Download Slide 1 - Washington State University

Application of Infrared
Spectroscopy
MatSci 571
2016
U. Mazur
Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS, DRIFT)
advantages: easy sample preparation, surface technique (1-10 depth), nondestructive
disadvantages: requires special accessories, surface technique (1-10 depth),
not universal
Praying Mantis accessory
powders, liquids, and suspensions (dyed textiles, cosmetic compounds, printed
papers, catalytic materials, agricultural and botanical specimen, etc.)
materials can be used neat and diluted in non-absorbing powders such as KBr,
KCl, Ge, and Si or ZnSe, ZnS, CdTe, and MgO
smaller particle size yields improved spectra
Diffuse reflectance can be used though out the IR-VIS-UV spectral range
DRIFT
Spectra of powders and rough surfaces can be recorded by illuminating these
surfaces and collecting scattered light for spectral analysis.
Schematic Diagram of Light Scattering from a
Powder Sample
In the UV-VIS integrating spheres and in the infrared ellipsoidal and parabolic
mirrors are used to collect the scattered radiation. Reaction chambers for diffuse
reflectance allow the study of catalysis and oxidation reactions in situ.
DRIFT spectra may exhibit features of transmission, external reflection, and/or
internal reflection.
Sample particle size and angle of illumination and/or observation can affect the
observed spectra.
Quantitative DRIFT
The Kubelka-Monk (K-M) transform is a mathematical transfrom for converting diffuse
reflectance intensity into absorbance.
K-M diffuse reflectance spectra can be used as one would use absorbance for
spectra measured in transmittance.
Where, R∞ is the absolute reflectance, K is the absorption coefficient, and S is the
scattering coefficient.
Due to the difficulty in measuring the absolute reflectance R∞, the comparative
reflectance r∞ with respect to a standard powder such as KBr or KCl (for which K is
near zero 0) is employed.
Quantitative DRIFT
Light emitted from the powder contains specular reflected light and diffuse reflected
light. The specular reflection must be reduced (ideally elliminated) to obtain accurate
diffuse reflectance spectra. To achieve this, the particle size must be reduced.
Reducing the particle size to a size  to the wavelength studied decreases the
proportion of specular reflection and maximizes the scattering efficiency.
The particle shape and packing status are other important factors, in addition to the
particle size.
Samples are normally not measured directly but in a form diluted to a suitable
concentration (1 % to 10 %) in a KBr or KCl standard powder.
After Kubelka-Monk (K-M) transformation
the spectral intensities vary linearly with
concentration (obey Beer’s Law)
Kubelka Munk Transform
The Kubelka-Munk equation creates a linear relationship for
spectral intensity relative to sample concentration.
It assumes
• infinite sample dilution in a non-absorbing matrix,
• a constant scattering coefficient (S), and
• an “infinitely thick” sample layer.
These conditions can be achieved for highly diluted, small
particle samples (the scattering coefficient is a function of
sample size and packing) and a sample layer of at least 1.5
mm. With proper sample preparation diffuse, reflectance
spectroscopy can provide ppm sensitivity and high quality
results.
Quantitative DRIFT
Diagram of the DRIFT optical path. IR
light comes from right hits M1 bounces of
M2 then M3 and shines infrared light on
the sample. M4 collects the diffuse
reflected light, which passes via M5 and
M6 to the detector. The sample plate is 6
mm in diameter and 1.5 mm in depth.
Highly sensitive measurements on a small
volume of sample can be made by filling
the sample plate with KBr powder and
putting the sample powder mixed with a
small quantity of KBr powder onto it.
Diffuse Reflectance Accessory
The Praying Mantis™ (Harrick)
This accessory incorporates two 90° offaxis ellipsoidal mirrors. One ellipsoid
focuses the incident beam on the sample
while the second collects the diffusely
reflected radiation from the sample. Both
ellipsoidal mirrors are tilted forward so the
diffusely reflected radiation is collected at
an azimuthal angle of 120°. Optical
geometry permits collection of the diffusely
reflected radiation, making the DRA quite
practical for routine measurements.
It is ideal for reliable diffuse reflectance
studies of powders and other rough
surface solid samples.
Low T chamber operates at 1-2 atm from
150ºC to 600ºC. High T, low pressure
chamber up to 910ºC (under vacuum) and
133 μPa (10-6 torr) to 133 kPa (1 ktorr).
