The observed effect of pressure in the multiphoton absorption and decomposition processes is analysed with the use of a master equation, with explicit consideration of the role of collisions in the remotion of the rotational and anharmonic restrictions to absorption. The influence of pressure of the reaction probability is also modelled to obtain information on the collisional relaxation of highly vibrationally excited molecules. The use of multiphoton absorption for the preparation of excited molecules in combination with optoacoustic detection and time resolved IR fluorescence in order to study energy transfer processes is also analysed.