Interpretation of the vacuum ultraviolet photoabsorption spectrum of iodobenzene by ab initio computations

Identification of many Rydberg states in iodobenzene, especially from the first and fourth ionization energies (IE₁ and IE₄, X²B₁ and C²B₁), has become possible using a new ultraviolet (UV) and vacuum-ultraviolet (VUV) absorption spectrum, in the region 29 000-87 000 cm⁻¹ (3.60-10.79 eV), measured at room temperature with synchrotron radiation. A few Rydberg states based on IE₂ (A²A₂) were found, but those based on IE₃ (B²B₂) are undetectable. The almost complete absence of observable Rydberg states relating to IE₂ and IE₃ (A²A₂ and B²B₂, respectively) is attributed to them being coupled to the near-continuum, high-energy region of Rydberg series converging on IE₁. Theoretical studies of the UV and VUV spectra used both time-dependent density functional (TDDFT) and multi-reference multi-root doubles and singles-configuration interaction methods. The theoretical adiabatic excitation energies, and their corresponding vibrational profiles, gave a satisfactory interpretation of the experimental results. The calculations indicate that the UV onset contains both 1¹B₁ and 1¹B₂ states with very low oscillator strength, while the 2¹B₁ state was found to lie under the lowest ππ^∗ 1¹A₁ state. All three of these ¹B₁ and ¹B₂ states are excitations into low-lying σ^∗ orbitals. The strongest VUV band near 7 eV contains two very strong ππ^∗ valence states, together with other weak contributors. The lowest Rydberg 4b₁6s state (3¹B₁) is very evident as a sharp multiplet near 6 eV; its position and vibrational structure are well reproduced by the TDDFT results.