High pressure chamber can withstand
pressures up to 3.44MPa (25.8 ktorr).
Comparison of DRIFT with Transmission IR Spectroscopy
1,2-Bis(diphenylphosphino)ethane (dppe) is a
commonly used bidentate ligand in coordination
chemistry.
Transmission spectrum of 1,2-bis(diphenyl
phosphino)ethane, KBr pellet, at 298 K.
DRIFT spectrum of 1,2-bis(diphenyl
phosphino)ethane, pure powder, at 298 K.
The diffuse reflection technique combines
the advantage of faster and simpler
sample preparation with the absence of the
spurious water band in the spectrum.
Oil & Gas Science and Technology – Rev. IFP, 2004, 59, 215
Boron Nitride Nanotubes (BNNT)
BNNT are great interest to biomedicine,
material science, and electronics. BNNT were
synthesized using the Laser Vaporization
Plume Chemistry approach, otherwise known
as the Pressurized Vapor/ Condenser (PVC)
Method. Tube Length≤ 200 µm band gap is 5.7
eV.
ATR FTIR spectrum of BNNT.
Bands at 1355 cm-1 and 804
cm-1, correspond to B–N
stretching and B–N bending,
respectively. 3220 cm-1
transition is assigned to the
stretching vibration of OH
groups of boric acid and was
found to be sensitive to
ambient environmental
conditions.
http://www.nanointegris.com/en/bnnt
FTIR for Quantitative Assessment of Bone Bioapatite Diagenesis*
DRIFT, ATR, and Transmission are used to
evaluate diagenesis by anthropological
geochemists and by examining the crystallinity
(IR-SF factor) and CO3:PO4 ratio of bone
material. Peak heights at 565 and 605 cm-1 and
the height of the minimum trough between them
gives the IR-SF factor:
IR  SF 
(565ht  605ht )
590ht
Modern fresh bone have IR-SF of 2.5 to 3.25.
Archaeological bone samples that contain a
measurable amount of collagen tend to have IRSF values less than fresh bone.
High IR-SF ratios indicate large ordered crystals,
low IR-SF values are consistent with smaller
sized crystals having irregular structure.
Spectra modern bone sample
obtained by different IR methods.
*Diagenesis is a complex process involving physical and chemical postmortem alterations to bones and teeth that is influenced by the burial
environment.
Journal of Archaeological Science 46 (2014) 16
Atmospheric Ageing of Nanosized Silicon Nitride Powders
540 days in humid (80%) air
540 days in dry air
90 days in Ar
DRFT spectra and TEM images of Si2N3
samples subjected to aging.
Ambient oxidation of nanosized
silicon nitride powder was studied
by DRIFT. S2N3 powder samples
were stored in Ar, in dry air and in
air of 80% humidity. No significant
changes were detected on storage
in argon gas for 90 days. Samples
stored in dry air and humid air
become oxidized and have N-H
bands. The nitride powder tends to
agglomerate in both dry and humid
air, and results in changes in the
particle size and morphology.
Nanosized silicon nitride powders,
therefore, should be processed by
excluding their contact with the
ambient atmosphere.
J. Mater. Chem., 2001, 11, 859
DRIFT Application to the in situ Analysis of Catalysts
Zeolite sample with Si/Al ratio of 13.6.
Intense band at 3741 cm–1 is due to the
free terminal silanols and the bands at
3630 and 3566 cm–1 are due to the bridged
Al-OH-Si acid hydroxyls located
respectively in the supercages and
hexagonal prisms.
A weak signal at 3670 cm–1 is due to Al-OH
of extra-lattice species, as well as two
shoulders at 3552 and 3526 cm–1. The
baseline shows a slope resulting from the
size of the particles causing diffuse
scattering.
Compared with the transmission spectrum,
the DRIFT spectrum exhibits a better
baseline. The 3670 band is more clearly
defined.
Oil & Gas Science and Technology – Rev. IFP, 2004, 59, 215
DRIFT Application to the in situ Analysis of Catalysts
Surface species on the catalyst surface (Pt/Al2O3) formed during 1 h of NO dosing at
423 K. In first few minutes, a series of bands appears, increasing in intensity during
the experiment up to 1hr of nitration. The bands at 1548 and 1584 cm−1 can be
attributed to N = O vibrations of the nitrate species and the intense band at 1836
cm−1 to linear NO on the oxidized platinum. The band at 1993 cm−1 could result from
the adsorption of NO linearly on Pt2+. At 423 K,a small quantity of CO was present.
