Gr. Zhuang et al., AN ULTRAHIGH-VACUUM CHAMBER FOR SIMULTANEOUS FOURIER-TRANSFORM INFRARED-SPECTROSCOPY AND MICROGRAVIMETRIC MEASUREMENTS, Review of scientific instruments, 65(8), 1994, pp. 2494-2499
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).