2.8. Raman and other Spectroscopies

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1 .8. Raman and other Spectroscopies The 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 (005)

2 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.

3 Infrared technology Infrared Reflectography The Fox Hunt, Winslow Homer Infrared Rendition Normal Kodak Ektachrome Infrared Kodak Ektachrome Infrared sensitive film & filter The Last Judgment, Hieronymus Bosch Franz Mairinger: The infrared examination of paintings; Radiation in Art and Archeometry, Elsevier 000; p. 40

4 Drawing techniques a& hidden secrets 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. Henry Inman ; Self portrait 1834 Visible light Infrared light

5 La Vie Pablo Picasso, Blue Period 1903 Optical light X-Ray radiograph Infrared Reflectograph High A material (Pb) Structure of underdrawing

6 Molecular Excitation Modes Provides a spectroscopic tool for analyzing molecular components in pigments stretching bending scissoring twisting Stretching mode between molecules 1 E h ; Phonon energy m m 1 1 K m m k c Wave number Δf=K ΔX K is spring constant Units K: N/m=kg/s

7 Infrared Spectroscopy Relies on molecular excitations of electromagnetic spectrum Infrared modes Infrared light reflects different modes of vibration & rotation of molecules

8 Example: O molecule O O O O k c K K What is the spring constant (bonding strength) of an O molecules with k=83 cm -1 for 16 O- 16 O and k= 788 cm -1 for 18 O- 18 O? 16 O m m1 1 1 c 16 K m m 8amu 9 amu O K K K 4 c m s 83cm 1 k m 1 cm 7 kg O O cm 100 9amu s cm m m Δx K=f chem /Δx 8amu kg amu amu Molecular spring constant is a constant for O molecules kg 330 s kg s

9 Principles of Raman Spectroscopy Molecular 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 deexcitation 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.

10 Raman Instrumentation Laser provides monochromatic photon excitation source Emitted 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 ~ mm 3 or 10-9 g!

11 Anion vibration in salts k 1000 cm -1 Wave number k=1/λ Lead white: k=1050 cm -1 (PbCO 3 ) Chalk: k=1085 cm -1 (CaCO 3 ) Bone white: k= 960 cm -1 (Ca 3 (PO 4 ) ) Red lead: k=6 cm -1, 313 cm -1, 390 cm -1, 549 cm -1 (Pb O 3 ) Raman spectrum of red lead Insufficient excitation energy (wavelength) for Pb O 3 Pb O 3 PbO Library of FT-Raman spectra of pigments, minerals, pigment media and varnishes, and supplement to existing library of Raman spectra of pigments with visible excitation Lucia Burgio, Robin J.H. Clark, Spectrochimica Acta Part A 57 (001) 1491

12 Probing for yellow pigments Historical yellow pigments: Yellow iron oxide FeO Orpiment As S 3 Lead tin antimony yellow Pb SnSbO 6.5 Lead antimonate Pb Sb O 7 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 L. Burgio et al. Anal Chem. 77, 161 (005)

13 Azurite and Malachite Different molecular components in complex molecules create certain Raman bands k=1000 cm -1 vibration between anion and kation in salt Lead white: k=1050 cm -1 (PbCO 3 ) Bone white: k= 960 cm -1 (Ca 3 (PO 4 ) ) azurite malachite Malachite: Cu + (CO 3 )(OH) Azurite: Cu + 3(CO 3 ) (OH) Generates vibration modes of three groups: O-H, C-O 3, Cu-O OH: k=95, 1035 cm -1 (bending mode) 3453, 347 cm -1 (stretching mode) CO 3 : 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)

14 Testing ink pigments of medieval monastery handwriting of letter R Lead white: k=1050 cm -1 (PbCO 3 ) Malachite: (Cu + (CO 3 )(OH) ) (k= 1490 cm -1 ) Azurite: (Cu + 3(CO 3 ) (OH) ) (k= 495 cm -1 ) Vermillion: k= 53 cm cm -1, 343 cm -1 (HgS) (cinnabar) Minium: k=6 cm -1, 313 cm -1, 390 cm -1, 549 cm -1 (Pb O 3 ) Best et al. Endeavour, New Series 16 (199) 66-73

15 Frescoes in Herod s Tomb in Jericho Roman fresco technique: lime wash, followed by pigment application Analysis of fragments with Raman spectroscopy Cinnabar (Persian Dragon s blood): HgS (vermilion) Provenance of HgS pigment (Pliny & Vitruvius claim Spain) CO 3 - calcite k=1086 cm -1 marble dust lime k=78k cm -1 Cinnabar, HgS k= 53 cm -1, 85 cm -1, 343 cm -1 Quartz k=463 cm -1 Fresco Tarna (Leon, Spain) Almaden (Cordoba, Spain) 1064 nm excitation H. G. M. Edwards et al. J. Raman Spectrosc. 30 (1999)

16 Saint Athanasios the Anthonite Visual image X-ray radiograph reconstruction

17 Analysis with Raman Spectroscopy Daniila et al., J. Raman Spectr. 33 (00)