Accurate ionization potentials of phenol and phenol-(H2O) from the electric field dependence of the pump–probe photoionization threshold

The adiabatic ionization potentials of phenol and phenol-(H2O) have been obtained from pump–probe photoionization threshold measurements in a supersonic molecular beam time-of-flight mass spectrometer. The observed photoionization thresholds were found to vary linearly with the square root of the ion-extraction electric field over fields ranging from 27 to 500 V/cm. Extrapolations to zero field yields IPs of 68 623±10 cm−1 for phenol and 64 035±10 cm−1 for phenol-(H2O). Thus, hydrogen bonding to one water molecule lowers the ionization potential of phenol by 4590±15 cm−1. The advantages of this technique of measuring ionization potentials are discussed.