Electronic and Cationic Spectroscopy of 9-Hydroxy-9-fluorene Carboxylic Acid

Resonance-enhanced multiphoton ionization spectroscopy of supersonically cooled gas-phase 9-hydroxy-9-fluorene carboxylic acid (9HFCA) is reported for its first electronic excited state, S-1. The UV-UV hole-burning experiment identifies a single conformer in the molecular beam, stabilized by an intramolecular hydrogen bond. For this C-s symmetric conformer, two low frequencies in the S-1 spectrum are assigned: an in-plane rocking mode of the carboxylic acid side chain lies at 58 cm(-1), and an in-plane fluorene bending mode appears at 183 cm(-1). The corresponding mode frequencies in the cation, 58 and 196 cm(-1), are measured by zero electron kinetic energy (ZEKE) spectroscopy upon pumping the S, vibronic states. The adiabatic ionization potential is measured to be 64 923 +/- 5 cm(-1). In addition, a feature established by ZEKE spectroscopy upon pumping the hot band is found at 67 cm(-1). This is assigned as a hot band of the HO-C9-COOH rocking mode in the neutral ground state.