The bands at 2128 and 2222 cm−1, respectively, are attributed to COlin/Pt and
AlNCO.
Oil & Gas Science and Technology – Rev. IFP, 2004, 59, 215
DRIFT Study of CuO−CeO2−TiO2 Oxides for NOx Reduction with NH3 at Low T
NOx are air pollutants contributing to acid
rain, photochemical smog and ozone
depletion.
A CuO−CeO2−TiO2 catalyst is used for
selective catalytic reduction of NOx with
NH3 at low T.
DRIFT spectra of CuCeTi catalyst after
dosing with NH3+NO+O2 at various T. In
N−H region, bands assigned to coordinated
NH3 on Lewis acid sites (3373, 3254 and
3161 cm−1) 1610, 1580, 1280 and 1225
cm−1 are assigned to nitrate species along
with σasNH3 on Lewis acid sites at 1600
cm−1. 1880 cm-1 weakly adsorbed NO on
the surface of the CuCeTi.
Studies indicate that most of the adsorbed
NH3 species react with NOx. Oxygen
oxidizes the Cu2+−NO species to nitrates,
which are stored on cerium sites or can be
directly reduced to N2.
ACS Appl. Mater. Interfaces 2014, 6, 8134
Attenuated Total Reflectance (ATR)
advantages: for solids, liquids, semisolids, thin films, easy sample preparation,
surface technique (1-5 depth), non-destructive
disadvantages: requires special accessories, surface technique (1-10 depth), not
universal
For ATR, light is introduced into a suitable prism at an angle exceeding the critical
angle for internal reflection. This produces an evanescent wave at the reflecting
surface. From the interaction of the evanescent wave with the sample, a spectrum
can be recorded with little or no sample preparation.
For thin films, ATR spectra are the same as transmission spectra. For thick films,
absorption bands are more intense at longer wavelengths.
As the angle of incidence approaches the critical angle, the bands tend to broaden on
the long wavelength side and the minima are displaced to longer wavelengths (lower
wavenumbers).
Dispersion type spectra are observed very close to and below critical angle.
ATR Materials
Note: KRS 5 (Thallium Bromide-Iodide)
Total Internal Reflection: Interface Between Water and Air
When light hits an interface between two different media, it can behave in two
different ways: the light partially refracts (bends) and partially reflects. The refractive
index of the material is what determines how fast light travels in a material. Light
travels about one and a third times faster in air than in water. Water has a higher
refractive index than air. When light is propagating from water to air, for all angles
less than a certain critical angle, the light gets transmitted through. But once the
angle is larger than the critical angle; all of the light gets reflected. This is what is
called total internal reflection.
2= 1.00
1= 1.33
 η2 
ac  sin  
 η1 
1  1.00 
ac  sin 

 1.33 
ac  48.7 
1
Total internal reflection
can be seen at the airwater boundary.
The refractive index of air is 1.0 and the refractive index of water is 1.33 so the
critical angle for total internal reflection for a water-air interface is 48.7°.
ATR
Crystal of IR transparent materials (KRS-5, ZnS, Ge)
•
solids, liquids, semisolids, and thin films
•
evanescent wave - it is slightly bigger than the crystal, so it penetrates a small
distance beyond the crystal surface into space
Reflectivity (fraction of radiation reflected)
R=(n2-n1)2/(n2+n1)2
n2: refractive index of air or thin sample
n1: refractive index of crystal
Snell’s law:
n1sin  = n2sin 
c: critical angle

 : angle of incidence
for   c internal reflectance occurs
c = sin-1n2/n1
when light enters material 2 (n2) from 1 (n1)
n2  n1 (glass to air) internal reflection (total reflection for 90    c)
For a typical ATR crystal in contact with air, c = 19
ATR
Depth of penetration (DP): depth at which the evanescent wave is attenuated to
13.5% of its total intensity
DP = 1/[2WNc(sin2 - 2sc)1/2]
W: wavenumber
c: crystal refractive index
: angle of incidence
sc:sample/crystal
Dp is defined as the distance between the
sample surface and the position where the
intensity of the penetrating Evanescent
wave dies off to (1/e)2 or 13.5%, or its
amplitude has decayed to 1/e.
average DP is 0.1-5 microns
common  is 45o and 60o
n2 > n1
n2
n1
ATR
Wavelength of light: the longer
the wavelength of the incident
light (lower wavenumber), the
greater the depth of penetration
into the sample. This yields
an ATR spectrum that differs from
the analogous transmission
spectrum, where band intensities
are higher in intensity at longer
wavelength. However, the ATR
spectrum is readily converted to
absorbance units
ATR and Angle of Radiation
 = 60
 = 45
 = 30
transmission spectrum
in mineral oil
ATR of ascorbic acid on KRS-5 prism.
