The direct overtone pumping of polyatomic molecules

The fs solid state tunable lasers that are becoming available operate at peak power in the near infrared (say, 700-1100 nm). We present a computation of the classical power spectrum of CH₃ONO at this frequency range in order to elucidate the factors that determine the absorption cross section. Fast pumping and an initially already excited molecule are two factors that significantly enhance the probability of a direct (CH) overtone transition. For a molecule in its ground state using continuous wave pumping the cross section for overtone transitions is significantly smaller as compared to the isolated CH chromophore. We attribute this to a dilution, by IVR, of the optically bright mode. The time constant for this is such that fs pumping can access the de facto isolated CH₃ group.