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

The authors wish to acknowledge: (a) beam time allocated at the ASTRID2 synchrotron at ISA Aarhus University, Denmark; (b) C. Puglia (Uppsala University, Sweden) and the Carl Tygger Foundation for making available the VG-Scienta SES-200 photoelectron analyser at the Gas Phase beamline, Elettra, Italy; (c) support of the NSCCS (National super-computing service in the UK to M.H.P.); (d) the financial support provided by the European Commission through the Access to Research Infrastructure action of the Improving Human Potential Programme, FP6-Transnational Access Programme IA-SFS: R113-CT-2004-50600; (e) M.B. acknowledges support by the Italian MIUR (under the Project No. PON01-01078/8) and PRIN 2011 DEMOCRITOS; (f) P.L.V. acknowledges partial funding from the research Grant Nos. PEst-OE/FIS/UI0068/2014 and PTDC/FIS-ATO/1832/2012 through the Portuguese Foundation for Science and Technology, FCT-MEC as well as support from the British Council for Portuguese-English joint collaboration. The research leading to these results has received funding from the European Community's Seventh Framework Programme (No. FP7/2007-2013) CALIPSO under Grant Agreement No. 312284. We all thank Professor Vincenzo Barone and Dr. Julien Bloino for helpful discussions and Professor N. J. Mason for access to an earlier recording of the VUV spectrum of PhI.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.publishersversionpublishe