Top  = 45 light entrance. Bottom  =
60 deg entrance.
ATR of NaClO3 on KRS-5 prism.
Applied Infrared Spectroscopy. J. Wiley & Sons.
s and p Polarization
The electric field vector for p polarized light lies in the plane of incidence.
The electric field vector for s polarized light is perpendicular to the plane of
incidence (s-polarization, from German senkrecht perpendicular)
Plane of incidence is the plane containing the surface normal and the
propagation vector (k vector)
p-polarization vector
s-polarization vector
plane of incidence
FTIR-ATR of Self-assembled (SAM) Thiol Monolayers on Gold

A 1-tetradecanethiol, TDT on a gold surface. Note that
the CH2-plane is perpendicular to the alkyl chain axis,
and that both sym(CH2) and asym(CH2) are in that plane,
where sym(CH2) and asym(CH2) are orthogonal to each
other.
ATR of C-H stretching region of TDT adsorbed on gold
with different angles of incidence 25 to 60. Selfassembled monolayers of TDT were prepared by
immersing the clean gold-coated substrate into a freshly
prepared solution of TDT (10-3 M in ethanol) for 20 h.
Supramolecular Science 1998, 5, 607
FTIR-ATR of SAM Thiol Monolayers on Gold
The tilt angle, , based on the interaction
of a polarized electric field with a pair of
orthogonal dipole moments of sym and
asym CH2 vibrations . The following
relates the dichroic ratio, D, with :
D
As
Ap
sin     D1 1 for  CH 
2
sym
Polarized FTIR-ATR spectra for
TDT SAMs: (1) parallel
polarized beam; (2)
perpendicular polarized beam
2

 D
for  asym CH 2 

 D 1
The orientation of the molecular vibrational plane is
determined by formalisms which relate this plane to the
surface coordinates via Euler angles.
 was calculated to be 34.
Supramolecular Science 1998, 5, 607
Polarization Measurement of Stretched PET Film Using ATR
Spectra of unstretched PET film. There
are hardly any differences between any of
the four spectra, indicating that there is no
molecular orientation.
Spectra of PET film stretched by a factor
of 3. 1,340 cm-1 corresponds to CH2
wagging and the mode in which the
molecules oscillate in the direction of their
long axes. In (a), this peak is intense for p
polarized light and weak for s polarized
light. This is reversed in (b), indicating
that the molecules are oriented in the
stretching direction. 1,410 cm-1 is an inplane bending vibration of benzene rings.
IR Combined with SPM
AFM-ATR and spectra of a polystyrene
AFM-IR technique combines an
atomic force microscope with an
IR spectrometer as shown in this
schematic. After illumination with a
pulsed OPO IR source, the
cantilever ringdown characteristics
from the sample provide both
spectral and mechanical data
about the sample.
In AFM-IR IR light absorbed by the
sample is converted to heat, causing
a rapid thermal expansion pulse
under the AFM tip, in turn exciting
resonant oscillation of the AFM
cantilever. IR absorption spectra
result by measuring the cantilever
oscillation amplitude (proportional to
to the sample absorption coefficient)
as a function of the wavelength of
the incident radiation.
Characterization of a polyethylene–polyamide multilayer film using nanoscale
infrared spectroscopy and imaging
Tapping mode AFM and AFM-IR spectra from seven layers (color coded) of a
multilayer film consisting of polyethylene (PE) and polyamide (PA). IR spectra
collected from layers labeled A and F are consistent with polyethylene. Spectra
collected from layers C, E, and G are consistent with a polyamide. The IR spectrum
recorded from layer D, which can be considered as the barrier layer in the film, is
consistent with polyethylene-co-(vinyl alcohol) (EVOH).
