A quantitative comparison of multiple-photon absorption in polyatomic molecules

Abstract

A quantitative comparison of multiple‐photon absorption for a number of polyatomic molecules has been performed. A basis for this comparison has been developed that takes into account the different experimental conditions and molecular parameters, and provides suitable normalization of the fluence and absorption parameters. The normalization, and consequently the generalized interaction process, can be specified in terms of the spectroscopic absorption cross section of the molecule σ₀(ν) and effective fraction of molecules that interact with the radiation field 〈f〉. The results of the study indicate that in terms of the normalized absorption variables, multiple‐photon absorption is a general phenomenon that is qualitatively the same for all molecules; quantitative differences can be related to differences in σ₀ and 〈f〉. The absorption of all polyatomic molecules can be described in terms of the number of photons absorbed per molecule η (Φ) at a fluence Φ. The data indicate that in spectral regions where σ₀(ν) ≳0.1 (max), η (Φ) can be represented by the function Φ^γ where γ=1 for σ₀Φ/〈f〉f〉≳1. Quantitative values of 〈f〉 and the functional dependence of η (Φ) on the exerimental parameters such as gas pressure, optical bandwidth, and optical pulse duration are derived and shown to be in good agreement with experimental data for SF₆. These results are also compared with the anharmonic oscillator model for multiple‐photon excitation of polyatomic molecules.