The physisorption of CO2 and the stepwise chemisorption of SO2 on the CsF(100) single crystal surface

The adsorption of CO2 and SO2 on the (100) surface of the extremely basic a nd hygroscopic insulator CsF, prepared in situ by cleaving under UHV, was s tudied for the first time using high-resolution polarization infrared surfa ce spectroscopy (PIRS) and low energy electron diffraction (LEED). The polarized infrared spectra of the asymmetric stretching vibration v(3) of the monolayer CO2-CsF(100), prepared at 80 K, show a correlation field d oublet, those of the bending vibration v(2) a crystal field doublet. The LE ED pattern reveals a (root 2 x root 2)R45 degrees superstructure with two g lide planes (symmetry pgg). Comparison of measured with computed spectra al lows the determination of the adsorbate orientational ordering. The tilt an gle of the molecules is 5 degrees +/- 5 degrees the intermolecular azimuth angle is about 170 degrees. The absorption behavior of SO2 is completely different, showing three phase s. At 101 K and 1.3 x 10(-9) mbar up to a dosage of similar to 7 L, phase I grows. Starting at 5-6 L, phase II replaces phase I, characterized by spec tral lines identified as the symmetric and asymmetric OSO-stretching mode o f the fluorosulfinate anion (SO2F-). Exposure of more than 20 L results in the additional formation of phase III, which was found to be physisorbed SO 2. These results could be relevant to applications in the widespread catalysis of organic synthesis by CsF and in the effective adsorption of the polluta nt SO2 in smoke gases. (C) 1999 Elsevier Science B.V. All rights reserved.