How it works IR spectroscopy, applications in Cultural Heritage Restoration and in Materials Science for the characterization of films (thin film deposited on a substrate and polymeric films - in particular how the deformation affects the dichroism ratio)

1. IR light and spectroscopy: some
applications
Carlotta Micale
Presentation for the exam of ‘Spectroscopy and Moulecular stucture’
22-06-2018

2. IR light and
spectroscopy:
some
applications
Introduction to IR spectroscopy: principles,
instrumental setup, interpretation of the spectra
Application in Cultural Heritage restoration: IR
imaging for diagnosis and an example of IR use in the
cleaning phase
Application in Material Science: FT-IR for the
evaluation of the thickness of a thin film deposited
on a substrate and Relation between deformation
and dichroism ratio in a polimeric film

3. Electromagnetic spectrum
X-RAY ULTRA-VIOLET VISIBLE INFRARED MICROVAWE RADIO
Increasing Frequency
Increasing Wavelength
NIR MIR FIR
50,000 cm-1
200 nm 380 nm 780 nm
12,820 cm-1
4,000 cm-1
400 cm-1
2,500 nm 25,000 nm
3 mm-20 cm
10 m-30 Km

4. What happen when IR light interacts with molecules?
• IR spectroscopy is based on IR absorption by molecules as undergo
vibrational and rotational transitions.
PotentialEnergy(E)
Interatomic Distance (r)
rotational transitions
Vibrational transitions
Potential energy resembles classic
Harmonic Oscillator: atoms are
considered as particles with a given
mass in the IR absorption process
equilibrium bond length
stretched
compressed
Spring force
Spring force
• rotational transitions have small energy
differences
≤ 100 cm-1, l > 100 mm
vibrational transitions occur at higher energies
• rotational and vibrational transitions often
occur together
E = ½ kx2
K=force constant
X= displacement

5. What happen when IR light interacts with molecules?
Vibrational frequency given by:
n=frequency
K=force constant
m=reduced mass – m1m2/m1+m2
If know n and atoms in bond, can get k
1/ 2 /k mn =
Harmonic Oscillations

6. What happen when IR light interacts with molecules?
Anharmonic oscillations
Anharmonic oscillation
Harmonic oscillation
Harmonic oscillation model is good at low energy levels
(n0, n1, n2, …)
It’s not good at high energy levels due to atomic
repulsions and attractions:
• as atoms approach, coulombic repulsion force adds
to the bond force making energy increase greater
then harmonic
• as atoms separate, approach dissociation energy and
the harmonic function rises quicker

7. What happen when IR light interacts with molecules?
Types of molecular vibrations
Source: www.wikipedia.org/
C—HStretching Bending
H
H
Symmetrical
2853 cm-1
H
H
Asymmetrical
2926 cm-1
H
H
H
H
Scissoring
1450 cm-1
Rocking
720 cm-1
H
H
H
H
Wagging
1350 cm-1
Twisting
1250 cm-1
C
O
H

8. What happen when IR light interacts with molecules?
Types of molecular vibrations
Stretching
frequency
Bending
frequency
•Stretching frequencies are
higher than bending
frequencies (it’s easier to bend
a bond than stretching or
compressing them)
•Bond involving Hydrogen are
higher in freq. than with
heavier atoms
•Triple bond has higher freq.
than double bond which has
higher freq. than single bond

9. What happen when IR light interacts with molecules?
• for non-linear molecules, number of
types of vibrations: 3N-6
• for linear molecules, number of
types of vibrations: 3N-5
• why so many peaks in IR spectra
• observed vibration can be less then
predicted because:
• symmetry (no change in
dipole)
• energies of vibration are
identical
• absorption intensity too
low
Examples:
1) HCl: 3(2)-5 = 1 mode
2) CO2: 3(3)-5 = 4 modes
Number of vibrational modes
- -+

10. What happen when IR light interacts with molecules?
IR active vibrations
In order for molecule to absorb IR radiation:
• vibration at same frequency as in light
• but also, must have a change in its net
dipole moment as a result of the
vibration
Example:
CO2: 3(3)-5 = 4 modes
-+
m = 0; IR inactive
m > 0; IR active
m > 0; IR active
m > 0; IR active
d-
d-
2d+
d-
d-2d+
d-
d-2d+
d-
d-2d+

11. IR spectroscopy: overall instrument design
Light Source:
• must produce IR radiation
• can’t use glass since
absorbs IR radiation
• several possible types
Detector:
• Thermal detectors (thermocouple)
• Photoconducting detectrors
• Pyroelectric detectors
Sample Cell
must be made of IR
transparent material
(KBr pellets or NaCl)
Monochromator
• reflective grating is
common
• can’t use glass prism,
since absorbs IR
Chopper is needed to discriminate
source light from background IR
radiation

12. IR spectra: absorbance, trasmittance and peaks
the stronger the
absorbance the
larger the peak
Infrared band shapes come in
various forms. Two of the most
common are narrow and broad.
sampleIR
source
Transmitted light
Detector
When a chemical sample is exposed to the action of
IR LIGHT, it can absorb some frequencies and
transmit the rest. Some of the light can also be
reflected back to the source.
When a chemical sample is exposed to the
action of IR LIGHT, it can absorb some
frequencies and transmit the rest. Some of the
light can also be reflected back to the source.
Detects the transmitted
frequencies, and by doing
so also reveals the
absorbed frequencies.

13. IR spectra: interpretation – Figerprint region
• organic molecules have a lot of C-C and C-H bonds within their structure
• spectra obtained will have peaks in the 1400 cm-1 to 800 cm-1 range
• this is referred to as the “fingerprint” region
• the pattern obtained is characteristic of a particular compound the frequency
of any absorption is also affected by adjoining atoms or groups.

