Spectroscopic Studies of 1,2-Dibromobenzene Using Synchrotron Radiation in the UV-VUV Region

The electronic absorption spectrum of the brominated benzene derivative 1,2-dibromobenzene (1,2-DBB) is studied using synchrotron radiation. Absolute absorption cross sections are measured in the ultraviolet (UV) and vacuum ultraviolet (VUV) regions 1200-2900 Å (34,500-83,300 cm^-1); the VUV spectrum in the 1200-1700 Å (∼58,800-83,300 cm^-1) region is reported for the first time. A detailed spectral analysis is performed, supported by time-dependent density functional theory (TDDFT) calculations. The S₀-S₁ transition in the UV region shows extensive vibrational structure, typical of benzene derivatives, tentatively assigned to ring deformation modes using calculated excited-state vibrational frequencies. The intensity profile of this band, simulated using Franck-Condon Herzberg-Teller simulations, shows a good agreement with the experimental spectrum. The VUV absorption spectrum is dominated by strong valence bands with weak and diffuse Rydberg transitions superimposed on the intensity profiles of the valence bands. Rydberg series converging to the first three ionization potentials of 1,2-DBB are assigned using quantum defect analysis. Vertical excitation energies calculated at the TDDFT/CAM-B3LYP/aug-cc-pVTZ level of theory help in corroborating the Rydberg assignments and assigning valence transitions and charge transfer transitions. Simulations of excited-state potential energy curves along the CBr bond provide some insights into the photodissociation dynamics of 1,2-DBB. The measured UV absorption cross-section data helps in estimating photolysis lifetimes of 1,2- and 1,3-DBB with respect to CBr bond breaking at different altitudes from the Earth's surface, thus providing valuable inputs toward modeling the ozone depletion processes in the upper atmosphere.