Infrared reflectance spectroscopy has developed into one of the primary methods of monitoring chemical structure and molecular orientation of thin films and monolayers adsorbed onto metal surfaces. At a high (grazing) angle of incidence the intensity of a reflected p-polarized infrared light beam is enhanced at a metal surface so that even submonolayer quantities of chemisorbed species can be observed in the p-polarized FTIR reflectance spectrum. In contrast, at a high angle of incidence an s-polarized reflected infrared beam has virtually no intensity at the metal surface. This polarization disparity leads to strong selection rules at the surface, and has been used to deduce the average molecular orientation and conformation for monolayers of long chain alkyl molecules adsorbed onto metals.
In addition to orientation measurements, the predominance of p-polarized light over s-polarized light at the metal surface has been utilized to obtain the differential reflectance spectrum of the adsorbed surface species, dR/R, by polarization modulation of the infrared light. Differential reflectance measurements have been obtained with both scanning spectrometers and FTIR instruments; the FTIR instruments have the advantage of very good resolution, rapid acquisition time and a wide spectral range. The differential reflectance measurement possesses an inherent surface sensitivity that makes it an ideal technique for in situ studies at condensed phase interfaces such as electrochemical surfaces.
Although PM-FTIR spectroscopy has been successfully employed in a
number of environments, the measurements from metal surfaces to date have
been restricted by some experimental limitations to this modulation
technique. Current implementations of the PM-FTIR scheme utilize a lock-in
amplifier to extract the differential interferogram from the detector
signal. In order to insure that the variations in the differential signal
do not exceed the time constant of the lock-in amplifier output
electronics, the mirror velocity is typically slowed down, reducing the
stability of the interferometer and diminishing the signal averaging
capabilities of the instrument. In addition, the spectral window of the
PM-FTIR reflectance spectrum is modulated by the wavelength dependence of
the photoelastic polarization modulator. A background spectrum is
typically taken to subtract off this variation as well as any other
instrumental artifacts. With Synchronous Sampling Demodulator (SSD)
electronics from
GWC Technologies,
the PM-FTIR differential reflectance spectrum can be obtained
at normal mirror velocities, and the need for obtaining a background
spectrum can be avoided.
Click Here for more information on how to set up a PM-FTIR reflection-absorption experiment.
Click Here to see the PM-FTIRRAS spectrum of a self-assembled alkanethiol monolayer on a gold surface.
Click Here for a list of recent of papers that employ the PM-FTIR reflection-absorption experiment and the SSD electronics.
For our polarization-modulation Fourier transform infrared
reflection-absorption (PM-FTIRRAS) experiments, we use macros to collect
and normalize the data. We use a RS-1 Mattson FTIR spectrometer hooked up
to an IBM PC (DOS) to collect the data, and then transfer the spectra over to a
Macintosh computer for the normalization and conversion to absorbance
units.
The collection and normalization macros are available on our
Calculations Page.