14. IR spectra: interpretation – Carbonyl compound
• carbonyl compounds show a sharp, strong absorption between 1700 and 1760 cm-1
• this is due to the presence of the C=O bond

15. IR spectra: interpretation – Alcohol
• alcohols show a broad absorption between 3200 and 3600 cm-1
• this is due to the presence of the O-H bond

16. IR spectra: comparison of spectra to recognize a
compound
O-H STRETCH
C=O STRETCH
O-H STRETCH
C=O STRETCH
AND
ALCOHOL
ALDEHYDE
CARBOXYLIC ACID

17. Restoration of Cultural Heritage:
diagnostic phase
FIR MIR VIS UV
It’s possible to obtain informations from IR
light (and other sources) also without
making a spectroscopy!
A. Cosentino "Infrared Technical Photography for Art Examination” e-Preservation
Science, 13, 1-6, 2016.

18. Restoration of Cultural Heritage:
cleaning phase
• Cleaning is the most delicate phase of the restoration:
it’s needed to remove the residues just on the surface
without penetrating in the deeper layers
• One of the most used method is the application of an
addensed solution on the substrate it needs to be clean
 limited to the planar objects (paintings)
• New method developed: use an addensed solution, and
then reticulate it directly on the substrate in order to
obtain a rigid gel, easy to remove by peeling, without
using any other solvent  no residues and possible
application on non-planar objects like sculptures

19. From the addensed solution to the rigid gel
Addensed
solution of Klucel
(HPC)
Organic solvent
(Etanol or Aceton)
Alginate
Application of
a Calcium
supersaturated solution
Formation of the
rigid gel due to
the ability of
Alginate to
formate hydrogels
with divalent
cations

20. How to be sure that there are no residues at the
end?
IR spectroscopy allows to verify that over the substrate no residues
are present after the treatment with the gel
It was used ATR  no sample preparation needed, could be used
with many substrates and materials, even no-treated powders
It was made a spectroscopy of the powders of Klucel and Aginate
and of the untreated substrate as a reference
The spectra were compared with a substrate of paper trated with
the gel

21. The IR spectra
Alginate Powder vs. Klucel Powder Klucel Gel

22. The IR spectra
• The spectra of the treated and the untreated paper are equal and they
overlap, which means that no residues of the gel remained on the
surface.
Untreated paper
Treated paper

23. Material Science: evaluation of the thickness of a thin
film deposited on a substrate
• Thickness dependent positions of the
absorbance lines due to simple Fresnel
refraction and multiple reflectance  the
interference fringes cause the oscillation
of the absorbance lines.
• To evaluate the thickness of the film it’s
used the formula
(𝑁 − 1)
2 ∗ (𝑛 ∗ ∆𝜆)
N=numbers of the maximum
𝑛=refractive index
∆𝜆= 𝜆max- 𝜆min=difference of wavelenght16000 18000 20000 22000
10
15
20
25
30
35
40
45
50
55
60
65
70
absorbanceintensity
wavelenght (cm-1)
FT-IR 80SiO2_5Er_5Yb_14mlEtOh
_A24h_L20_1100_30s
Gunde, M. K. (1992). Optical effects in IR spectroscopy: thickness-dependent
positions of absorbance lines in spectra of thin films. Applied spectroscopy, 46(2),
365-372.

24. Polimeric film to identify by ATR (attenuated total reflectance)
spectroscopy
Various deformations are applied to the film
ATR in the parallel and perpendicular direction of the deformation
Evaluation of the Dichroism Ratio 𝐷 =
𝐴↑
𝐴→
between the absorbance
in the different direction
Objective: understand if the film absorbs the light in a preferential
direction of in an isotropic way (D=1)
Material Science: deformation of a polymeric film
and dichroism

25. Identification of the polymer
• HDPE – the spectrum miss the LDPE
bands (1300-1400 cm-1).
• 2800-2900 cm-1 C-H asimmetric
and simmethric stretchings
• 1460-1470 cm-1 bending 
amorphous phase
• 730-720 cm-1rocking crystalline
phase
• The sinusoid on the bottom of the
spectra give information about the
thickness of the film
IR spectrum of the polimeric film without any deformation
applied

26. Analysis of the peaks after the deformation
IR spectrum of the polimeric film with various deformations
applied
• The peaks in absorbance
deacrease due to the
deformation
• In the amophous zone we have
elastic retirementthe effect is
increasing the orientation of the
chains in the polymer
• We expect that D=1 for the
amorphous phase, and increase
for the crystalline one due to his
platic behaviour

27. Results
Amorphous peak (1473 cm-1):
Dichroism ratio decrease in the parallel way and increase in
the perpendicular way of deformation.
This behaviour is due to the % of amorphous and crystalline
phase and probably to a realative motion between the two
phases.
Crystalline peak (730 cm-1):
Dichroism ratio increase in the parallel way and decrease in
the perpendicular way of deformation.
This behaviour is due to the plastic deformation of the
polymeric chains

28. Conclusions
IR specroscopy is a very versatile technique that can be
used in many fields of research
With IR light is possible to make diagnosis (i.e. in the
field of the cultural heritage conservation)
It’s adaptable to many types of samples (powders, thin
films, pellets..) and no particular preparation of the
sample is needed and it’s a non-destructive analysis
Paired with other techniques such as NMR or mass
spectometry gives (almost) complete informations
about the sample

29. THANKS FOR THE ATTENTION!
Carlotta Micale