AN ULTRAHIGH-VACUUM CHAMBER FOR SIMULTANEOUS FOURIER-TRANSFORM INFRARED-SPECTROSCOPY AND MICROGRAVIMETRIC MEASUREMENTS

A small UHV chamber that fits in the sample compartment of a commercia l Fourier transform infrared spectrometer (IBM-98, Brucker) has been d esigned and constructed to acquire reflection/absorption spectra of sp ecies adsorbed and/or condensed on metal films vapor deposited in situ on the surface of a quartz-crystal microbalance (QCM). The QCM is mou nted on a variable-temperature Au-plated Cu block, which houses a seco nd (reference) QCM, attached in turn to a liquid-He cold finger. The u nique capabilities of this apparatus make it possible to correlate qua ntitatively various spectral characteristics, including peak widths an d intensities, with the absolute mass of adsorbed and/or condensed mat erial per unit area m, or coverage, over a wide range of m and tempera tures. Results obtained using tetrahydrofuran (THF) condensed on fresh ly vapor deposited gold films have shown that the sensitivity of the i nstrument is sufficient to detect m(THF) on the order of a single mono layer. Plots of the intensity of the most prominent THF peak at 1064 c m(-1) as a function of m(THF) were found to deviate significantly from linearity in the range (3.1<m(THF)<2.8 mu g/cm(2). In addition, the r elative intensities of the most prominent THF peaks for even the large st m(THF) examined differed markedly from those of bulk-like THF, an e ffect that can be attributed to a preferential orientation of THF micr ocrystals with respect to the underlying substrate. In the case of cov erages on the order of one monolayer, such spectral behavior may arise from specific adsorbate/substrate interactions, which can cause a low ering in the molecular symmetry and/or distortions in the structure of the adsorbate. Evidence in support of a Au-induced THF activation was obtained from independent temperature programmed desorption (TPD) mea surements involving perdeuterated THF (TDF) performed in a separate UH V system. In particular, for exposures of about 1.5 L, the TPD spectra for the largest m/e features, i.e., 80, 48, and 30, revealed the pres ence of a peak centered at a higher temperature (similar to 180 K) tha n that found for much larger exposures (120 K).