Surface Vibrational Spectroscopy

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Surface Vibrational Spectroscopy

• Vibrational spectroscopy provides the most
  definitive means of identifying the surface
  species generated upon molecular adsorption and
  the species generated by surface reactions.
• In principle, any technique that can be used to
  obtain vibrational data from solid state or gas
  phase samples (IR, Raman etc.) can be applied to
  the study of surfaces - in addition there are a
  number of techniques which have been specifically
  developed to study the vibrations of molecules at
  interfaces (EELS, SFG etc.).

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• There are, however, only few techniques that are
  routinely used for vibrational studies of
  molecules on surfaces - these are
• IR Spectroscopy (of various forms)
• Raman spectroscopy
• Electron Energy Loss Spectroscopy ( EELS )

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Metal Surface optics and external reflection IR
spectroscopy Two polarization components- s
(parallel to the plane of incidence) and p
(perpendicular to the plane of incidence). The
reflection of an s-polarized wave from a metal
surface has a phase shift of ?, while the
reflection of a p wave has a phase shift which
depends on the angle of incidence.

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The outcome- the electromagnetic field at the
surface is completely normal to it and almost 4
times as intense as the incoming field.
Thus normal modes having a dipole moment parallel
to the surface will not be IR-active, while those
with the dipole moment perpendicular to the
surface will be IR-active. This is the Surface
Selection Rule.
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A complementary surface selection rule involves
the image dipole. A dipole above a metal creates
an image dipole within the metal in order to zero
the potential on the surface. If the dipole is
parallel to the surface the dipole image cancels
it

-
molecule
-

metal
If the dipole is perpendicular to the surface
there is no cancellation
-
molecule

-
metal

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In the case of non-metal surfaces

• The is a restriction on which modes can be
  excited, since modes polarized along the
  direction of the light propagation cannot be
  excited.
• The electromagnetic field oscillate only in
  directions perpendicular to the lights
  propagation.

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• There are a number of ways in which the IR
  technique may be implemented for the study of
  adsorbates on surfaces.
• For solid samples possessing a high surface area
  
• Transmission IR Spectroscopy employing the same
  basic experimental geometry as that used 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 )
  in which the diffusely scattered IR radiation
  from a sample is collected, refocused and
  analysed. This modification of the IR technique
  can be employed with high surface area catalytic
  samples that are not sufficiently transparent to
  be studied in transmission.

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• For studies on low surface area samples (e.g.
  single crystals)
• Reflection-Absorption IR Spectroscopy ( RAIRS )
  where the IR beam is specularly reflected from
  the front face of a highly-reflective sample,
  such as a metal single crystal surface.
• Multiple Internal Reflection Spectroscopy ( MIR )
  in which 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
  - hence the alternative name of Attenuated Total
  Reflection (ATR).

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Attenuated total reflection infrared (ATR-IR)
spectroscopy
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• To obtain internal reflectance, the angle of
  incidence must exceed the so-called critical
  angle. This angle is a function of the real parts
  of the refractive indices of both the sample and
  the ATR crystal
• Where n2 is the refractive index of the sample
  and n1 is the refractive index of the crystal.
• The evanescent wave decays into the sample
  exponentially with distance from the surface of
  the crystal over a distance on the order of
  microns. The depth of penetration of the
  evanescent wave d is defined as the distance form
  the crystal-sample interface where the intensity
  of the evanescent decays to 1/e(37) of its
  original value. It can be given by
• Where ? is the wavelength of the IR radiation.

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• For instance, if the ZnSe crystal (n12.4) is
  used, the penetration depth for a sample with the
  refractive index of 1.5 at 1000cm-1 is estimated
  to be 2.0µm when the angle of incidence is 45.
  If the ZnSe crystal (n12.4) is used under the
  same condition, the penetration depth is about
  0.664µm.
• The depth of penetration and the total number of
  reflections along the crystal can be controlled
  either by varying the angle of incidence or by
  selection of crystals. Different crystals have
  different refractive index of the crystal
  material.
• By the way, it is worthy noting that different
  crystals are applied to different transmission
  range (ca. ZnSe for 20,000650cm-1, Ge for
  5,500800cm-1).

