Quantum-state resolved lifetime of triplet ([formula omitted]) formaldehyde

Abstract

We report state-resolved lifetimes of formaldehyde in the first excited triplet ã3A2 electronic state. Pump-laser excitation within 2800 cm−1 of the triplet origin is followed by variably delayed F2-laser ionization. Lifetimes fluctuate from >100 μs to ∼100 ns and a large isotope effect is observed. The average decay rate increases exponentially with increasing vibrational energy above the electronic origin, consistent with a mechanism involving tunneling through the barrier on the electronic ground state to the CO+H2/D2 continuum. By accessing the triplet state we are able to investigate the molecular elimination reaction down to ∼25 kJ/mol below the zero-point-energy corrected activation barrier. A model based on empirical RRKM rates allows the intersystem crossing matrix elements to be estimated. The T1–S0 intersystem crossing matrix elements are comparable to the S1–S0 internal conversion matrix elements observed at similar vibrational excitation.