On the valence shell spectroscopy of 1,2-dichlorobenzene

We report high-resolution vacuum ultraviolet (VUV) photoabsorption spectrum of 1,2-dichlorobenzene in the photon energy range 4.0–10.8 eV (310–115 nm). The electronic state spectroscopy of ortho-C₆H₄Cl₂ has been investigated together with quantum chemical calculations at different levels of theory, also providing vertical excitation energies and oscillator strengths. The valence, mixed valence-Rydberg and Rydberg character of the electronic transitions is accompanied by fine structure which has been mainly assigned to in-plane breathing with C–Cl stretching, $v_7^{\prime }\left(a_1\right)$, ring breathing and C–C stretching, $v_8^{\prime }\left(a_1\right)$, in-plane ring breathing, $v_9^{\prime }\left(a_1\right)$, C–Cl symmetric stretching, $v_{10}^{\prime }\left(a_1\right)$, and in-plane C–Cl bending $v_{11}^{\prime }\left(a_1\right)$ modes. The experimental absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of 1,2-dichlorobenzene in the Earth’s atmosphere (0–50 km), showing that solar photolysis is expected to be a weak sink at altitudes lower than 20 km relative to OH radical reactions. Potential energy curves for the lowest-lying excited electronic states, as a function of the C–Cl stretching and in-plane C–Cl bending coordinates, were also obtained employing the time dependent density functional theory (TD-DFT) method. The results show the importance of the complex quasi-degenerate nature of the lowest-lying electronic states which in the intricate nuclear dynamics of the reaction coordinates, yield relevant internal conversion from Rydberg to valence character and in the asymptotic limit bond excision.