Multiphoton ionization photoelectron spectroscopy of phenol: Vibrational frequencies and harmonic force field

A molecular beam of phenol, cooled by a supersonic expansion, is crossed at right angles by the output of a pulsed frequency-doubled dye laser, causing 1 + 1 resonance enhanced multiphoton ionization. The kinetic energy of the resulting photoelectrons is determined as a function oflaser wavelength with time-of-flight analysis, permitting the assignment of 11 vibrational frequencies for the 2BI phenol-h6 cation and ten vibrational frequencies for phenol-d5. Of these, all but the lowest frequency one in each case are in-plane vibrations of which phenol has a total of 19. An approximate harmonic force field for the in-plane modes of the phenol cation is derived along with its associated frequencies and mode forms. This in turn facilitates the vibrational analysis. Analogous force field calculations have been carried out on the ground eA d and first excited (I B2 ) states of the neutral parent, permitting conclusions to be reached concerning bonding changes upon removal of an electron from the phenol electron system.