Abstract
Potential energy surfaces for the first singlet and triplet excited states of methane have been studied using multireference configuration interaction (MRCI) and equation-of-motion coupled cluster (EOM-CCSD) ab initio molecular orbital calculations. The vertical excitation energies for the 1T2 and 3T2 states are computed to be 10.64–10.66 and 10.25–10.30 eV, respectively. Two minima are found on the first excited singlet surface, 1 (∼C3v) and 2 (C2v), with adiabatic excitation energies of 9.16–9.25 and 8.39–8.52 eV, respectively. No minima is located on the triplet surface. Vibronic spectra, calculated based on the geometries, vibrational frequencies, and normal modes of the ground and excited states, reproduce well the experimental results. The spectra due to the 3s(C2v)←1t2 transition start at ∼8.63 eV and form a broad underlying continuum. The 3s(C3v)←1t2 transition is shown to be responsible for the minor fine structure observed in the experimental absorption spectra between 9.5 and 10.6 eV. Dissociation pathways leading to various photofragmentation products are discussed on the basis of the calculated minimal energy pathways of H and H2 elimination. Production of CH3(2A2″) and fast hydrogen atoms, the major channel observed experimentally, is speculated to occur either via the S0←S1 internal conversion or, more likely, via the S1(1A″)→T1(3A) intersystem crossing followed by fast dissociation in the triplet state. Spin–orbit coupling between S1 and T1 has been calculated to be about 45 cm−1.
Published Version
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