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• RAIRS - the Study of Adsorbates on Metallic
  Surfaces by Reflection IR Spectroscopy
• It can be shown theoretically that the best
  sensitivity for IR measurements on metallic
  surfaces is obtained using a grazing-incidence
  reflection of the IR light.
• Furthermore, since it is an optical (photon
  in/photon out) technique it is not necessary for
  such studies to be carried out in vacuum. The
  technique is not inherently surface-specific, but
  there is no bulk signal to worry about, the
  surface signal is readily distinguishable from
  gas-phase absorptions using polarization effects.

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• 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 ( - 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.

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Electron Energy Loss Spectroscopy (EELS)

• This is a technique utilising the inelastic
  scattering of low energy electrons in order to
  measure vibrational spectra of surface species
  superficially, it can be considered as the
  electron-analogue of Raman spectroscopy.
• To avoid confusion with other electron energy
  loss techniques it is sometimes referred to as
• HREELS - high resolution EELS   or   VELS -
  vibrational ELS

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EELS Experimental set-up
Since the technique employs low energy electrons,
it is necessarily restricted to use in high
vacuum (HV) and UHV environments - however, the
use of such low energy electrons ensures that it
is a surface specific technique and, arguably, it
is the vibrational technique of choice for the
study of most adsorbates on single crystal
substrates. The basic experimental geometry is
fairly simple as illustrated schematically - it
involves using an electron monochromator to give
a well-defined beam of electrons of a fixed
incident energy, and then analysing the scattered
electrons using an appropriate electron energy
analyser.
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• A substantial number of electrons are elastically
  scattered ( E Eo ) - this gives rise to a
  strong elastic peak in the spectrum.
• On the low kinetic energy side of this main peak
  ( E superimposed on a mildly sloping background.
  These peaks correspond to electrons which have
  undergone discrete energy losses during the
  scattering from the surface.
• The magnitude of the energy loss, DE (Eo - E),
  is equal to the vibrational quantum (i.e. the
  energy) of the vibrational mode of the adsorbate
  excited in the inelastic scattering process. In
  practice, the incident energy ( Eo ) is usually
  in the range 5-10 eV (although occasionally up to
  200 eV) and the data is normally plotted against
  the energy loss (frequently measured in meV).

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Selection Rules

• The selection rules that determine whether a
  vibrational band may be observed depend upon the
  nature of the substrate and also the experimental
  geometry specifically the angles of the incident
  and (analysed) scattered beams with respect to
  the surface.
• For metallic substrates and a specular geometry,
  scattering is principally by a long-range dipole
  mechanism.  In this case the loss features are
  relatively intense, but only those vibrations
  giving rise to a dipole change normal to the
  surface can be observed.

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• By contrast, in an off-specular geometry,
  electrons lose energy to surface species by a
  short-range impact scattering mechanism.  In this
  case the loss features are relatively weak but
  all vibrations are allowed and may be observed.
• If spectra can be recorded in both specular and
  off-specular modes the selection rules for
  metallic substrates can be put to good use -
  helping the investigator to obtain more
  definitive identification of the nature and
  geometry of the adsorbate species.
• The resolution of the technique (despite the
  HREELS acronym !) is generally rather poor
  40-80 cm-1 is not untypical. A measure of the
  instrumental resolution is given by looking at
  the FWHM (full-width at half maximum) of the
  elastic peak.

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• This poor resolution can cause problems in
  distinguishing between closely similar surface
  species - however, recent improvements in
  instrumentation have opened up the possibility of
  much better spectral resolution ( will undoubtedly enhance the utility of the
  technique.
• In summary, there are both advantages and
  disadvantages in utilising EELS, as opposed to IR
  techniques, for the study of surface species It
  offers the advantages of- high sensitivity,
  variable selection rules, spectral acquisition to
  below 400 cm-1 , both vibrational and electronic
  spectra can be recorded.
• but suffers from the limitations of -
• use of low energy electrons (requiring a HV
  environment and hence the need for low
  temperatures to study weakly-bound species, and
  also the use of magnetic shielding to reduce the
  magnetic field in the region of the sample)
• requirement for flat, preferably conducting,
  substrates
• lower resolution

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Surface Plasmons

• Plasmons are the Quanta associated with
  longitudinal waves propagating in matter through
  the collective motion of large numbers of
  electrons. Surface plasmons are a subset of these
  'eigen-modes' of the electrons, which are bound
  to the interface region in the material between a
  dielectric and conducting medium. Because of the
  long range of the medium the quantum nature of
  the excitation is relaxed and it exists over the
  entire frequency range from zero to an asymptotic
  value determined by the classical surface plasmon
  energy.
• Because the momentum of the plasmons is in plane,
  they cannot coupled simply to the electromagnetic
  field. This problem can be overcome by several
  methods as will be discussed.
• We currently use this resonant effect as a means
  of probing the optical properties of materials
  and interfaces.

