Analysis of Spectral Properties of Photosynthetic Pigments and Water Molecule with a Method of Quantum Chemistry and its Application to Remote Sensing

In remote sensing, many spectral indices such as NDVI (Normalized Difference Vegetation Index) or NDWI (Normalized Difference Water Index) have been developed for plant management in agriculture. However an essential problem exists that the remote sensing indices are not based theoretically on the spectral properties of the molecules of photosynthetic pigment, nutrient or water in plant. This article presents that spectral properties of the pigments and water were analyzed with a molecular orbital method derived from quantum chemistry and related to remote sensing indices. The electromagnetic wave absorption spectrum calculations for a single molecule of chlorophyll a, lutein and water were carried out with semi-empirical molecular orbital method software. The chlorophyll a is main photosynthetic pigments in plants and lutein is one of carotenoids served as sub material of the pigments. Water molecule is the solvent in pant cells. Chlorophyll a showed two main absorption bands in 360 nm and 600 nm, and lutein in 410 nm. The molecules showed their main absorption bands moved low in a region of 50 nm to 100 nm approximately in comparison with the actual bands in vitro measured by a spectrometer. This difference was thought due to a solvent effect. Both molecules are actually surrounded with water, protein and other molecules. The NDVI uses reflectance from a red absorption band around 660 nm and a nearinfrared around 860 nm, suggesting that the NDVI dependents strongly on the spectral properties of chlorophyll a. Water molecule showed the three main absorption bands in k1=3,850 cm -1 (2,600 nm), k2=1,750 cm -1 (5,700 nm), k3=4,000 cm -1 (2,500 nm), ranging in infrared and caused from the oscillations among the three atoms of H2O. Those fundamental absorption bands created two characteristic compound absorption bands at red color region around 600 nm and at infrared around 900 nm. For examples, k1+3k3 =15,850 cm -1 (630 nm) and 2k1+k3=11,700 cm -1 (850 nm). The NDWI is defined as the reflectance ratio of (860 nm 1,240 nm) / (860 nm + 1,240 nm), implying a relationship with the characteristic compound absorption band in 850 nm of water molecule.