Resonance-enhanced two-photon ionization time-of-flight spectroscopy of cold perfluorinated polyethers and their external and internal van der Waals dimers

Abstract

Perfluorinated polyethers of the type R a (OCF 2CF 2CF 2) n F, where R a is the end group C 6H 6HCH 2CH 2O(CO)CF 2CF − 2-, 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 polymer distributions extending to 7000 Da, with minimal fragmentation. Under the appropriate expansion and desorption conditions parent masses of van der Waals dimers of these polymers were also observed. By scanning the ionization laser and monitoring particular mass-to-charge ratios, resonance-enhanced two photon ionization (R2PI) spectra were obtained for the jet-cooled polymers and their dimers near the electronic origin. Polymers with the two end groups, present as an impurity in the samples, were detected exclusively in an internally dimerized form. In both the internal and external cases, the dimerization occurs only at the phenoxy chromophore. The R2PI spectra of a series of model compounds were measured and used to characterize the evolution of the spectra from phenol toward the polymer. The model compound spectra revealed the role of multiple conformations and molecular size in the polymer spectra, which are ultimately broadened by low frequency motions of multiple conformers. The results are discussed relative to teh general problem of the photoionization of large molecules.