Nonlinear coloration of photochromic spiropyran solutions

An investigation was made of the two-photon photochromism of spiropyran solutions subjected to ruby laser radiation. The two-photon absorption cross section of spiropyran was calculated. At high laser radiation intensities the dependence of the optical density of the photoinduced form of spiropyran on the incident radiation intensity deviated from the square law. The cause of this deviation was considered.