Physical Methods for Cultural Heritages

Introduction to Vibrational Spettroscopy G. Valentini

Gianluca Valentini

Vibrational spectroscopy

• Vibrational spectroscopy gives information on chemical bounds that are present in a compound

It provides analytical info because the measurement results can be compared with reference spectra and with “spectral signatures” that are collected in large databases

• Vibrational spectroscopy can be divided in:

Infrared Absorption Spectroscopy, also called FTIR (Fourier Transform Infrared) Spectroscopy

Ramam Spectroscopy

• Raman spectroscopy and absorption spectroscopy are complementary investigation techniques

Some molecular vibrations are Raman “active” and FTIR “inactive” and vice versa

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Gianluca Valentini

Infrared Absorption Spectroscopy

• It is based on interactions of the electromagnetic radiation with chemical bounds

• The main field of application is the analysis of organic molecules with high molecular weight (polymers)

• IR spectrum provides information about the presence of functional groups (sulphate, carbonate, carbonyl, hydroxyl, amine, etc.)

• In the medium-IR band it is possible to identify both organic and inorganic compounds

• Minimal quantity of the sample are required for the analysis (mg)

4000 3000 2000 1500 1000 400cm-1

16.0

20.0

25.0

30.0

35.0

41.3

%T

Q

Q

C

C

C

Vy

Vy

VyVy

CQ

FT-IR spectrum of a sample of plaster taken from fresco staccati dell’Oratorio Visconteo di Albizzate , XIV secolo (micro-pastiglia di NaCl)

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Gianluca Valentini

Simple model of the infrared absorption

• In molecules, the chemical bound exertsan elastic force between atoms

• If the barycentre of the positive charge + doesnot match that of the negative charge – the molecule can absorb IR radiation and vibrate

• Absorption takes place only for discrete frequencies that correspond to the energy separation of vibrational levels

• In the same way the rotation of a moleculecan be triggered by the absorption ofinfrared radiation (this effect mainly takesplace in gases)

• Homonuclear molecules do not show anycharge separation +/- and do not absorbinfrared radiation

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Molecular vibrations

• Molecular vibrations can be divided in two basic types Streatching Bending

• For a molecule made of two atoms havingmasses m1 e m2

• The energetic separation between 2 vibrationalslevels is:

• In terms of wave numbers

• The strenght constant k depends on the bond type (simple, double,..)

• Equation (#) allows one to estimate the spectral band for the IR absorption by a great number of molecular structures

• Beyond the “fundamental band” other absorption bands corresponding to higher harmonics (2, 3) are present with lower intensities

21

21

mmmmk

21

khE

c1

kc2

1(#)

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Gianluca Valentini

Measurement devices for IR absorption

• A FTIR spectrometer is made of a radiation source a dispersive element (e.g. diffraction grating) a detection subsystem

• Source usually a blackbody emitter with temperature between 1500 and 2200 K

• tungsten lamp for normal measurements (NIR and MID-IR)• special lamps for far infrared measurements

• Dispersive element It can be based on a diffraction grating like it is in UV/VIS spectrometers

• Usually the double beam configuration is used to compensate for water vapour and e CO2 absorption

Inteferometric method based on the Fourier transform (FTIR)

• Detectors Thermal detectors

• thermocouple, bolometer, pyroelectric detectors Photoconductive detectors

• Thin slabs of semiconductor materials: PbS, PbSe, HgCdTe (77 K)

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Gianluca Valentini

The FTIR Spectrometer

• When a monocromatic radiations enters aMichelson interferometer having a mirror thatmoves at speed v, the radiation is modulatedat frequency f:

• Every wavenumber is “encoded” by a frequency f in the range of some kHz

• If the radiation is not monocromatic the oscillations sum up

• The oscillatory components can be recovered by the Fourier Transform

v2v2

f

v = costante < 0.1 m/s

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The FTIR Spectrometer

• One mirror of the Michelson interferometer is moved with great precision

• The laser in the figure emits redradiation to be used for reference

• The sample (2-5 mg) is finely grinded and snterized in a tabletof alkaline halide (KBr)

• The typical measurement range of medium class FTIR is:

Top class instruments:

Resolution

Measurement time

1-1- cm 350 cm 7800 m 29 m 3.1

1-1- cm 10 cm 00502 m 1000 mn 400

1-1- cm 0.01 cm 8

minutes some sec1t

1-1-tipica cm 0.1 cm 1

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Gianluca Valentini

FTIR spectra interpretation

• Region of the group frequencies Belongs to the band 3600 cm-1 1250 cm-1 (2.5-8 m) Included absorption peaks of bondings: C=O, C=C, C–H, CC, O–H Allows one to detect the presence of the most important functional

groups of organic molecules Seldom allows the complete identification of a compound

• Region of fingerprints Belongs to the band 1200 cm-1 700 cm-1 (8-14 m) Contains information about the fine structure “fine” of a molecule Sometimes allows the complete identification of a compound

• The identification of a specific material can be performed through pattern matching within suitable digital libreries

