Raman Spectroscopy of Ocular Tissue - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 294

Fig. 12.8. Layer structure of the central retina, with indication of light paths used
to measure macular pigment

three lowest lying energy states. Absorption, fluorescence, and Raman tran-
sitions occur at fixed nuclear positions of the molecule's atoms (fixed con-
figuration coordinate) as vertical transitions. A characteristic strong, electric
dipole allowed absorption transition occurs between the molecule's delocalized
π

-orbital from the 1 1 A g singlet ground state to the 1 1 B u singlet excited state,

Fig. 12.9. a Molecular structure of lutein and zeaxanthin, which both feature a
linear, conjugated, carbon backbone with alternating carbon single (C-C) and dou-
ble bonds (C=C). Resonance Raman spectroscopy detects the vibrational stretch
frequencies of the carbon bonds; b absorption spectrum of a lutein solution corre-
sponding to the molecule's 1 1 A g → 1 1 B u transition, showing the characteristic
broad absorption with vibronic substructure in the blue-green spectral range; c con-
figuration coordinate diagram for three lowest lying energy levels of carotenoids,
with indication of optical and non-radiative transitions between all levels. Lumi-
nescence transitions are very weak due to the existence of a low-lying excited even
parity state that has the same parity as the ground state, which allows one to detect
the relatively weak resonance Raman responses of the molecule without an otherwise
overwhelming intrinsic luminescence background