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Abstract

We report state-resolved lifetimes of formaldehyde in the first excited triplet (a) over tilde (3)A(2) electronic state. Pumplaser excitation within 2800 cm(-1) of the triplet origin is followed by variably delayed F-2-laser ionization. Lifetimes fluctuate from > 100 mu s to similar 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 + H-2/D-2 continuum. By accessing the triplet state we are able to investigate the molecular elimination reaction down to similar 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 T-1-S-0 intersystem crossing matrix elements are comparable to the S-1-S-0 internal conversion matrix elements observed at similar vibrational excitation.