**Figure 3.** (a) The experimental (red) and convoluted TDDFT (black) photoabsorption cross section of pyridazine. The calculated partial final continuum contributions (‐ ‐ ‐ a_{1}, a_{2}, ‐‥‐‥ b_{1}, − − b_{2}) are shown. The marked ionization limits are those onto which observed Rydberg series converge. (b) The experimental photoabsorption cross section of pyridazine. The theoretical predictions are plotted as vertical bars: optically allowed transitions (full lines using the results given in table 3 and other omitted predicted excitations) and electric-dipole-forbidden and/or spin-forbidden transitions (dashed lines using the results given in table 4).

**Abstract**

The valence shell electronic states of pyridazine have been studied experimentally, by recording the photoabsorption spectrum, and theoretically, by calculating oscillator strengths and excitation energies. The absolute photoabsorption cross section has been measured between 4 and 40 eV, using synchrotron radiation, and is dominated by prominent bands associated with intravalence transitions. In contrast, structure due to Rydberg excitations is weak. One Rydberg state, belonging to a series converging onto the {\rm \tilde X}\;{}^2{\rm B}_{\rm 2} state limit has been observed and assigned. The accompanying vibrational structure has been characterized by analogy with that in the corresponding photoelectron band. Vibrational progressions associated with Rydberg states belonging to one or more series converging onto the {\rm \tilde A}\;{}^2{\rm A}_{\rm 2} state limit have also been observed. The absorption structure associated with these series is complex and only tentative assignments have been proposed for the Rydberg states. The time-dependent version of density functional theory has been used to calculate oscillator strengths and excitation energies for the optically allowed singlet–singlet valence transitions and also to obtain the excitation energies for electric-dipole-forbidden and/or spin-forbidden transitions. The valence shell photoionization dynamics have been investigated theoretically by calculating photoelectron angular distributions and photoionization partial cross sections of the four outermost orbitals. In addition, the ground state outer valence electronic configuration has been obtained at the complete active space self-consistent field and the N-electron valence state perturbation theory to second-order levels of theory.