Vibrational spectroscopy

1
Vibrational spectroscopy

• Chemical composition finger print
• Bonding orientation adsorption structure on
  surfaces

Infrared Spectroscopy (IR) High Resolution
Electron Energy Loss Spectroscopy
(HREELS) Surface Enhanced Raman Spectroscopy
(SERS) Second Harmonic Generation
(SHG) Photo-acoustic Spectroscopy (PAS) Inelastic
electron tunneling Spectroscopy (IETS) Inelastic
Neutron Scattering (INS)
2
Surface Infrared spectroscopy

• Refs Y.J. Chabal, Surf. Sci. Rep. 8, 211 (1988)
• F.M. Hoffman, Surf. Sci. Rep. 3, 107
  (1983)
• Transmission IR Spectroscopy
• supported metal cataysts
• IR transparent samples (Si)
• Diffuse Reflectance Infrared Fourier Transform
• Spectroscopy (DRIFTS)
• refocus the diffusively scattered IR beam
• high surface area catalytic samples
• low surface area single crystals
• Reflection-Absorption IR Spectroscopy ( RAIRS )
• specular reflected IR beam
• single crystal samples
• Multiple Internal Reflection Spectroscopy ( MIR )
  or
• Attenuated Total Reflection (ATR)
• total internal reflection
• SAM , polymer films

3
Background
-
I
I0

-

• Transmission and absorption mode
• Transmittance T I/I0 exp(kcl)
• Absorbance A ecl
• k absorption coefficient e absorptivity
• c concentration l cell thickness
• Imaginary part of refractive index n k
• n n ik for absorbing medium
• n n for dielectric non-absorbing medium
• needs to take reference and sample spectra
• not popular for surface studies due to the large
  bulk contribution

4
Reflection
The reflection angles Snells law n1/n2
sinqi/sinqt Crtical angle qc sin-1(n2/n1) Inten
stiy of the reflected light - Depend on
polarizations Fresnels equations n n
ik s-polarized light the plane of
incidence Rs (n-secq)2k2/
(nsecq)2k2 p-polarized light ? the plane
of incidence Rp (n-cosq)2k2/
(ncosq)2k2 - qi must be large grazing
incidence for thin films on reflective surface
the plane of incidence
x
Ep
Es
x
qi
qr
qt
5
Phase shift , electric field, intensity of
p-polarized light as a function of incidence
angle from a metal surface
s-pol q?
n 3, k30
Surface intensity function
180 0
Surface electric field E/E0
Phase shift on refelctions
(E/Eo)2secq
60
40
p-pol q
20
0 incidence angle 90
0 incidence angle 90
s-polarized light at the surface - uniform
phase shift - vanishing E field at the
surface p-polarized light at the surface -
dependent on incidence angle - strong E field
at large incidence angle, ie, grazing
incidence
Absorbance is proportional to E2 and area of
surface as 1/cos q I E2/cos q
6
Adsorbate covered surface
Dielectric constant e (nik)2
Ro
R
Vcauum e1n1
d
Adsorbate e2(n2, k2)
Ro
R
Metal e3(n3, k3)
Absorption function A (R- R0) /R0 DR/R e3
e21, de3)/(1-e2)) DRp/Rp (8pdcosq/ l)Im((e2
e3)(1-(1/ e2 e3)(e2 e3)sin2q/
(1-e2)(1/e3) )(1 e3)sin2q Reflectivity change
of s-polarized light is negligibly small Assuming
e3 e2 and cosq e3-1 DRp/Rp
(8pdsin2q/lcosq)Im(-1/e2)
a large reflectivity change at high incidence
angle
7
Surface selection rule
mfi ? 0, dm/dr ? 0

• The electric field of light and the molecule
  interact with surface electrons
• The incident light must be p-polarized
• Only vibrations with a dipole moment
  perpendicular to the surface
• The incident light should be reflected at grazing
  incidence

mM

mM

-
-
-

-
mimage
mimage

IR inactive IR active

• for lying down molecules, molecular and image
  dipoles are cancelled out
• for upright molecules, molecular and image
  dipoles are enhanced

8
Surface IR spectra of adsorbed molecules
Identification of adsorbate high resolution
2-4 cm-1 Orientation of adsorbed molecule by
surface dipole selection rule How to confirm the
metal-adsorbate bond ? - frequency shift of
internal modes compared to gas-phase spectra
- additional metal-molecule vibration cm-1 Frequency shift of internal and external
modes for adsorbed layers - weakening of
metal-molecule bond n decreases as coordination
of surface atoms increases - formation
of adsorbate islands - compression
structures DR/R 0.110-3 often small
sufficient for submonolayer sensitivity for
molecule with strong dynamic dipole moment DR/R
roughly linear with coverage, but not a good
indicator of population
9
Peak width and intensity
homogeneous broadening - coupling to phonon
- electron-hole creation inhomogeneous
broadening - inhomogeneous distribution of
harmonic oscillator - intermolecular
interaction energy transport between molecule and
surface dipole-surface interaction dynamic
dipole interaction
10
Instrumentation RAIRS
J.E. Reutt-Robey et al, JCP 93, 9113 (1990)
11
Instrumentation MIR IR
12
IR finger print
13
Modes of vibration
14
IR spectra of CO on Pd(100)

