ANOMALOUS EMISSION OF CHLOROPHYLL, STIMULATED BY RUBY LASER

IN THE course of a study on molecular excitation to higher states, produced by energy summation of two photons, we subjected solutions of chlorophyll a, methylchlorophyllide a+b, chlorophylline, Mg-phthalocyanine in ethanol (conc. mole/l.), and metal-free phthalocyanine in dioxane (conc. mole/l.) to a ruby laser beam. The laser had a light energy of 1 J per impulse of 1 msec duration, their frequency being 2 per sec. The solutions were contained in rectangular glass cells. The laser beam was filtered through a 5-mm thick red glass, eliminating wave-lengths shorter than 660 nm, and focussed by a lens on to the cell. The effect produced was observed at right-angles to the incident beam through a 5-mm thick blue-green glass filter which completely eliminated the scattered laser light (694.3 nm). For chlorophyll a, methylchlorophyllide a + b and chlorophylline, a distinct bluegreen emission was observed in the narrow path of the laser beam. For the phthalocyanines this emission was absent. Evacuation of the solutions did not affect in any way the observed emission. On freezing the ethanol solution to 77°K the effect disappeared. To record the spectrum of this emission we used a motor-driven prism monochromator (f1 : 5 ) with a photomultiplier sensitive to the range 600-360 nm, cooled by liquid nitrogen vapour. A red glass filter (7 mm thick) was placed at the exit of the laser beam, and a blue-green one (5 mm thick), complementary to the first (transmitting the range 580-360 nm) was set before the monochromator slit. A photon-counter technique was used and to increase its precision at separate wavelengths a photon-count was carried out for equal times. The emitted spectrum thus recorded is shown for methylchlorophyllide in Fig. 1 (full curve). The pigment-absorption bands are represented by the dashed curve, the position of the laser line being indicated by an arrow. The spectrum thus consists of a band between 510 and 430 nm with a maximum at 480, i.e. at definitely shorter wavelength than that of the exciting light. The 480 nm band is situated near and on the red side of the Soret absorption band at 430 nm. It can be assumed that the observed emission represents a radiative transition from the level of the Soret band to the ground state. To reach this singlet level the summation of the energies of two red photons is necessary (Fig. 2). Several alternatives can be proposed to explain this effect, in line with those advanced