Two-photon excitation spectroscopy of phenanthrene singlet states below 50000 cm-1

Abstract

The 2-photon excitation spectrum of phenanthrene in liq. solns. is reported in the energy range 29,000-49,000 cm-1. Comparison with the 1-photon spectrum and extended CNDO/S calcns. allows assignment of 8 singlet states. The strongest 2-photon band is assigned to the Bb state not seen directly in the UV spectrum. This high intensity and other features of the spectrum are in sharp contrast to the pairing selection rules which forbid 2-photon transitions to plus states in alternant hydrocarbons.