Vibrational Spectroscopy 82, 2016, 10
AFM-IR of a Biomaterial
AFM-IR spectra of a wood cross-section; blue markers in the middle lamella
region where there is a higher content of lignin (~1504 cm-1 ) than the layer with
red markers.
ATR VideoMVP™ (Harrick)
ATR Spectra from two locations
on a credit card. The inset
photographs are bordered by the
color of the corresponding
spectrum.
The VideoMVP™ has high performance diamond ATR accessory plus video imaging and
optional force sensing capabilities. It has a a sampling area of less than 500 μm diameter
on its monolithic diamond. Also a heatable sampling plates for operation up to 200°C with
diamond, Si or ZnSe ATR crystals, 100°C with Ge.
Good for hard samples, abrasive powders, fibers, beads, and even corrosive materials.
The video imaging system provides a real-time magnified view of the sample through the
ATR crystal. This image can be seen on the built-in display or digitally captured. Solid
samples are compressed against the ATR crystal using the built-in pressure applicator. A
force sensor measures force applied to the sample.
Specular Reflectance
Reflection Absorption Infrared Spectroscopy (RAIRS)
advantages: easy sample preparation, surface technique, non-destructive
disadvantages: requires special accessories, sample must have smooth reflective
surface (Al, Au), not universal
Light is reflected from a smooth (mirror-like) sample.
Non-destructive, non-contact technique (Francis and Elison in 1959); particularly
useful for film thickness and refractive index measurements, as well as recording
spectra of thin films on metal substrates.
Spectra may look different from transmission spectra in many ways, e.g. bands may
be shifted to higher wavenumbers, spectra may follow the dispersion in the refractive
index, and spectral contrast may not depend linearly on sample thickness.
RAIRS
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.
The surface signal is readily distinguishable from gas-phase or solute absorptions
using polarization effects.
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. Small signals (0.01% - 2% absorption) can still be recorded at
relatively high resolution (ca. 1 cm-1).
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. This is because of the low sensitivity of detectors in
this region, NOT an inherent characteristic of the technique.
RAIRS
To detect sub-monolayer coverages RAIRS, uses near grazing incidence geometry
Optimum angle of incidence for
RAIRS is 75 to 86 parallel
polarization, depending on the
properties of the surface.
Greenler RG. 1966.J. Chem. Phys.44:310–15
RAIRS of PNBA as a function
of angle of incidence.
Fresenius J Anal Chem (1998) 362 : 15–20
RAIRS Surface Selection Rule
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.
Vibration of a polar molecule oriented parallel to the
substrate surface induces an image charge in the
surface that cancels out the dipole, whereas it is
enhanced for vibrations perpendicular to the surface.
s and p Polarization
The electric field vector for p polarized light lies in the plane of incidence.
The electric field vector for s polarized light is perpendicular to the plane of
incidence (s-polarization, from German senkrecht perpendicular)
polarization
vector
surface normal
polarization
vector
surface normal
IR ray
IR ray


plane of incidence
plane of perpendicular polarized light
plane of incidence
plane of parallel polarized light
RAIRS: Choosing Angle of Incidence
RAIRS spectra of 70 nm thick SiO2
on an AL substrate at various angles
of incidence with parallel (p)
polarized light.
RAIRS spectra of 70 nm thick SiO2
on an Al substrate at various angles
of incidence with perpendicular (s)
polarized light.
Adsorption of CO on Cu(100) Surface
FT-RAIRS spectra of CO/Cu(100).
A. Adsorption at 23 K showing both
monolayer chemisorbed CO peak and
multilayered physisorbed CO.
B. Effect of warming to 26 K to desorb
multilayers, leaving a single
physisorbed monolayer.
C. Effect of warming to 35 K and
recooling to 23 K.
D. Surface described in c following
further adsorption of CO at 23 K. 128
co-added scans.
J. Chem. Soc. Faraday Trans. 1997, 93, 2315.
Nitric Oxide (NO) Adsorption on a Pt Surface
The sequence of spectra demonstrate how IR spectroscopy can clearly reveal
changes in the adsorption geometry of chemisorbed molecules. In this particular
instance, all the bands are due to the stretching mode of the N-O bond in NO
molecules adsorbed on a Pt surface, but the vibrational frequency is sensitive to
changes in the coordination and molecular orientation of the adsorbed NO molecules.
