Optically detected zero field magnetic resonance studies of the photoexcited triplet states of chlorophyll a and b

The zero-field EPR transitions for the triplet states of chlorophyll a and chlorophyll b in n-octane solutions at 2°K have been observed by optical detection methods. Both triplet absorption detection of magnetic resonance and fluorescence-microwave double resonance techniques were used. Sharp-line (5–20 MHz) zero-field EPR spectra were recorded for both molecules. The rates of depopulation for the individual triplet state spin sublevels were determined by microwave-modulated fluorescence intensity measurements. These experiments show that the middle spin sublevel is the most active in triplet state intersystem crossing for both chlorophyll a and chlorophyll b.