SumFrequency Generation SFG

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Sum-Frequency Generation (SFG)

• Chemistry at the Interface
• Jakoah Brgoch
• 11.29.2007

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Sum-Frequency Generation (SFG)

• Molecular Interfaces
• Development of SFG
• Theoretical Background
• Application
• Carbon monoxide binding and oxidation

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Molecular Interfaces

• The boundary separating and connecting any two
  bulk phases
• One to several molecular layers thick
• All chemical and physical processes involving
  interactions and energy or mass transfer must
  cross the interface
• The interface has distinct and different
  properties than the bulk material
• Originate from the difference of chemical
  compositions and asymmetry of the forces that
  molecules and atoms experience at the interface

Wang, H. Gan, W. Lu, R. Rao, Y. Wu, B., Int.
Rev. in Phys. Chem. 2005, 24, 191-256.
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Problems with Studying the Interface

• Arises from interface regions existing in such a
  small area
• Makes it difficult to characterize by
  conventional experimental methods
• Very easily contaminated with small levels of
  impurities
• Surface and interfaces studies were completed on
  the macroscopic level until the 1950s

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Problems with Studying the Interface

• New techniques rely on high-vacuum, laser,
  sensitive and fast electronics
• Uses scattering, absorption, or emission of
  X-rays, neutrons, electrons, atoms and ions
• Techniques can be applied to flat surfaces in a
  high-vacuum environment
• Limitation of microscopic study of surfaces under
  high pressure and temperature
• Material and biological systems

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Development of SFG

• Tunable IR laser and visible laser at fixed
  frequency
• Used to obtain the vibrational spectrum at the
  sum frequency of the molecular groups at a
  molecular interface
• the vibrational spectrum originates mainly from
  the interface

Dellwig, T. Rupperchter, G. Unterhalt, H.
Freund, H.-J. 2000, 85, 776-779.
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Development of SFG
n
?Vis
g
2?
?IR
e

• Two laser beams (?vis ?IR) co-linear
• The IR frequency coincides with a vibrational
  frequency of an adsorbed molecule
• The molecule absorbs an IR photon and a visible
  photon excited to a high-energy virtual state ngt

Vidal, F. Tadjeddine, A. Rep. Prog. Phys. 2005,
68, 1095-1127.
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Development of SFG

• Second-order non-linear optical processes
• two photons of certain frequencies interact
  simultaneously
• produce a new photon with the sum of the two
  frequencies
• SFG vibrations must be both IR and Ramen active
• Ramen polarizability IR selection rules for the
  normal mode
• Coherent second-order optical processes are
  symmetrically forbidden in media having a center
  of inversion
• The centrosymmetry is naturally broken at the
  interface layer, but it is conserved for the
  isotropic bulk
• SFG becomes intrinsically interface specific

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Theoretical Background

• ?, ?1 and ?2 are the frequencies of the SF
  signal, visible and IR laser beams respectively
• nm (?i) is the refractive index of the bulk
  medium m (m1,2,) at frequency ?i (?0,1,2)
• ß is the incident or reflection angle from the
  interface normal to the beam
• I(?i) is the intensity of the SFG signal or the
  input laser beams respectively
• ê(?i) is the unit electric field vector for the
  light beam at frequency
• ?eff is the effective second-order nonlinear
  susceptibility of the interface
• Contributions from the interface and bulk phases

Vidal, F. Tadjeddine, A. Rep. Prog. Phys. 2005,
68, 1095-1127.
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Theoretical Background

• ?s(2)eff is a third rank tensor having 27
  elements
• 33 where three dimension of space (xyz) can vary
  in three dimensions of the non-linear crystal
  (ijk)
• Most vanish due inversion symmetry
• Tensor a physical property which connects two
  physical quantities.

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Theoretical Background

• A second-order process involving two input
  lasers E(?1) and E(?2)
• ?(2) is the second order nonlinear susceptibility
  tensor of the medium and ?sfg ?1 ?2
• At the interface between two media with inversion
  symmetry, centrosymmetry is broken
• ?s(2)?0, where ?s is the nonlinear susceptibility
  of the surface or interface

Vidal, F. Tadjeddine, A. Rep. Prog. Phys. 2005,
68, 1095-1127.
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Application

• Molecular adsorption isotherms can be obtained
• They reveal how the adsorbate structure changes
  as a function of coverage that is pressure
  dependent in equilibrium with the gas phase
• To monitor the surface during catalytic reaction
• determine which of the species turn over and are
  reaction intermediates and which are only
  spectators
• Weakly adsorbed species that are reaction
• Absent from the surface at low pressure