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• Experimentally two methods have been developed to
  couple light with plasmons, Attenuated total
  reflection and grating coupling.

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• Grating coupling
• The incident electromagnetic radiation is
  directed towards a medium whose surface has a
  spacial periodicity (D) similar to the wavelength
  of the radiation, for example a reflection
  diffraction grating.
• The incident beam is diffracted producing
  propagating modes which travel away from the
  interface and evenescent modes which exist only
  at the interface. The evenscent modes have
  wavevectors parallel to the interface similar to
  the incident radiation but with integer 'quanta'
  of the grating wavevector added or subtracted
  from it. These modes couple to Surface Plasmons,
  which run along the interface between the grating
  and the ambient medium.  

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Surface plasmon resonance

• The surface plasmon resonance (SPR) technique is
  an optical method for measuring the refractive
  index of very thin layers of material adsorbed on
  a metal. In case of e.g. protein-adsorption the
  difference between the refractive index of the
  buffer (i.e. water) and the refrative index of
  the adsorbate can be easily converted into mass
  and thickness of the adsorbate as all proteins
  have almost identical refractive indices.The
  SPR-technique exploits the fact, that, at certain
  conditions, surface plasmons on metallic slabs
  can be excited by photons, thereby transforming a
  photon into a surface plasmon. The conditions
  depend on the refractive index of the adsorbate.

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• The most common geometrical setup in the
  Kretschmann configuration. The incoming light is
  located on the opposite side of the metalic slab
  than the adsorbate. This is due to the fact, that
  photons cannot excite surface plasmons on the
  surface being hit (can easily be seen by
  comparing the curves of dissipation for incoming
  light, and for surface plasmons on the incident
  surface). The photons will however induce an
  evanescent light field into the metallic slab.
  Normally no transport of photons takes place
  through this field, but photons incident at a
  certain angle are able to tunnel through the
  field and to excite surface plasmons on the
  adsorbate side of the metallic slab. Whenever a
  plasmon is excited, one photon disappears,
  producing a dip in reflected light at that
  specific angle. The angle, which is dependent on
  refractive index of the adsorbate, is measured
  with a CCD-chip.

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• The advantages of the Kretschmann configuration
  are, that it is not necessary to shine light
  through the adsorbate and that it is easy to
  build (relative to other possible
  SPR-configurations).The mass-resolution of SPR
  is in the order of nano-grams of molecules, and
  the surface plasmons typically extend in the
  order of 200nm into the adsorbate, thus 'sensing'
  a weighted mean refractive index of this
  volume.SPR is mainly used to measure the
  change in refractive index, during adsorption of
  cells and proteins. From these data, properties
  such as mass and thickness of adsorbed layers can
  be deduced. Also conformational changes in the
  internal structure of the layers can be seen,
  which is invaluable information in the
  exploration of binding-behavior of proteins and
  cells.

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Surface Raman spectroscopy

• Reflection Raman
• Similar to reflection IR.
• Surface enhanced Raman
• Surface plasmons coherent electron-hole pair
  oscillating waves on the surface of a metal. SP
  excitations cannot be created by light or emit
  from a flat surface, because of the mismatch of
  the dispersion relations. However, if the
  surface is rough (grating, roughened electrode,
  colloid) coupling to light can occur.

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For example, for a sphere the resonance frequency
?R is obtained by solving
?(?R) is the dielectric constant at ?R, and ?0 is
the dielectric constant of vacuum.
SP waves are created both by incident light and
by a radiating surface species. Therefore the
SERS enhancement is the combination of both
where EI and EL are the incident and local field.
G can be as large as 1012 under very specific
conditions, but regularly reaches 106 . The
surface selection rule can be used here too. For
example, for the special case of spherical
colloids it is found that