Molecular Adsorption and Fragmentation of Bromoform on Polyvinylidene Fluoride with Trifluoroethylene
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Molecular Adsorption and Fragmentation of Bromoform on Polyvinylidene Fluoride with Trifluoroethylene Carolina C. Ilie1, Jie Xiao2, and Peter A. Dowben2 1 Physics, SUNY Oswego, 7060 Route 104, Oswego, New York 13126-3599, U.S.A. 2 Physics, University of Nebraska at Lincoln, 116 Brace Lab., Lincoln, NE, U.S.A. ABSTRACT Bromoform absorption on crystalline polyvinylidene fluoride with 30% of trifluoroethylene, P(VDF-TrFE 70:30) was investigated by photoemission and inverse photoemission and found to be associative and reversible. As on other surfaces, bromoform decomposition does occur, under electron irradiation, and one possible mechanism is dissociative electron attachment. INTRODUCTION The adsorption of halocarbons on metal surface is an active area in surface science for more than two decades [1]. Despite the fact that the electronic structure of the haloforms is very similar [2], most of the emphasis has been on chloroform adsorption [3-8], although recently, bromoform [9,10] and chloroform [11,12] adsorption and photodecomposition on molecular films like ice have been investigated. While chloroform (CHCl3) adsorption on calixarene molecular films has been studied [13], bromoform (CHBr3) adsorption on the copolymer P(VDFTrFE 70:30) provides a more compelling comparison [14] as this is a ferroelectric polymer as well as an excellent dielectric [15-17]. EXPERIMENTAL Ultrathin ferroelectric thin films of copolymer 70% vinylidene fluoride with 30% of trifluoroethylene, P(VDF-TrFE 70:30) were fabricated by Langmuir-Blodgett (LB) deposition technique on freshly-cleaved highly ordered pyrolytic graphite (HOPG) substrates from the water subphase [16-17]. The 5 molecular layers thick films (25 Å) were annealed in ultrahigh vacuum at 110º C before and after each experiment for 30 minutes [18-21]. The LangmuirBlodgett (LB) method produces highly crystalline films of P(VDF-TrFE) as is evident from the surface structure investigated by STM and experimental band structure mappings [18-21].The bromoform (CHBr3) was admitted to the ultrahigh vacuum system through a standard leak valve and the exposures are denoted in Langmuirs (1 L = 10-6 torr.sec). The combined ultraviolet photoemission (UPS) and inverse photoemission (IPES) spectra were used to characterize the molecular orbital placement of both occupied and unoccupied orbitals of the polymer substrate and the bromoform adsorbate. In both photoemission and inverse photoemission measurements, the binding energies are referenced with respect to the Fermi edge of gold or tantalum, in contact with the sample surface. The UPS data taken in this work was done with a helium lamp He I at hν = 21.2 eV and a large PHI hemispherical analyzer with an angular acceptance of ±100 or more [14,19-21]. The IPES were obtained by using variable incident energy electrons while measuring the emitted photons at a fixed energy (9.7 eV) using a Geiger-Müller detector with an instrumental linewidth of ~ 400 meV, as described elsewhere [14,19-21].
RESULTS AND DISCUSSION As seen in Figur
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