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Here, the entrance channel complex in the exothermic OH + CH₄ → H₂O + CH₃ reaction has been isolated in helium nanodroplets following the sequential pick-up of the hydroxyl radical and methane. The a-type OH stretching band was probed with infrared depletion spectroscopy, revealing a spectrum qualitatively similar to that previously reported in the gas phase, but with additional substructure that is due to the different internal rotation states of methane (j_CH4 = 0, 1, or 2) in the complex. We fit the spectra by assuming the rotational constants of the complex are the same for all internal rotation states;more » however, subband origins are found to decrease with increasing j_CH4. Measurements of deuterated complexes have also been made (OD–CH₄, OH–CD₄, and OD–CD₄), the relative linewidths of which provide information about the flow of vibrational energy in the complexes; vibrational lifetime broadening is prominent for OH–CH₄ and OD–CD₄, for which the excited OX stretching state has a nearby CY₄ stretching fundamental (X, Y = H or D).« less

Here, sequential capture of OH and CO by superfluid helium droplets leads exclusively to the formation of the linear, entrance-channel complex, OH-CO. This species is characterized by infrared laser Stark and Zeeman spectroscopy via measurements of the fundamental OH stretching vibration. Experimental dipole moments are in disagreement with ab initio calculations at the equilibrium geometry, indicating large-amplitude motion on the ground state potential energy surface. Vibrational averaging along the hydroxyl bending coordinate recovers 80% of the observed deviation from the equilibrium dipole moment. Inhomogeneous line broadening in the zero-field spectrum is modeled with an effective Hamiltonian approach that aims tomore » account for the anisotropic molecule-helium interaction potential that arises as the OH-CO complex is displaced from the center of the droplet.« less