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Application

• The adsorption and oxidation of CO on a Pd(111)
  surface

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Experimental Setup

• Active mode-locked NdYAG laser
• 20 ps pulse _at_ 1064 nm, 20 Hz rate, 50 mJ
• Nonlinear optics
• Frequency doubled with a KDP nonlinear
  crystal-532nm
• IR generated with tunable optical parametric
  generator amplifier
• The lights are aligned and focused concentrically
  on metal catalyst
• UHV/Batch Reactor-vary the pressure
• NOT SHOWN
• Polarizer, filters and monochromator to remove
  523 nm contribution

Somorjai, G. A. Rupprechter, G. J. Phys. Chem.
B, 1999, 103, 1623-1638.
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CO Adsorption and Surface Restructuring

• SFG studies of Co on a Pt(111) surface
• Peaks
• 10-7 torr
• 1845 cm-1 CO bridge
• 2095 cm-1 CO atop
• 1 torr
• 2105 cm-1 CO atop
• 11.5-150 torr
• 2105 cm-1 CO atop
• Higher pressures
• 2045 cm-1 terminal Pt-CO bond

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Atop Bridge Bonding
Atop
Bridging
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CO Adsorption and Surface Restructuring

• SFG studies of Co on a Pt(111) surface
• Peaks
• 10-7 torr
• 1845 cm-1 CO bridge
• 2095 cm-1 CO atop
• 1 torr
• 2105 cm-1 CO atop
• 11.5-150 torr
• 2105 cm-1 CO atop
• Higher pressures
• 2045 cm-1 multiple CO-Pt bonds

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CO Adsorption and Surface Restructuring

• High Pressure
• Some of the Pt atoms are pulled
• out of the crystal lattice
• Form multiple bonded CO Pt
• complexes
• Ptm-(CO)n with n/m gt 1
• Low Pressure
• Repulsive interactions of CO ligands cause
  complex to not be stable

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CO Adsorption and Surface Restructuring

• The increased pressure the interaction of the
  CO-Pt surface overcame the CO-CO repulsion
• A layer in lower symmetry locations became
  occupied
• Shown by the broad background at higher pressures
• Elevated CO pressure consistent with displacive
  reconstruction

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CO oxidation on Pt(111) SurfaceExcess O2

• At low temp. lt600K
• 2095 cm-1 CO atop
• Low turn-over rate (TOR)
• Above ignition temp. gt600K
• CO atop peak lost
• High TOR
• 3 new peaks
• 2045 cm-1 multiple bonded CO-Pt clusters
• 2240 cm-1 CO2 species or Pt
• 2130 cm-1 CO stretch

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CO oxidation on Pt(111) SurfaceExcess O2

• At 600K the reaction becomes self-sustained
• Ignition temperature
• Increase temperature
• The atop CO intensity decreased due to thermal
  desorption
• Turn-over rate increases

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CO oxidation on Pt(111) SurfaceExcess CO

• 2095 cm-1 CO atop
• 2045 cm-1 multiple CO-Pt bonds
• CO atop peak lost
• T gt1000K
• Higher TOR at increased temp.
• Loss of peak at 2240 cm-1
• CO2 species or Pt

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CO oxidation on Pt(111) SurfaceExcess CO

• Top reaction rate vs atop CO coverage _at_ 590
• Bottom reaction rate vs new CO species _at_ 720
• Increased coverage of the atop decreased the rate
• The rate oxidation is proportional to the
  concentration of the of new species (2045 cm-1)

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CO oxidation on Pt(111) Surface

• Decrease of atop CO coverage (2095 cm-1)
• Not an active intermediate in oxidation
• Reaction rate was proportional to the
  concentration at 2045 cm-1
• Active species for oxidation to CO2

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Sum-Frequency Generation

• Advantages
• Allows detection of transition states
• Can monitor surfaces under ambient conditions
• Disadvantages
• Needs very uniform surface free from defects
• Would be very difficult for liquid/liquid or
  liquid/gas interface

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Questions?
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EXAM QUESTION

• Why is it possible to monitor reactions on the
  surface with SFG?
• Because the inversion symmetry is broken when the
  sample is adsorbed onto the surface, allowing the
  coupling of a photon.

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SHG SFG (summary slide)

• Second-order non-linear optical processes are
  forbidden under the electric-dipole approximation
• If inversion symmetry is present
• At the interface of centrosymmetric surfaces the
  symmetry is broken
• SHG and SFT are allowed on these surfaces
• The technique is interface specific and can be
  used to study interfaces not accessible by light
• Use of ultra-short laser pulses allow for
  time-resolved studies of interfacial dynamics