Variation of the absorption cross section of high-power infrared laser radiation in homologous series of CnH2n+1OH molecules

An experimental investigation was made of variation of the characteristics of infrared multiphoton absorption in a homologous series of C_nH_2n+1OH alcohols (n = 1–5) excited with CO₂ laser pulses. The dependences of the energy absorbed by the molecules on the frequency and energy density of laser radiation were determined by the optoacoustic method. It was found that the multiphoton absorption cross section decreases on increase in the radiation energy density at a rate which becomes slower on increase in the molecular size. A model is proposed for multiphoton excitation of molecules in a homologous series. This model is based on an analysis of a resonant mode interacting with the infrared radiation field and coupled to a reservoir of modes that do not interact with the field. The model predicts correctly the change in the multiphoton absorption cross section on increase in the number of the degrees of freedom of a molecule.