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Gianluca Valentini

Raman Scattering

• The Raman effect has been discovered by Indian physicists Raman and Krishnan and is caused by the interaction of e.m. radiation with vibrations The spontaneous Raman scattering is a very faint effect that gives rise to

a frequency shift of the scattered radiation In a typical Raman scattering experiment the fraction of the intensity that

undergoes the effect is 10-6-10-7

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CCl4Hg lamp

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Spontaneous Raman vs. stimulated Raman

• When the radiation excites molecular vibrations “Stokes” lines appear

• When the radiation interacts with molecules in vibrational excited state “Anti-Stokes” lines appear

• In spontaneous Raman scattering the radiation is scattered in all the directions by independent molecular emitters (no phase relations) The linewidth corresponds to that of the vibrational level

• In stimulated Raman scattering the radiation is scattered in a cone with define aperture and has good coherence properties The molecules emit with phase relations The linewidth depends on the Raman gain and the peak frequency

correspond to the maximum spectral gain

• The linewidth is lower than that of spontaneous Raman scattering

The efficiency of the process can be very high (e.g. 30 %) and the Raman intensity can be comparable with that of the incident laser radiation

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Gianluca Valentini

Classical model for Raman scattering

• Every molecule (or unit cell of a crystal) consists of 2 or more nuclei bounded by the clouds of electronic orbitals

• When an e.m. field interacts with the molecule a dipole momentum is induced

• According to a simplified model the dipole momentum can be ascribed to the oscillation of the electronic cloud around nuclei If nuclei remain still the elettronic polarizzability is constant In general nuclei moves because part of the e.m. Energy is transferred

to a molecular vibration The elettronic polarizzability ij changes with

the nuclear configuration (distance) and can beexpressed as a series of terms (normal modes of vibration)

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tcosE~E jijiiji ij = elettronic polarizzability tensor

tcosQ~Q

QQ v

0

ij0ij

0

ij0ijij

Q = q2-q1 = vibrational coordinatesv = vibrational frequency

Let’s assume electric and mechanic armonicity

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Q

• The dipole momentum induced according to this approximation is:

• The polarization of the medium emits diffused radiation at frequency equal (Raleigh) lower (Stokes) and higher (anti-Stokes) that that of the incidentfield

• The Raman effect is proportional to If the la polarizzability changes with the vibrazional

coordinate (case I) the Raman effect takes place If the la polarizzability does not change (caso II) the

Raman effect is not present (first order) In general the presence or the absence of the

Raman effect depends on the symmetry group of the molecule or crystal

• The strength of the effect is greater for covalent bonds than with ionic ones

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tcostcosE~Q~Q2

1tcosE~P vivij

0

ijj

0ijij

Stokes Anti-StokesRayleigh

0

ij

Q

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Biatomic omonuclear molecules

• A molecular bond is infrared active if the derivative of at least a component of the dipole momentum with respect to Q is different from zero

• The biatomic omonuclear molecules do not show any dipole momentum at rest

• Even following symmetric stretching the dipole momentum is zero

They are NOT infrared active

• The molecular polarizability can be represented by means of a rotation ellipsoid with an axis oriented as the bond ( and )

• Let’s define for a given internuclear distance:

• To understand the properties of and of let’s consider the hydrogen molecule H2

• When the internuclear distance goes to zero the polarizability become similar to that of a helium atom

• When the internuclear distance goes to the polarizability become similar to that of 2 hydrogen atoms

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||

Average polarizability = 231 || Anisotropy = ||

0|| Q 0Q

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Hydrogen molecule

• Since the polarizability of 2 hydrogen atoms H is greater that that of 1 He atom the polarizability has positive derivative at the equilibrium distance

• In case of H2

and have different derivativewith respect to Q

• and

• H2 is Raman active

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Atom H → ≠ 0 = 0

Molecole H2 → ≠ 0 ≠ 0

0Q0|| 0Q 0

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Biatomic and linear triatomic molecules

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NON linear triatomic molecoles and more complex molecoles

• In general, in molecules with low symmetry all the vibrations are both Raman and infrared active

• Molecules with high symmetry can have vibrations inattive either Raman or infrared

• According to a realistic scenario where the armonicity hypothesis does not hold, overtones and combinatory frequencies can be observed In case of Raman scattering

these lines have negligible amplitude

In case of IR absorption thay can give contributes to the spectrum

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Comparison of Raman and FTIR spectra

• The Stokes Raman shift coincides with IR absorption lines

• Some lines are Raman active, other are only IR active The two spectroscopy methods are somehow complementary

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Gianluca Valentini

Instrumentation for Raman spectroscopy

• Excitation source The Raman scattering intensity increases with the forth power of the

laser radiation frequency An NIR laser (typically a Nd:YAG @1064 nm) is preferred to avoid

the fluorescence phenomenon

• The analysis of the Raman lines can be performed with a grating uno spectrometer or with a FTIR spectrometer

• Usually only the Stokes lines are considered since they are more intense

• Raman digital libraries greatly simplify the identification of compounds

• To increase the S/N ratio in Raman spectroscopy more complex methods that guarantee greater sensitivity can be used Resonant Raman (the laser frequency is close to the electronic

assertion band) Stimulated Raman scattering and Coherent Raman Anti-Stokes

(CARS) Surface enhanced Raman scattering (SERS)

• The above mentioned methods are seldom used in Cultural Heritages

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