Physical Methods for Cultural Heritages Introduction to Vibrational Spettroscopy G. Valentini

1. Physical Methods for Cultural Heritages Introduction to Vibrational Spettroscopy G. Valentini

2. 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 G. Valentini 2

3. 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) FT-IR spectrum of a sample of plaster taken from fresco staccati dell’Oratorio Visconteo di Albizzate , XIV secolo (micro-pastiglia di NaCl) G. Valentini 3

4. 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 G. Valentini 4

5. 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 (2n, 3n) are present with lower intensities (#) G. Valentini 5

6. 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) G. Valentini 6

7. 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 v = costante < 0.1 m/s G. Valentini 7

8. 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 G. Valentini 8

9. FTIR spectra interpretation Region of the group frequencies Belongs to the band 3600 cm-1 1250 cm-1 (2.5-8 mm) Included absorption peaks of bondings: C=O, C=C, C–H, CC, 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 mm) 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 G. Valentini 9

10. Hg lamp CCl4 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

11. 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

12. Classical model for Raman scattering aij = elettronic polarizzability tensor Let’s assume electric and mechanic armonicity Q = q2-q1 = vibrational coordinateswv = vibrational frequency • 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 aij changes with the nuclear configuration (distance) and can beexpressed as a series of terms (normal modes of vibration)

13. Q Anti-Stokes Stokes Rayleigh • 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 vibrazionalcoordinate (case I) the Raman effect takes place • If the la polarizzability does not change (caso II) theRaman effect is not present (first order) • In general the presence or the absence of theRaman 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

14. Biatomic omonuclear molecules Average polarizability = Anisotropy = • 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

15. 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  a and g have different derivativewith respect to Q and H2 is Raman active Atom H → a ≠ 0 g = 0 Molecole H2→ a ≠ 0 g ≠ 0 G. Valentini 15

16. Biatomic and linear triatomic molecules G. Valentini 16

17. 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 G. Valentini 17

18. 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 G. Valentini 18

19. 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 G. Valentini 19