The electronic excited states of dichloromethane in the 5.8-10.8 eV energy range investigated by experimental and theoretical methods

We present a comprehensive experimental high-resolution vacuum ultraviolet (VUV) photoabsorption spectrum of dichloromethane, CH₂Cl₂, with absolute cross sections determined for the full 5.8–10.8 eV energy-range. The calculations on the vertical excitation energies and oscillator strengths were performed using the equation-of-motion coupled cluster method, restricted to the single and double excitations level (EOM-CCSD), and were used to help analyse the valence and Rydberg structures in the photoabsorption spectrum. The present spectrum additionally reveals several new features not previously reported in the literature, with particular reference to the valence σ_CCl^*(10a₁)←n_Cl(7b₂)(1¹B₂←X˜¹A₁) and (σ_CCl^*(10a₁)←n_Cl(9a₁)+σ_CH^*(11a₁)←n_Cl(7b₂))(1¹A₁←X˜¹A₁) transitions at 7.519 and 7.793 eV. A vibrational progression of the CCl₂ symmetric stretching, ν₃^′, and CCl₂ scissoring, v₄^′(a₁), modes have also been assigned for the first time in the 7.4–8.6 eV energy range. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of dichloromethane in the Earth's atmosphere (0–50 km). Potential energy curves as a function of the C–Cl coordinate, for the four lowest-lying excited A′ and A″ electronic states, have additionally been calculated at the EOM-CCSD level of theory.