Mechanism of four-level laser action in solution excimer and excited-state proton-transfer cases

Four-level laser energy level schemes are compared from the mechanistic spectroscopic viewpoint: (i) noble-gas excimer, (ii) solution molecular excimer, (iii) conventional dye laser, and (iv) intramolecular proton transfer. The lasing action of the chlorophyll special pair is discussed as an example of a solution excimer laser, and the lasing action of 3-hydroxyflavone and other molecules is discussed as an example of an intramolecular proton-transfer laser.