ν(N-O) spectra obtained from a Pt surface
subjected to a fixed exposure of NO at various
temperatures.
Bridged NO species, bonded to the step edge,
is the most stable species on the surface, a
1610-1620 cm-1 band. 1801 cm-1 is tentatively
assigned to the formation of an O-NO complex.
This species forms when a NO molecule bonds
on top of an O atom, which results from the
dissociation of NO on the Pt surface at RT.
Note - the surface coverage of adsorbed NO
molecules decreases as the temperature is
raised and little NO remains adsorbed at
temperatures of 450 K and above.
J. Phys. Chem. B, 2004, 108, 289
CO Chemisorbed on a Pt Surface
RAIRS spectrum shown below was obtained for a saturation coverage of CO on a
Pt surface at 300 K.
The reduction in the stretching
frequency of terminally-bound CO from
the value observed for the gas phase
molecule ( 2143 cm-1 ) can be explained
in terms the bonding of CO to metals:
A : σ-bonding -- overlap of a filled  "lone pair" orbital on C with empty
metal orbitals (of the correct symmetry)
- charge transfer from the CO molecule
to the metal center.
Type of Molecule
CO ( gas phase )
(C-O)
2143 cm-1
Terminal CO
2100 - 1920 cm-1
Bridging ( 2f site )
1920 - 1800 cm-1
Bridging ( 3f / 4f site )
< 1800 cm-1
The majority of the CO molecules are bound
terminally to a single Pt surface atom
B : π bonding -- overlap of filled metal
dπ (and pπ) orbitals with the π*
antibonding MO of the CO molecule.
Leads to charge transfer into the CO
antibonding orbital and reduction in the
CO bond strength and its vibrational
frequency (relative to the isolated
molecule).
HCN adsorption on a Pt Surface
The RAIRS spectra of HCN adsorption on Pt at sub-ambient temperatures; the surface
species give rise to much weaker absorptions than NO, and signal/noise is important.
Spectra also illustrate the surface normal selection rule for metallic surfaces.
a) 0.15 L HCN at 100 K: HCN is weakly coordinated to the
surface in a linear end-on fashion via the nitrogen; the ν(HCN) mode is seen at 3302 cm-1 but the ν(C-N) mode is too
weak to be seen and the δ(HCN) mode expected at ca. 850
cm-1 is forbidden by the surface selection rule. The overtone of
the bending mode, 2δ(HCN), is however allowed and is
evident at ca. 1580 cm-1.
(b) 1.50 L HCN at 100 K: Higher exposures lead to the
physisorption of HCN molecules into a second layer. These
molecules are inclined to the surface normal and the HCN
bending mode (820 cm-1) of these second layer molecules is
no longer symmetry forbidden. Hydrogen bonding between
molecules in the first and second layers also leads to a
noticeable broadening of the ν(H-CN) band to lower
wavenumbers.
Langmuir, 1 L = 1x10-6 mbar s.
(c) 30 L HCN at 200 K: At the higher temperature of 200 K
only a small amount of molecular HCN remains bound in an
end-on fashion to the surface. The relatively strong band at
2084 cm-1 suggests that some dissociation has also occurred
to give adsorbed CN groups, which give rise to a markedly
more intense ν(C-N) band than the HCN molecule itself.
Infrared Spectroscopy from Phonons
Phonons = lattice vibrations
total number of phonon modes: dn N
N: number of unit cells in crystal
n: number of atoms in unit cell
d: dimensionality
arranged in dn branches: d acoustic branches
d (n-1) optical branches
IR spectroscopy couples
only to optical phonons
Example: diatomic chain
optical phonon, “internal” vibration of unit cell
In 3-dimensional crystals, one distinguishes between transverse and longitudinal
optical phonons:
TO phonons usually have lower energy.
Phonons in metals are difficult to see, because
free electrons screen E-field
from interior of sample in IR spectral region
(R1) IR phonon spectroscopy mostly
in insulators and semiconductors.
Comparison of Phonons & Photons
PHONONS
Quantized normal modes of lattice
vibrations. The energies & momenta
of phonons are quantized
E phonon 
p phonon 
h s

h

Phonon wavelength:
λphonon ≈ 10-10 m
PHOTONS
Quantized normal modes of
electromagnetic waves. The energies
& momenta of photons are quantized
E photon 
hc
p photon 
h


Photon wavelength (visible):
λphoton ≈ 10-6 m
Infrared Spectra of AIN Films Prepared by Ion-Beam Deposition
N-H and the C-H stretches appear
near 3250 and 2900 cm-1,
respectively. Band near 2200 cm-1
is due to Al-N=N stretch.