• threefoldsite 18001900 cm-1
• bridge site 19002000 cm-1
• on top site 20002100 cm-1

Lower frequency shift compared to that of gas
phase ? - Interaction with the vibrating
dipole with the image dipole \ - Chemical effect
due to backdonation, which change the CO bond
strength Higher frequency shift as coverage? -
vibrational coupling dipole-dipole,
dipole-metal electrons - chemical effect
reduced backdonation into antibonding orbitals
- electrostatic effect due to charge transfer
between the metal and moelcule -
intermolecular repulsion
15
IRRAS spectra
CO on Pd(111)
16
Diffuse reflectance IR spectra
17
High Resolution Electron Energy Loss Spectroscopy

• Inelastic scattering of low energy electron beam
• Energy loss due to the vibrational excitation
• observe vib. modes parallel and perpendicular to
  the surface
• Lower resolution 3meV (24 cm-1 )(compare with
  IR 2 cm-1)
• Submonolayer sensitivity
• can observe surface-atom vib. freq.

Eo-E hv
I
E
Eo
v

-
Eo
E
18
Scattering mechanism
Dipole scattering Impact scattering Resonance
scattering

• Dipole scattering
• electrons interact with the long range field at
  surface
• electron momentum perpendicular to the surface
  normal is condserved
• forward scattering by adsorbate
• peaked in the specular position
• elastic electrons specular
• inelastic electrons near specular
• vibration perpendicular to the surface normal
  can be excited
• larger cross section for smaller Eo(5 eV)

E
Eo
g
mM

mM
g

-
-
ki

ki
kf
-

-
?
mimage
mimage
?

19
Impact scattering

• short range interaction( a few A) of electron
  with atomic core potential
• of surface
• strong multiple scattering
• Isotropic angular distributions
• scattering probability depends on surface dipole
  amplitude and electron
• energy
• - favored by high incident electron energy 50
  eV
• - off specular angle
• lower scattering cross section the the dipole
  scattering

Negative ion resonance scattering

• short range interaction
• electron trapped in empty Rydberg state of
  adsorbate
• temporary negative ion
• enhancement of vib. Intensity over relatively
  narrow range of Ei
• very small cross- section off resonance
• molecular orientation on surface

20
Peak positions for different adsorption states
21
Instrumentation
22
Examples CO on W(100)
565 cm-1 W-C stretching 630 cm-1 W-O
stretching 363 cm-1 W-CO (on top) 2081 cm-1 CO
stretching CO(g) 2140 cm-1
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(No Transcript)
24
Interaction ions with solid

• Charge transfer neutralization of ion and
  electronic excitation
• Kinetic energy transfer sputtering, scattering

e
Auger neutralization
Evac
Resonance ionization
EF
Resonance neutralization
Ei
Quasi-resonance neutralization
25
Atomic and nuclear collision
Impact parameter (b) scattering process
energy transfer (Tc) 1 A
inelastic excitation 10eV
of valence electrons 10-1 A
inelastic excitation 100eV of
L-shell electrons 10-2 A
inelastic excitation 1 keV of
K-shell electrons 10-4 A
elastic scattering 100keV from
nuclei
26
Ion scattering spectroscopy
Low energy ion scattering (LEIS) 0.5 3
keV Medium energy ion scattering (MEIS) 10500
keV High energy ion scattering (HEIS)
or Rutherford backscattering spectroscopy (RBS)
0.5 5 MeV
Binary elastic collision Kinematic factor K
E1/Eo E1/Eo ((M22 M12)sinq)1/2M1cosq)
/(M2M1)2 M1,M2 mass of incident atom and
target q scattering angle
27
Ion trajectroy
28
Blocking, shadowing, and channeling effect

• scattering cross section
• at a certain angle depend
• on atomic potentials of incident
• and substrate atoms
• scattering depend on incident
• angle and impact parameter
• lower ion energy,
• larger shadow cone

29
Scattering cross section
2pbdb s(q) 2psinqdq s(q) b(db/d q)/sin q
of scattered paricles into dW/total of
incident particles
Rutherford formula ds /dW Z1 Z2e2/4Ecsinqc/22
Ec M2/(M1M2)Eo
dq
q
b
db
30
Quantitative analysis
Total of particles of impurity mas M3, atomic
number Z3, surface density N3(atoms/cm2) The
measured yield Y3 Y3 N3 (ds /dW) DW Q Q
measured of incident particles DW solid angle
accepted by detector - N3 can be determined
typically with an accuracy better than 10
31
Stopping power and depth resolution

• the rate of energy loss dE/dx depends on mass of
  projectiles, traget, and
• incident energy
• for 0.52.0 MeV, dE/dx is independent of energy
• Depth resolution 30100 Å

Electronic stopping during going in
Elastic scattering
Electronic stopping during going out
Final Energy of a particle at normal incidence E1
Eo DEin Es - DEout
32
Energy spectrum
33
Channeling and blocking
34
Surface peaks
35
Energy distribution of sputtered species
36
Sputtering yield ion energy dependence
37
Sputtering yield dependence on element
38
Sputtering yield angle dependence

