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 commercial Fourier transform infrared spectrometer (IBM-98, Brucker) has been designed and constructed to acquire reflection/absorption spectra of species adsorbed and/or condensed on metal films vapor deposited in situ on the surface of a quartz-crystal microbalance (QCM). The QCM is mounted on a variable-temperature Au-plated Cu block, which houses a second (reference) QCM, attached in turn to a liquid-He cold finger. The unique capabilities of this apparatus make it possible to correlate quantitatively various spectral characteristics, including peak widths and intensities, with the absolute mass of adsorbed and/or condensed material per unit area m, or coverage, over a wide range of m and temperatures. Results obtained using tetrahydrofuran (THF) condensed on freshly vapor deposited gold films have shown that the sensitivity of the instrument is sufficient to detect m_THF on the order of a single monolayer. Plots of the intensity of the most prominent THF peak at 1064 cm^-1 as a function of m_THF were found to deviate significantly from linearity in the range 0.1<m_THF<2.8 μg/cm². In addition, the relative intensities of the most prominent THF peaks for even the largest m_THF examined differed markedly from those of bulk-like THF, an effect that can be attributed to a preferential orientation of THF microcrystals with respect to the underlying substrate. In the case of coverages on the order of one monolayer, such spectral behavior may arise from specific adsorbate/substrate interactions, which can cause a lowering 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) measurements involving perdeuterated THF (TDF) performed in a separate UHV 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 presence of a peak centered at a higher temperature (∼180 K) than that found for much larger exposures (120 K).