2'7' Raman and other Spectroscopies

1
2.7. 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 (2005) 1261-1267
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
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.
Infrared light
Visible light
4
La ViePablo Picasso, Blue Period 1903
Optical light
X-Ray radiograph
Infrared Reflectograph
Structure of underdrawing
High A material (Pb)
5
Molecular Excitation Modes
Provides a spectroscopic tool for analyzing
molecular components in pigments
Stretching mode between molecules
K is spring constant Units K N/mkg/s2
Phonon energy
Wave number
6
Infrared Spectroscopy
Relies on molecular excitations of
electromagnetic spectrum
Infrared modes
Infrared light reflects different modes of
vibration rotation of molecules
http//www.cem.msu.edu/reusch/VirtTxtJml/Spectrpy
/InfraRed/infrared.htm
7
Example O2 molecule
What is the spring constant (bonding strength) of
an O2 molecules with k832 cm-1 for 16O-16O and
k 788 cm-1 for 18O-18O?
Molecular spring constant is a constant for O2
molecules
8
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 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.
9
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 510-7 mm3
or 10-9 g!
10
Anion vibration in salts k1000 cm-1
Raman spectrum of red lead
Insufficient excitation energy (wavelength) for
Pb2O3
Wave number k1/?
PbO
Lead white k1050 cm-1 (PbCO3) Chalk k1085
cm-1 (CaCO3) Bone white k 960 cm-1
(Ca3(PO4)2) Red lead k226 cm-1, 313 cm-1, 390
cm-1, 549 cm-1 (Pb2O3)
Pb2O3
11
Probing for yellow pigments
Historical yellow pigments Yellow iron oxide
FeO Orpiment As2S3 Lead tin antimony
yellow Pb2SnSbO6.5 Lead 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.
12
Azurite and Malachite
Different molecular components in complex
molecules create certain Raman bands
k1000 cm-1 vibration between anion and kation in
salt Lead white k1050 cm-1 (PbCO3) Bone white
k 960 cm-1 (Ca3(PO4)2) Malachite
Cu22(CO3)(OH)2 Azurite Cu23(CO3)2(OH)2
Generates vibration modes of three groups O-H,
C-O3, Cu-O OH k952, 1035 cm-1 (bending mode)
3453, 3427 cm-1 (stretching mode) CO3
k817,837,1090 cm-1, 1415, 1490 cm-1, 747, 769
cm-1 (vibrational modes) Cu-O k345, 455 cm-1
(bending mode), k400, 495 cm-1 (stretching mode)
azurite
malachite
13
Testing ink pigments of medieval monastery
handwriting of letter R
Lead white k1050 cm-1 (PbCO3) Malachite
(Cu22(CO3)(OH)2 ) Azurite (Cu23(CO3)2(OH)2
) Vermillion k 253 cm-1 285 cm-1, 343 cm-1
(HgS) (cinnabar) Minium k226 cm-1, 313 cm-1,
390 cm-1, 549 cm-1 (Pb2O3)
Best et al. Endeavour, New Series 16 (1992) 66-73
14
Frescoes in Herods Tomb in Jericho
Roman fresco technique lime wash, followed by
pigment application
Analysis of fragments with Raman spectroscopy
Cinnabar (Persian Dragons blood) HgS
(vermilion)
Provenance of HgS pigment (Pliny Vitruvius
claim Spain)
H. G. M. Edwards et al. J. Raman Spectrosc. 30
(1999) 361-377
15
Saint Athanasios the Anthonite
Visual image
X-ray radiograph
reconstruction
16
Analysis with Raman Spectroscopy
Daniila et al., J. Raman Spectr. 33 (2002) 807-814