Rydberg and valence excited states of dibromomethane in 35,000–95,000 cm−1 region studied using synchrotron radiation

The UV–VUV photoabsorption spectrum of dibromomethane (CH2Br2) in the energy region 4.3–11.8 eV (35,000–95,000 cm−1) is investigated using synchrotron radiation. Rydberg series converging to the first four ionization limits at 10.52, 10.74, 11.21 and 11.30 eV corresponding to excitations from the 3b1, 2b2, 1a2, and 4a1 orbitals of CH2Br2 are identified and analyzed. Quantum defect values are observed to be consistent with excitation from the bromine lone pair orbitals. Assignments of the ns Rydberg series are revised and the np and nd Rydberg series are assigned for the first time. Observed vibrational features accompanying the 5p and 4d Rydberg states are assigned exclusively to the totally symmetric (a1) –CBr symmetric stretching mode (ν3) in contrast to the earlier assignment to ν3 and –CH2 bending (ν2) modes. The Rydberg and valence transitions observed in the present experiment are found to be in good agreement with the vertical excited states calculated using the TDDFT method. The calculations are further used to infer the valence transitions responsible for the broad intensity pedestals underlying the Rydberg transitions. The assignments are confirmed using isotopic substitution studies on CD2Br2 whose UV–VUV photoabsorption spectrum is reported here for the first time. This work presents a consolidated analysis of the UV–VUV photoabsorption spectrum of dibromomethane. - Highlights: • VUV spectroscopy of CH2Br2 using synchrotron radiation and TDDFT calculations. • Assignments of np and nd series for the first time and revised assignments of ns series. • Vibrational bands assigned to ν3 in contrast to earlier assignments of ν2 and ν3. • Valence states identified responsible for underlying intensity in VUV spectrum. • First VUV study of CD2Br2, used to consolidate Rydberg and vibronic assignments