UC Berkeley

A joint experimental and theoretical study is reported on the low-lying vibronic level structure of the ground state of the methoxy radical using slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled, mass-selected anions (cryo-SEVI) and Köppel-Domcke-Cederbaum (KDC) vibronic Hamiltonian calculations. The KDC vibronic model Hamiltonian in the present study was parametrized using high-level quantum chemistry, allowing the assignment of the cryo-SEVI spectra for vibronic levels of CH₃O up to 2000 cm^-1 and of CD₃O up to 1500 cm^-1 above the vibrational origin, using calculated vibronic wave functions. The adiabatic electron affinities of CH₃O and CD₃O are determined from the cryo-SEVI spectra to be 1.5689 ± 0.0007 eV and 1.5548 ± 0.0007 eV, respectively, demonstrating improved precision compared to previous work. Experimental peak splittings of <10 cm^-1 are resolved between the e_1/2 and e_3/2 components of the 6₁ and 5₁ vibronic levels. A pair of spin-vibronic levels at 1638 and 1677 cm^-1 were predicted in the calculation as the e_1/2 and e_3/2 components of 6₂ levels and experimentally resolved for the first time. The strong variation of the spin-orbit splittings with a vibrational quantum number is in excellent agreement between theory and experiment. The observation of signals from nominally forbidden a₁ vibronic levels in the cryo-SEVI spectra also provides direct evidence of vibronic coupling between ground and electronically excited states of methoxy.