Theoretical investigation of coherent bending vibrations in SO₂

We theoretically investigated strong-field IR pump - IR probe dissociative ionization of SO₂ including all vibronic degrees of freedom (symmetric stretch, bending, and antisymmetric stretch modes). We model the IR-pulse pump process by solving the coupled-channel Schrödinger equation on the Born Oppenheimer (BO) potential energy surfaces of the SO₂[X¹A₁]  and SO₂⁺[X²A₁] electronic ground states neglecting the photoelectron kinetic energy. The BO potential surfaces are ab initio calculated by applying the multi-configurational self-consistent-field quantum-chemistry code GAMESS. We account for multiple ionization in the delayed probe pulse by vertically projecting the SO₂ and SO₂⁺ nuclear wave functions onto the lowest potential energy surface of the triply-charged molecule, which we either approximate as purely Coulombic or (for higher accuracy) derive from molecular dynamics calculations. We propagate the nuclear motion to sufficiently large internuclear distances to obtain the fragment kinetic energy releases by Fourier transformation of the real-space nuclear wave function to momentum space. We provide the kinetic energy of S⁺, O⁺, and O⁺ and the angle between O⁺, and O⁺ fragments as functions of the pump-probe delay. We find coherent bending vibrations in the SO₂⁺[X²A₁] and SO₂[X¹A₁] states with oscillation periods of 85 and  64 fs, respectively, in agreement with experimental data.