RESONANCE-ENHANCED 2-PHOTON IONIZATION TIME-OF-FLIGHT SPECTROSCOPY FOCOLD PERFLUORINATED POLYETHERS AND THEIR EXTERNAL AND INTERNAL VAN-DER-WAALS DIMERS

Perfluorinated polyethers of the type R(a)-(OCF2CF2CF2)n-F, where R(a) is the end group C6H6-O-CH2CH2-O-(C=O)CF2CF2--, were laser-vaporized and entrained in a pulsed jet expansion. Two photon ionization of the jet-cooled polymers via the phenoxy chromophore was combined with time of flight (TOF) mass spectrometry. Mass spectra were obtained for pol ymer distributions extending to 7000 Da, with minimal fragmentation. U nder the appropriate expansion and desorption conditions parent masses of van der Waals dimers of these polymers were also observed. By scan ning the ionization laser and monitoring particular mass-to-charge rat ios, resonance-enhanced two photon ionization (R2PI) spectra were obta ined for the jet-cooled polymers and their dimers near the electronic origin. Polymers with two end groups, present as an impurity in the sa mples, were detected exclusively in an internally dimerized form. In b oth the internal and external cases, the dimerization occurs only at t he phenoxy chromophore. The R2PI spectra of a series of model compound s were measured and used to characterize the evolution of the spectra from phenol toward the polymer. The model compound spectra revealed th e role of multiple conformations and molecular size in the polymer spe ctra, which are ultimately broadened by low frequency motions of multi ple conformers. The results are discussed relative to the general prob lem of the photoionization of large molecules.