Al-O stretch is at 950 cm-1. Below
1000 cm-1, only the LO AlN mode
can be observed in the reflectance
spectrum. This result is expected
for near grazing incidence
reflectance data. TO component is
near 640 cm-1. Broadness of the
LO and TO bands is due to the
amorphous or nanocrystalline
nature of the AlN film.
Vibrational spectra of AIN films synthesized with
25% H2-enriched N2: (a) absorbance spectrum of a
25-nm-thick film deposited on a KBr window; (b)
RAIRS spectrum (at 80 incidence) of 8-nm-thick
film on an Al mirror; (c) IET spectrum of a -0.4-nmthick film.
The tunneling vibrational bands in
(c) near 730 and 1820 cm-1 are
associated with AI-H motions in a
hydrogen-implanted alumina
barrier.
J. Phys. Chem. 1990, 94, 189
Photoacoustic Spectroscopy (PAS)
advantages: can work on any gas, liquid, or solid, nondestructive
disadvantages: requires special accessories, not very sensitive
Spectra are not dependent on sample size or shape.
The signal to noise ratio may increase with sample surface area. Sample loading and
unloading can be tedious.
A final advantage of PAS is that there is little sample preparation, simple place the
sample in the accessory and start taking spectra.
APS is particularly sensitive to interfering spectra from evolved gases. Extreme
caution must be exercised to minimize vibration of the apparatus.
PAS
• Sample literally 'talks' to the observer. Light entering the photoacoustic cell
passes through undetected if the sample is non-absorbing, but heats and expands
the gas in the cell if the light is absorbed. This expansion (and subsequent
contraction) of the gas makes an audible sound whenever absorption occurs.
Sound is detected by a microphone.
• APS is particularly sensitive to interfering spectra from evolved gases. Extreme
caution must be exercised to minimize vibration of the apparatus.
• Spectrum is a direct measure of the absorption of radiation by the sample
• Detection of pressure waves is related to properties such as thermal diffusivity
which can vary between samples. Thermal diffusivity is not necessarily constant
with sample depth.
• Background spectra are obtained from carbon black. Normally, one avoids taking
infrared spectra of carbon black because it has broad, featureless absorbance
bands that are not very useful. However, since carbon black absorbs at all infrared
wavenumbers, it has a PAS signal at all wavenumbers making it useful as a
background material.
PAS
PAS drops into the sample compartment of
most FTIRs. Samples are placed in a
sample cup (~1/2 inch across and 1/2 inch
deep), and KBr window is sealed above the
sample.
Helium (or air) is the gas used.
PAS has optics to direct the radiation
through the IR window and focus it on the
sample  sample heats up.
A diagram of a photoacoustic
accessory.
Heat from different layers in the sample
diffuses towards the sample's surface. This
movement of heat is known as a thermal
wave. Once a thermal wave reaches the
surface of the sample, the gas above the
sample is heated and expands. This
expansion causes a pressure wave to
propagate through the gas, literally, the wind
blows inside the sample cell.
PAS accessory contains a sensitive
microphone (originally developed for hearing
aids) to hear the pressure wave.
PAS: Depth Profiling
The sampling depth in a PAS experiment is defined as the depth from which 63%
of the thermal wave that reaches the surface originates. The equation used to
calculate the sampling depth is:
L  D / πF 
1/ 2
where:
L = sampling depth in cm
D = sample thermal diffusivity, in cm2/sec
F = frequency of modulation of radiation
When one needs to know the sampling depth in a PAS experiment, this equation
can be used to calculate the value. Now, the frequency with which infrared
radiation is modulated by an interferometer is given as follows:
where:
f = frequency of modulation
 = moving mirror velocity, in cm/sec
 = wavenumber of infrared radiation
F  2νυ
Then
L  D / 2νυ
1/ 2
PAS: Depth Profiling
The sampling depth is dependent on the moving mirror speed of the interferometer.
It is possible to obtain spectra from different depths within the same sample by
simply changing the scan speed.
