The Raman spectra of chlorophyll a (1700-1000 cm(-1)) in the S-0, S-1, and T-1 states were recorded for the species of natural-abundance (NA) isotopic composition and those species that were totally labeled with the N-15, C-13, C-13 + N-15, H-2, and H-2 + N-15 isotopes. An empirical normal-coordinate analysis using the S-0 Raman lines of all the isotope species was performed to establish their assignments and to determine a set of force constants including the stretching, bending, nonbonded repulsive force constants and their cross-terms. The C-a-C-m and C-a-N stretching force constants in the S-1 and T-1 states were also determined, on the basis of the assumption that other force constants were the same as those in the S-0 state, to achieve the classification of the observed Raman lines of the NA-, C-13-, and H-2-Chl a into two groups : those which shift largely upon the N-15-substitution and are associated with the C-a-N stretching vibrations and those which do not and are associated with the carbon-oxygen and carbon-carbon stretching and the carbon-hydrogen bending vibrations. The resultant stretching force constants were regarded as a scale of the bond orders in the macrocycle in the different electronic states : In the S-0 state, the C-a-C-m bond orders are higher than the C-a-N bond orders, and weak positional dependence is found in both of them. Upon excitation to the S-1 state, the C-a-C-m bond orders decrease, whereas the C-a-N bond orders increase; both of them converge into a single value. Upon excitation to the T-1 state, drastic reorganization of the bond orders takes place in the macrocycle, and the averaged C-a-N bond orders become larger than the averaged C-a-C-m bond orders.