Autoionization-detected infrared spectroscopy of jet-cooled naphthol cations

We applied autoionization-detected infrared (ADIR) spectroscopy in order to observe OH stretching vibrations of jet-cooled 1- and 2-naphthol cations. In this technique, high Rydberg states, the vibrational levels of which are essentially the same as those of the corresponding bare molecular ion, wer e prepared by two-color double-resonance excitation. Vibrational transition s in the ion core of the high Rydberg states were measured by detecting the vibrational autoionization signal. For rotational and structural isomers o f naphthol, similar low-frequency shifts of the OH frequencies upon ionizat ion were found. The OH frequency shifts of naphthols were much smaller than that found for phenol, although both molecules have quite similar OH frequ encies in their neutral ground state. This remarkable difference in frequen cy shifts was qualitatively explained in terms of the charge delocalization in the aromatic ring. In addition, the OH stretching vibrations of the 1- and 2-naphthol-Ar cluster cations were observed by infrared photodissociati on spectroscopy. It was found that perturbations from the Ar atom to the hy droxyl group of naphthol are negligible in both the neutral and cationic gr ound states.