Stimulated-emission pumping spectroscopy of highly excited states of CH_3O (X˜ ^2E): zero-order vibrational states at 3000 cm^−1 ≤ 9000 cm^−1

By the technique of stimulated-emission pumping, CH_3O (X˜^2E) radicals were excited to high-lying rotation-vibration states at energies of 3000 cm^−1 ≤ E_v ≤ 9000 cm^−1. Over this range the total vibrational density of states increases from 0.05 states/cm^−1 ≤ ρ_v ≤ 4 states/cm^−1. Stimulated-emission pumping spectra were recorded at a resolution of Δ_ν ≍ 2 cm^−1 with the pump laser tuned to the à ? X˜, 3^1_0, ^pP_11(2.5, +1) transition. The spectra were found to be dominated by distinctive vibrational band progressions. The main progression was assigned to the ν_3 (C—O stretch) zero-order states with 3 ≤ v_3 ≤ 9. The spin-orbit splittings of the excited ν_3 states were resolved. Higher-resolution spectra show that Franck-Condon intensity is carried by only few bright zero-order vibrational states, which, with higher excitation, are increasingly coupled to a manifold of dark background states. At energies of E_v ≳ 6500 cm^−1, above the adiabatic H—CH_2O dissociation energy, the spectra show an underlying continuum, which can be attributed to effective intramolecular vibrational redistribution, promoted by the strong anharmonicity of the potential energy surface in the vicinity of the CH_3O dissociation and isomerization barriers, and coupling of CH_3O resonance states to the H + H_2CO continuum.