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Operational
modes odes
of operation
For samples
with large surface area the following methods are applicable:
- Transmission IR Spectroscopy
. It employs the same basic experimental geometry for
liquid samples and mulls. This is often used for studies on
supported metal catalysts where the large metallic surface area
permits a high concentration of adsorbed species to be sampled.
The solid sample must, of course, be IR transparent over an
appreciable wavelength range.
- Diffuse Reflectance IR Spectroscopy
( DRIFTS ) . The diffusely scattered IR radiation
from a sample is collected, refocused and analysed. This modification
of the technique can be employed with high surface area catalytic
samples that are not sufficiently transparent to be studied
in transmission.
For samples with small surface area one should consider:
- Attenuated Total Reflection
(ATR) . The IR beam is passed through a thin,
IR transmitting sample in a manner such that it alternately
undergoes total internal reflection from the front and rear
faces of the sample. At each reflection, some of the IR radiation
may be absorbed by species adsorbed on the solid surface.
- Reflection-Absorption IR Spectroscopy
( RAIRS ). The IR beam is specularly reflected
from the front face of a highly-reflective sample, such as a
metal single crystal surface. Best surface sensitivity is achieved
using a grazing-incidence reflection of the IR light. It is
most popular for studying adsorbates on metallic surfaces and
is often called Surface FTIR.
The technique is not inherently surface-specific, but one
should not that
(a) there is no bulk signal to worry about
(b) the surface signal is readily distinguishable from gas-phase
absorptions using polarization effects.
One major problem, is that of sensitivity (i.e. the signal
is usually very weak owing to the small number of adsorbing
molecules). Typically, the sampled area is ca. 1 cm2
with less than 1015 adsorbed molecules (i.e. about
1 nanomole). With modern FTIR spectrometers, however, such small
signals (0.01% - 2% absorption) can still be recorded at relatively
high resolution (ca. 1 cm-1 ). For a number of practical
reasons, low frequency modes ( < 600 cm-1 ) are
not generally observable - this means that it is not usually
possible to see the vibration of the metal-adsorbate bond and
attention is instead concentrated on the intrinsic vibrations
of the adsorbate species in the range 600 - 3600 cm-1.
Selection Rules
The observation of vibrational modes of adsorbates on metallic
substrates is subject to the surface dipole selection rule.
This states that only those vibrational modes which give rise
to an oscillating dipole perpendicular (normal) to the surface
are IR active and give rise to an observable absorption band.
Further information on the selection rules for surface IR spectroscopy
can be found in the review by Sheppard & Erkelens [Appl. Spec.
38, 471 (1984)]. It also needs to be remembered that even if a
transition is allowed it may still be very weak if the transition
moment is small.
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