Electronic state spectroscopy of diiodomethane (CH₂I₂): experimental and computational studies in the 30,000-95,000 cm⁻¹ region.

The electronic absorption spectrum of diiodomethane in the 30,000-95,000 cm(-1) region is investigated using synchrotron radiation; the spectrum in the 50,000-66,500 cm(-1) region is reported for the first time. The absorption bands in the 30,000-50,000 cm(-1) region are attributed to valence transitions, while the vacuum ultraviolet (VUV) spectrum (50,000-95,000 cm(-1)) is dominated by several Rydberg series converging to the first four ionization potentials of CH2I2 at 9.46, 9.76, 10.21, and 10.56 eV corresponding to the removal of an electron from the outermost 3b2, 2b1, 1a2, and 4a1 non-bonding orbitals, respectively. Rydberg series of ns, np, and nd type converging to each of the four ionization potentials are assigned based on a quantum defect analysis. Time dependent density functional theory calculations of excited states support the analysis and help in interpretation of the Rydberg and valence nature of observed transitions. Density functional theory calculations of the neutral and ionic ground state geometries and vibrational frequencies are used to assign the observed vibronic structure. Vibronic features accompanying the Rydberg series are mainly due to excitation of the C-I symmetric stretch (ν3) and CH2 wag (ν8) modes, with smaller contributions from the C-H symmetric stretch (ν1). UV absorption bands are assigned to low lying valence states 1(1)B2, 1(1)B1, 2(1)A1, 3(1)A1, 2(1)B1, and 2(1)B2 and the unusually high underlying intensity in parts of the VUV spectrum is attributed to valence states with high oscillator strength. This is the first report of a comprehensive Rydberg series and vibronic analysis of the VUV absorption spectrum of CH2I2 in the 50,000-85,000 cm(-1) region. The VUV absorption spectrum of CD2I2 which serves to verify and consolidate spectral assignments is also reported here for the first time.