of China, the International Partnership Program of CAS, and the Key Program of Research and Development of Hefei Science Center of CAS.

Mass-selected photoelectron matrices, slow photoelectron spectra (black solid lines) and photoionization yields (blue dotted lines). (Image by LIN Xiaoxiao) 

Optimized structures of H₂SO₄ (trans and cis conformers) and HSO₃, as well as their individual cations, in the ground electronic states. (Image by LIN Xiaoxiao)