The ability of PAS to obtain spectra from different depths within a sample is known
as depth profiling. Depth profiling can produce interesting results if a sample's
composition changes with depth.
It is important to realize that PAS cannot do a layer by layer spectra within a sample
(unlike NMR imaging). Instead, when L is increased, the entire depth from which the
signal is obtained increases. Thus, when L is small, the spectrum of the top layer of
the sample is obtained. When L is large, the top layer and lower layers contribute to
the infrared spectrum.
PAS of Nanomaterials
PA spectrum of Eu2O3 nanopowder
(~60 nm diameter).
The PA technique offers a unique
method to study the spectra of
powdered rare earth materials. Their
spectra are atomic in nature due to the
shielding of the electrons in the
unfilled f orbitals by a closed electronic
5s25p6 shell.
Well-defined peaks corresponding to
transitions from the ground state (7F0)
to various excited states are obtained,
and the observed peaks are assigned
based on the data available in the
literature. Although some transitions
such as 5D1 ← 7F0 and 5D3 ← 7F0 are
not allowed, the proximity of 7F1 level
to the 7F0 level causes mixing of levels
making these transitions allowed.
J. Chem. Ed. 2009, 86,1238
PAS of Nanomaterials
PA spectra of bulk and as-prepared
CdS (semiconductor) nanowires are
shown in Figure 2. The band gap of
bulk CdS powder occurs at 2.39 ±
0.04 eV, which agrees with the
literature value of 2.42 eV. The
absorption edge of CdS nanowires is
much steeper and occurs at a slightly
larger value of 2.49 ± 0.04 eV. The
increased steepness might be
attributed to the relatively well-ordered
structure and size distribution.
PA spectra of bulk and CdS nanowires.
The average diameter of the nanowires
was about 50 nm.
J. Chem. Ed. 2009, 86,1238
PAS of Materials
PASof a carbon fiber/epoxy which is
used in the manufacture of carbon
fiber materials.
PAS of a vinyl acetate/polyethylene
copolymer in the form of a small pellet.
The different spectra are of pellets with
differing amounts of vinyl acetate.
Several of the bands vary in intensity
with vinyl acetate concentration, which
means these spectra can be used for
quantitative analysis.
Applied Infrared Spectroscopy. J. Wiley & Sons.
PAS: Depth Profiling
The PAS spectrum of a vinyl acetate/polyethylene copolymer in the form of a small
pellet. The different spectra are of pellets with differing amounts of vinyl acetate.
Several of the bands vary in intensity with vinylacetate concentration, which means
these spectra can be used for quantitative analysis.
Applied Infrared Spectroscopy. J. Wiley & Sons.
Analysis of biological materials
Depth profiling of cells for localization of ligands
Presence of aromatic amino acid in proteins
Monitoring malaria parasite
Investigation of nanosized magnetic particles for drug delivery systems
Evaluation of elasticity and integrity of pharmaceutical tablets
Quantitative analysis of drug content in semisolid formulation
Monitoring electron transfer processes
Organic semiconductors imbedded in polymer matrix
Gas phase analysis
Analysis of condensed matter
Analysis of highly concentrated textile dyes
Analysis of
Eurasian J. Anal. Chem. 2010, 5, 87
Emission Spectroscopy (ES)
advantages: non-destructive, particularly useful for films on rough metal surfaces
and catalysts, impurities in microscopic craters and crevices.
disadvantages: requires special accessories, not very sensitive, depends on
particle size
ES are affected by the nature of the sample, e.g. particle size which controls the
surface to volume ratio and concentration which controls reabsorption of emitted
radiation and tends to invert emission peaks.
Adsorption bands tend to be narrower in emission than in transmission or diffuse
reflection. In a few cases, selective reflection is responsible for inversion of peaks.
This arises from the situation where in the expression for emissivity
E=1-T-R
the reflectivity at certain frequencies is not low enough to be neglected.
FT-IR Sampling Techniques
Sample
TS
RAIRS
ATRS
DRS
PAS
Gas
Liquid
Powder
Single Crystal
Thin Film on Metal
Rough Films on Metal
Thin Film on Dielectric
Rough Surfaces
Optical Constants
Film Thickness
Anisotropic Samples
Micro Craters
Micro and Nano Sampling
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TS:
RAIRS:
ATRS:
DRS:
PAS:
transmission
refection absorption
attenuated total reflectance
diffuse reflectance
photoacoustic