2.7. Raman and other Spectroscopiesnsl/Lectures/phys10262/art-chap2-7.pdf · 9 8 2 1 2 1 K c k K c c k O O K m m m m K ⇒ = ... Microsoft PowerPoint - art-chap2-7.ppt Author: mwiesche

2.7. Raman and other SpectroscopiesThe analysis and identification of the pigment chemistry of paint! Identifies radiation which is characteristic for molecular excitation modes.

L. Burgio et al., Anal. Chem. 77 (2005) 1261-1267

Infrared Reflectography

Incoming light is reflected on surfaces between material layers of different densities. A fraction of light is scattered back, the rest penetrates layer and is either absorbed (depending on energy dependent absorption coefficient) or scatters back on next surface layer. Light particles with certain wavelengths are absorbed out of incoming spectrum. Different pigments have different reflection and absorption coefficients at different wavelengths (see X-ray example). Scattering decreases with increasing wavelength, UV light is primarily scattered back on surface. IR light penetrates deeper and is a good tool for investigating underlayers of painting.

Penetration of incoming infrared light through paint layer with subsequent absorption on the charcoal underdrawing.

Drawing techniques a& hidden secrets

Henry Inman 1801-1846; Self portrait 1834

Visible light Infrared light

Method for studying underdrawing techniques for paintings. Underdrawing can be clearly visualized using infrared reflectography because carbon black pigments absorb infrared light, whereas opaque pigments such as lead white are transparent with infrared light.

La ViePablo Picasso,Blue Period 1903

Optical light X-Ray radiograph Infrared Reflectograph

High A material (Pb) Structure of underdrawing

Molecular Excitation Modes

stretching bending twistingscissoring

Stretching mode between molecules

ckhE

mmmm

K

νν

πν

=⋅=

⎟⎟⎠

⎞⎜⎜⎝

⎛+⋅⋅

=

;

21

21

21

Phonon energy Wave numberK is spring constantUnits K: N/m=kg/s2

Δf=K·ΔX

Provides a spectroscopic tool for analyzing molecular components in pigments

Infrared SpectroscopyRelies on molecular excitations of electromagnetic spectrum

Infrared light reflects different modes of vibration & rotation of molecules

Infrared modes

http://www.cem.msu.edu/~reusch/VirtTxtJml/Spectrpy/InfraRed/infrared.htm

Example: O2 molecule

222

1818

1616

21

21

42

1

98

21

21

kcKKcc

k

OOOO

K

mmmm

K

⋅⋅⋅⋅=⇒⋅⋅

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=−

=−

⋅=

⎟⎟⎠

⎞⎜⎜⎝

⎛+⋅⋅

=

μπμπ

ν

μ

μ

μππν

What is the spring constant (bonding strength) of an O2 molecules with k=832 cm-1 for 16O-16O and k= 788 cm-1 for 18O-18O?

( )

( ) 227

21

2821818

227

21

2821616

3301066.191007881034

3301066.181008321034

skg

amukgamu

mcmcm

smOOK

skg

amukgamu

mcmcm

smOOK

=⋅⋅⋅⎟⎠⎞

⎜⎝⎛ ⋅⋅⎟

⎠⎞

⎜⎝⎛ ⋅⋅=−

=⋅⋅⋅⎟⎠⎞

⎜⎝⎛ ⋅⋅⎟

⎠⎞

⎜⎝⎛ ⋅⋅=−

−−

−−

π

π

Molecular “spring constant” is a constant for O2 molecules

K=fchem/Δx

Δx

Principles of Raman SpectroscopyMolecular excitations are associated with vibrations or rotation of molecules which correlate with low frequency modes. Raman spectroscopy relies on the interaction of monochromatic light produced by a laser (in the infrared to near ultraviolet range) exciting an electron from its molecular bonding configuration with subsequent de-excitation to lower vibrational (rotational) excitation mode.

Emitted radiation from the de-excitation is shifted in energy (frequency, wavelength) with respect to laser light energy.

Challenge is to filter weak Raman transitions from strong Rayleigh scattering transition signals.

Raman Instrumentation

Laser provides monochromatic photon excitation sourceEmitted photons are optically focused onto diffraction grating for spectroscopic analysis and are recorded by CCD detector

Microscope facilitates sample resolution of ~0.5 μm, Minimum required sample size is ~5·10-7 mm3 or 10-9 g!

Lead white: k=1050 cm-1 (PbCO3)Chalk: k=1085 cm-1 (CaCO3)Bone white: k= 960 cm-1 (Ca3(PO4)2)

Red lead: k=226 cm-1, 313 cm-1, 390 cm-1, 549 cm-1 (Pb2O3)

Wave number k=1/λ

Anion vibration in salts k≈1000 cm-1

Raman spectrum of red lead

Insufficient excitation energy (wavelength) for Pb2O3

PbO

Pb2O3

Probing for yellow pigmentsHistorical yellow pigments: Yellow iron oxide FeO

Orpiment As2S3Lead tin antimony yellow Pb2SnSbO6.5Lead antimonate Pb2Sb2O7

Clear identification of lead based yellow mixed with Calcite as used by Vermeer during his late period of painting ~1700 AD shortly before his untimely death in the age of 43 in 1705.

Azurite and MalachiteDifferent molecular components in complex molecules create certain Raman bands

k=1000 cm-1 vibration between anion and kation in saltLead white: k=1050 cm-1 (PbCO3)Bone white: k= 960 cm-1 (Ca3(PO4)2)

Malachite: Cu2+2(CO3)(OH)2

Azurite: Cu2+3(CO3)2(OH)2

Generates vibration modes of three groups: O-H, C-O3, Cu-OOH: k=952, 1035 cm-1 (bending mode) 3453, 3427 cm-1 (stretching mode)CO3: k=817,837,1090 cm-1, 1415, 1490 cm-1, 747, 769 cm-1 (vibrational modes)Cu-O: k=345, 455 cm-1 (bending mode), k=400, 495 cm-1 (stretching mode)

azurite malachite

Best et al. Endeavour, New Series 16 (1992) 66-73

Lead white: k=1050 cm-1 (PbCO3)Malachite: (Cu2+

2(CO3)(OH)2 )Azurite: (Cu2+

3(CO3)2(OH)2 )Vermillion: k= 253 cm-1 285 cm-1, 343 cm-1 (HgS) (cinnabar)Minium: k=226 cm-1, 313 cm-1, 390 cm-1, 549 cm-1 (Pb2O3)

Testing ink pigments of medieval monastery handwriting of letter R

Frescoes in Herod’s Tomb in JerichoAnalysis of fragments with Raman spectroscopy

Cinnabar (Persian Dragon’s blood): HgS (vermilion)

Roman fresco technique: lime wash, followed by pigment application

1064 nm excitation

CO32- calcite

k=1086 cm-1

marble dustlimek=78k cm-1

Cinnabar, HgSk= 253 cm-1,

285 cm-1,343 cm-1

Fresco

Tarna (Leon, Spain)

Almaden (Cordoba, Spain)

Quartz k=463 cm-1

Provenance of HgS pigment (Pliny & Vitruvius claim Spain)

H. G. M. Edwards et al. J. Raman Spectrosc. 30 (1999) 361-377

Saint Athanasios the Anthonite

Visual image X-ray radiograph reconstruction

Analysis with Raman Spectroscopy

Daniila et al., J. Raman Spectr. 33 (2002) 807-814