This dissertation describes new quantum mechanical methods for studying the photon-induced dissociation of three-atom molecules, and is divided into three parts.

The first part studies the infrared multiphoton dissociation of ozone. A rotating-wave-like method is used to calculate the probabability that an ozone molecule dissociates as a function of the laser frequency. The effects of vibrational and rotational motion on the dissociation probabilities are studied.

The second part studies single-photon dissociation of HCN and ICN into H + CN and I + CN. A method based on quantum R matrix scattering theory, which propagates Franck-Condon overlap information, is developed to calculate changes in rotational and vibrational quantum states of the CN fragment during the dissociation half collision.

The third part casts the single-photon dissociation theory into the finite basis representation and the discrete variable representation. The discrete variable representation simplifies the calcuation, without a loss of accuracy, by taking advantage of the fact that the CN fragment rotates little in the region where it interacts strongly with the H or I atom.