The excited states of azulene: a study of the vibrational energy levels for the lower ππ*-valence states by configuration interaction and density functional calculations, and theoretical studies of the Rydberg states

A new vacuum ultraviolet absorption spectrum of azulene vapour has been obtained by using a synchrotron radiation source.The onset of the ultraviolet spectrum, previously reported by Sidman et al has been analysed in detail by Franck-Condon (FC) and Herzberg-Teller (HT) methods. The PES profile identifies the 3px-Rydberg state 0⁰ band as 131 cm^-1 from the VUV maximum. Excited state energy levels were calculated by three independent methods: the wide scan VUV spectrum was correlated with symmetry adapted cluster configuration interaction (SAC-CI) calculations. The low energy portion of the spectrum was studied by both time dependent density functional theoretical methods (TDDFT) and multi-reference multi-root CI (MRD-CI). Equilibrium structures were determined for valence states at the TDDFT level. Rydberg states were determined by both TDDFT and MRD-CI. The FC+HT analyses were performed on the TDDFT wave-functions. The HT intensity profile is generally low in intensity, relative to the FC ones; however, HT is dominant in the second singlet state (S₂, 1¹A₁). As a result, numerous non-symmetric modes, their overtones and combination bands show considerable intensity in that band. Extremely diffuse s-, p-, d- or f-character functions enabled extrapolation to the IE₁ for many Rydberg states (RS). The lowest RS based on IE₂ (3b₁3s) lies at 4.804 eV with quantum defect 0.714. Differentiation between valence and RS is readily made using the second moments of the charge distribution.