Symmetry of rydberg transitions in 1,4-cyclohexadiene: Multiphoton ionization investigation

Abstract

Although much work has been done concerning the absolute ordering of the occupied π molecular orbitals of 1,4-cyclohexadiene, this ordering has not yet been directly proven. Consequently, the ordering of the vacant π* orbitals cannot be said to be directly proven. Most recently, an electron impact spectroscopic investigation of the electronic transitions of 1,4-cyclohexadiene revealed that only two clear Rydberg transitions are present. Both are to p-Rydberg orbitals and each is from a different occupied π orbital. However, since only indirect assignments of those transitions were possible, the absolute ordering of the occupied π molecular orbitals is still unproven. The application of multiphoton ionization spectroscopy can directly answer this problem. For molecules like 1,4-cyclohexadiene, that have a center of symmetry, transitions which involve an even number of photons have selection rules gg or uu, while transitions with an odd number of photons have selection rules gu or ug. In the two-photon spectrum of this molecule only one strong signal, at 6.42 eV, has been observed. (The second transition at 7.30 eV is not accessible with two-photon excitation.) In the three-photon spectrum only one signal, at 7.30 eV, has been observed. From these results the signal at 6.42 eV can be unambiguously assigned as a uu transition, while the signal at 7.30 eV is assigned as a gu transition. Since all p-Rydbergs are of “u” symmetry, these assignments thus confirm the predicted ordering of the occupied π molecular orbitals: πu (SS) above πg(SA).