Transcript Vibrational spectroscopy

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)
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
Background
I0
-
I
+
-
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
Reflection
The reflection angles
Snell’s law
n1/n2 = sinqi/sinqt
Crtical angle: qc= sin-1(n2/n1)
Intenstiy of the reflected light
- Depend on polarizations
Fresnel’s equations
n = n + ik
s-polarized light : || the plane of incidence
Rs = [(n-secq)2+k2]/ [(n+secq)2+k2]
p-polarized light : ㅗ the plane of incidence
Rp = [(n-cosq)2+k2]/ [(n+cosq)2+k2]
- qi must be large: grazing incidence for thin
films on reflective surface
the plane of incidence
x
Ep
x
Es
qi
qr
qt
Phase shift , electric field, intensity of p-polarized light as a
function of incidence angle from a metal surface
p-pol :q||
0
Surface electric field E/E
180
n =3, k=30
(E/Eo) secq
Phase shift on refelctions
Surface intensity function
s-pol: qㅗ
60
2 40
20
0
0
incidence angle
90
0
incidence angle
90
s-polarized light at the surface
Absorbance is proportional to
- uniform phase shift
E2 and area of surface as 1/cos q
- vanishing E field at the surface
2/cos q
I
~
E
p-polarized light at the surface
- dependent on incidence angle
- strong E field at large incidence angle,
ie, grazing incidence
Adsorbate covered surface
Dielectric constant e = (n+ik)2
Ro
d
R
Vcauum e1n1
Adsorbate e2(n2, k2)
Ro
Metal e3(n3, k3)
R
Absorption function A = (R- R0) /R0 = DR/R
e3 >> e2~1, d<<l
DRs/Rs = (8pdcosq/l)Im((e2– e3)/(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
Surface selection rule
mfi = <f|m|i>  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
+
+
-
-
-
+
-
mimage
+
mM
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
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: <800 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: 0.1~10-3 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
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
Instrumentation: RAIRS
J.E. Reutt-Robey et al, JCP 93, 9113 (1990)
Instrumentation: MIR IR
IR finger print
Modes of vibration
IR spectra of CO on Pd(100)
-threefold:site :
1800~1900 cm-1
-bridge site:
1900~2000 cm-1
-on top site:
2000~2100 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
IRRAS spectra
CO on Pd(111)
Diffuse reflectance IR spectra
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. <800 cm-1
E
Eo
+
I
Eo-E = hv
v
Eo
E
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)
Eo
g||
E
g ||
ki
||
ㅗ
ki
kf
ㅗ
mM
+
+
-
-
-
+
-
mimage
+
mM
mimage
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
Peak positions for different adsorption states
Instrumentation
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
Interaction ions with solid
- Charge transfer: neutralization of ion and electronic excitation
- Kinetic energy transfer: sputtering, scattering
e
Evac
Auger neutralization
Resonance ionization
EF
Resonance neutralization
Ei
Quasi-resonance neutralization
Atomic and nuclear collision
Impact parameter (b)
scattering process
energy transfer (Tc)
~1 A
inelastic excitation
of valence electrons
10eV
~10-1 A
inelastic excitation
of L-shell electrons
100eV
~10-2 A
inelastic excitation
of K-shell electrons
1 keV
~ 10-4 A
elastic scattering
from nuclei
~100keV
Ion scattering spectroscopy
Low energy ion scattering (LEIS): 0.5 ~ 3 keV
Medium energy ion scattering (MEIS): 10~500 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)1/2+M1cosq) /(M2+M1)]2
M1,M2 : mass of incident atom and target
q = scattering angle
Ion trajectroy
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
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/2]2 Ec = [M2/(M1+M2)]Eo
dq
q
b
db
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%
Stopping power and depth resolution
-the rate of energy loss dE/dx depends on mass of projectiles, traget, and
incident energy
-for 0.5~2.0 MeV, dE/dx is independent of energy
-Depth resolution: 30~100 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
Energy spectrum
Channeling and blocking
Surface peaks
Energy distribution of sputtered species
Sputtering yield: ion energy dependence
Sputtering yield: dependence on element
Sputtering yield:
angle dependence
-varies 1/cosq
-Drop at grazing incidence angle
Secondary Ion Mass Spectrometry (SIMS)
detect sputtered species (neutrals, ions)
from the sample
Ion beam
S+
S
-Sensitive to top most layers
-Chemical composition
-Structural informations
-Very high sensitivity
-Imaging: 100~1000nm
-Depth profiling: 5nm
-Ion yield depends on surface concentration and sputtering yield
-Organic anlaysis: m/z = 5000~40,000
-Matrix effect: secondary ionization mechanism
-Destructive: implantation, mixing, sputtering, ion beam induced
surface chemistry, radiation induced atomic redistribution
SIMS modes
-Static SIMS
-Dynamic SIMS
- low sputter rate
~1nA/cm2
<10 Å/hr
- nondestructive
- Submonolayer analysis
- high sputter rate
~10 mA/cm2
~100 mm/hr
- destructive
- Depth profiling
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
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
Mass spectrometer
Quadrupole
-inexpensive, compact
Double focusing electrostatic
/magnetic sector
-high transmission
-High mass resolution
Time of flight
-high molecular weight
From Jeol
Example
Imaging SIMS
-scan ion beam or
ion detector
-Beam size <10nm
-Resolution ~100mm
Thermal desorption spectroscopy
Temperature 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
Analysis of TPD
Ed,ko’ q, b : Desorption temperature
ko’ q,n: peak shape
q: Peak area:
coverage
Intensity
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 rate[K/s]
rd = -dq/dT = (1/b)koqn exp(-Ed/kT)
kd=k0eEa/kT
TPD spectra
Temperature
Ea = 24kcal/mol
b= 10 K/sec
n=1
ko=1013 1/sec
Zero-order desorption kinetics
rd = -dq/dt = ko exp(-Ed/kT)
Intensity
-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
T/K
First-order desorption kinetics n =1
rd = -dq/dt = koqexp(-Ed/kT)
Intensity
-rate proportional to coverage
-balance between q and exp(-Ed/kT)
-Tpeak independent of q
-Asymmetric line shape
-Tpeak  as Ed 
-Molecular desorption
exp(-Ed/kT)
q
T/K
Second order desorption kinetics n=2
rd = -dq/dt = koq2 exp(-Ed/kT)
Intensity
-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
T/K
Fractional order desorption kinetics
Indicate cluster formation on the surface
Desorption from edge of clusters
Intensity
Effect i of activation barrier Ed=50~400kJ/mol
n
d
i
c E
d
10
a
20
30
40
50
Ed  Tpeak  peak width 
At saturation coverage
Ed/RTp= 30kJ/mol
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
Intensity
CO/Ni(110)
b= 18.5
T/K
b= 17.5
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
Tp
ko/b=1014/K
ko/b=1010/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)