Reassessing the orbitals of pi systems using photoinduced Ryberg ionization spectroscopy

The molecular orbital energy ordering of most molecules has been establishe d primarily by comparing non-vibrationally-resolved photoelectron spectra w ith electronic structure calculations. The advent of techniques involving t he use of Rydberg states has now allowed the aquisition of vibrationally re solved cationic spectra from vibrationally selected lower states and thus e nabled the unambiguous assignment of cation states. The technique of photoi nduced Rydberg ionization (PW) spectroscopy enables electronic spectroscopy to proceed from various vibrational levels of the ground state of the cati on. From this method, data is presented on various substituted benzenes (ph enol, fluorobenzene, and chlorobenzene), as well as butadiene. For the subs tituted benzenes, in each case the analysis of the (B) over tilde state pro duced results at variance with previous work. In the case of phenol, it was found that, the molecule is non-planar, while for the halobenzenes the orb ital ordering had to be altered. The PIRI method thus provides a revised pi cture of some molecular pi systems. (C) 2000 Elsevier Science BN. All right s reserved.