• varies 1/cosq
• Drop at grazing incidence angle

39
Secondary Ion Mass Spectrometry (SIMS)
Mass
Ion beam
detect sputtered species (neutrals, ions) from
the sample
S
S

• Sensitive to top most layers
• Chemical composition
• Structural informations
• Very high sensitivity
• Imaging 1001000nm
• Depth profiling 5nm
• Ion yield depends on surface concentration and
  sputtering yield
• Organic anlaysis m/z 500040,000
• Matrix effect secondary ionization mechanism
• Destructive implantation, mixing, sputtering,
  ion beam induced
• surface chemistry, radiation induced atomic
  redistribution

40
SIMS modes

• Dynamic SIMS
• high sputter rate
• 10 mA/cm2
• 100 mm/hr
• destructive
• Depth profiling

• Static SIMS
• low sputter rate
• 1nA/cm2
• nondestructive
• Submonolayer analysis

1nA/cm2 10-9A/cm2/1.6x10-19 C 6.3x109
ions/sec-cm2 6.3x109 ions/sec-cm2
1015atoms/cm2 1.6x10-5 ML
41
Instrumentation

• Ionization methods
• electron impact
• microwave
• field ionization
• laser ablation
• Ion source
• Ar ion
• O2 for electropositive elements
• Cs for electronegative elements
• Liquid metal Ga, In
• - small beam size

42
Mass spectrometer

• Quadrupole
• inexpensive, compact
• Double focusing electrostatic
• /magnetic sector
• high transmission
• High mass resolution
• Time of flight
• -high molecular weight

From Jeol
43
Example
44
Imaging SIMS

• scan ion beam or
• ion detector
• Beam size
• Resolution 100mm

45
Thermal desorption spectroscopyTemperature
programmed desorpion
-measure desorbing molecules by heating the
surface using mass spectrometer
Quadrupole mass spectrometer
Adsorbed molecules
heater

• Heat of adsorption if Eads Edes
• Surface coverage peak area
• Adsorption sites peak position
• Intermolecular interaction
• Kinetics of desorption peak shape

46
Analysis of TPD
Redhead, Vacuum 12, 203 (1963) The rate of
desorption rd -dq/dt koqn exp(-Ed/kT)
n order of reaction ko prexponential
factor q coverage Ed activation
barrier for desorption The sample temperature
varies linearly T(t) T0 bt b
dT/dt heating rateK/s rd -dq/dT
(1/b)koqn exp(-Ed/kT)
coverage
kdk0eEa/kT
Intensity
TPD spectra
Temperature
Ea 24kcal/mol b 10 K/sec n1 ko1013 1/sec
Ed,ko q, b Desorption temperature ko q,n
peak shape q Peak area
47
Zero-order desorption kinetics
rd -dq/dt ko exp(-Ed/kT)

• -independen of coverage
• exponential increase with T
• common leading edge
• Rapid drop
• Tmax move to higher T with coverage
• Pseudo zerp-order for strong intermolecular
• interactions between adsorbates

Intensity
T/K
48
First-order desorption kinetics n 1
rd -dq/dt koqexp(-Ed/kT)

• -rate proportional to coverage
• balance between q and exp(-Ed/kT)
• Tpeak independent of q
• Asymmetric line shape
• Tpeak ? as Ed ?
• Molecular desorption

Intensity
exp(-Ed/kT)
q
T/K
49
Second order desorption kinetics n2
rd -dq/dt koq2 exp(-Ed/kT)

• -rate proportional to coverage
• balance between q2 and exp(-Ed/kT)
• Tpeak varies with q
• symmetric line shape
• Common trail of peaks
• Recomnative desorption
• Pseudo-2nd order for strong
• intermolecular interactions

Intensity
T/K
50
Fractional order desorption kinetics
Indicate cluster formation on the
surface Desorption from edge of clusters
- indica
Effect of activation barrier Ed50400kJ/mol
Ed
Intensity
10
20
30
40
50
Ed ? Tpeak ? peak width ? At saturation
coverage Ed/RTp 30kJ/mol
51
Effect of pre-exponential factor k0 1011 1015
1/sec
-oscillation frequency for adsorbate particles
Intensity
k0 1015
k0 1011
T/K
Effect of heating rate
b dT/dt 18.5
CO/Ni(110)
Intensity
b 17.5
b 18.5
T/K
52
Determination of activation barrier Ed
The maximum rate in the desorption rate drd/dt
0, konqn-1 exp(-Ed/kT) b Ed/kTp2 -Ed/kT ln
(b/ kTp2 )ln(Ed/ konqn-1 ) Plot of ln vs 1/T at
constant initial coverage Ed
ko/b1014/K
Tp
ko/b1010/K
Ed
Other methods Chan, Aris, Weinberg, Appl. Surf.
Sci. 1, 360 (1978) Habenschaden, Kuppers, Surf.
Sci., 138 L148 (1984) D.A. King, Surf. Sci. 47,
384 (1975)