Zero-kinetic-energy (ZEKE) photoelectron spectroscopy of the hydrogen-bonded phenol–methanol complex

Abstract

The two-colour, two-photon (1 + 1′) resonance-enhanced multiphoton ionisation (REMPI) spectrum of the S₁ state of the hydrogen-bonded phenol–methanol complex has been recorded. The region around the S₁ origin is in good agreement with previously reported spectra. In this work, further discussion of this region is presented and all the observed spectral features are attributed to intermolecular vibrations (and combinations) of a single conformal isomer. Additionally, further bands have been observed in the higher energy region and are assigned to intramolecular (phenol-localised) modes in combination with intermolecular vibrations.

Zero-kinetic-energy (ZEKE) photoelectron spectra have been recorded using different intermediate vibrational levels in the S₁ state. The spectrum recorded via the vibrationless level of the S₁ state shows beautiful structure and indicates a substantial change in complex geometry on ionisation. The observed structure is dominated by progressions of a low-frequency intermolecular bending vibration (34 cm^–1) in combination with other intermolecular vibrations. In particular, the intermolecular stretch (278 cm^–1) demonstrates a progression, each component of which is in combination with progressions of the low-frequency bending mode. Excitation via other intermediate, intermolecular vibronic states also gives rise to structured spectra and these lead to the assignment of the six intermolecular vibrational frequencies of the phenol–methanol cation. In contrast, excitation via the intramolecular phenol ν_6a level gives rise to a featureless spectrum. It is proposed that the latter observation is due to rapid intramolecular vibrational relaxation (IVR) in the S₁ state. The energy of the lowest band observed in the ZEKE spectra exciting via the S₁ vibrationless level, the intermolecular stretch and also two other intermolecular vibrations, in all cases allowed a (field-corrected) adiabatic ionisation energy of 63 207 ± 4 cm^–1(7.8367 ± 0.0005 eV) to be derived.