Mass analyzed threshold ionization detected infrared spectroscopy: isomerization activity of the phenol–Ar cluster near the ionization threshold

Abstract

The structure of the phenol–argon cluster (PhOH–Ar) in high-n Rydberg states is investigated by the newly developed technique of mass analyzed threshold ionization detected infrared (MATI-IR) spectroscopy. This method selectively measures IR spectra of molecular clusters in very high-n Rydberg states (n > 100) utilized in zero kinetic energy (ZEKE) photoelectron and MATI spectroscopy, whose ionic cores are essentially the same as the corresponding bare cation. The MATI-IR spectrum exhibits only the free OH stretching vibration (ν^π_OH) when the π-bound cluster of the neutral ground electronic state (S₀) is resonantly excited via the S₁ origin to Rydberg states converging to its adiabatic ionization energy level, IE₀(π). When Rydberg states converging to vibrationally excited levels of the local π-bound minimum are prepared, in addition to ν^π_OH also the hydrogen-bonded OH stretching vibration (ν^H_OH) of the H-bonded global minimum is observed in the MATI-IR spectra, even for vibrational excitation of only 14 cm⁻¹ above IE₀(π). These results show that the π → H site switching reaction of the Ar ligand from the aromatic ring to the OH group proceeds only from vibrationally excited states in the π-bound cation core with a small barrier of less than 14 cm⁻¹ from IE₀(π). On the other hand, directly photoionized PhOH⁺–Ar shows both ν^H_OH and ν^π_OH in the IR spectra, even when it is just ionized to IE₀(π). This result implies that the ionization-induced π → H site switching occurs without excess energy in the H-bound or π-bound cations, in contrast to very high-n Rydberg states converging to levels of the π-bound cation. The different efficiencies of the site switching for the Rydberg ion core and the bare ion and the mechanism for the π → H site switching are interpreted by direct ionization from the π-bound to the H-bound structures in addition to the conventional vertical ionization and transitions to high-n Rydberg states.