PAIR FORMATION OF PHENOL IN THE VICINITY OF AN AQUEOUS-SOLUTION SURFACE STUDIED BY MEANS OF LIQUID BEAM MULTIPHOTON IONIZATION MASS-SPECTROMETRY

An aqueous solution of phenol was introduced into vacuum as a continuo us liquid flow (liquid beam) and was irradiated with a laser beam at a wavelength of 272 nm. Ions produced by multiphoton ionization in the liquid beam and ejected from it were analyzed by a time-of-flight mass spectrometer. The mass spectrum of ions ejected from the liquid beam exhibits peaks assignable to C6H5OH+(H2O)(n), H3O+(H2O)(n), and C6H5OC6H5OH. The ions C6H5OH+ and H3O+ are considered to be produced in the liquid beam by multiphoton ionization of phenol and proton transfer f rom phenol to a water molecule, respectively; these ions are ejected i nto vacuum with the solvent water molecules. On the other hand, C6H5OC6H5OH is considered to be produced mainly from a pair of phenol molec ules in the vicinity of the solution surface. This ion is regarded as the precursor for formation of phenoxyphenol, C6H5OC6H4OH, which is kn own as a product of the ion-molecule reaction of C6H5O+ + C6H5OH in th e liquid. An abrupt rise in the abundance of C6H5O+C6H5OH above 0.55 M indicates that the surface structure starts to change at this concent ration to a new one where two phenol molecules are paired.