Time of flight (TOF) mass spectrometry is used in conjunction with a variable repelling
voltage technique to elucidate the mechanism by which phenol ionizes and dissociates
under 266 nm pulsed laser irradiation in combination with a 532 nm or 355 nm pulsed
laser. The results suggest that, like benzene, the molecular ion is the predominant
precursor of all ionic species generated in the process. Predominance of C5Hx+ species
at relatively low powers confirms the presence of a low energy dissociation channel
involving the elimination of CO. The use of a second laser at 532 nm is found to
selectively destroy the C5Hx+ (as compared to the parent ion) species. The parent ion
is found to be protected from the radiation of the second laser pulse at 532 nm but
not at 355 nm if the second laser pulse is delayed by 50 ns. This is explained in terms
of relaxation within the parent ion energy levels, the location of a low energy dissociation
channel and the wavelengths of the lasers used. The main aspects of the fragmentation
pattern are discussed in terms of the statistical theory of Rebentrost and Ben-Shaul.
