Unimolecular reactions induced by vibrational overtone excitation

A continuous-wave dye laser has been used to activate molecules above threshold for unimolecular reaction. Visible photons selectivity excite high overtone transitions of inequivalent CH oscillators. Thus, a known amount of energy is deposited directly into vibrational degrees of freedom of the molecule in its electronic ground state in a single photon absorption. The rate of unimolecular reaction has been measured as a function of internal energy, type of CH overtone transition excited and pressure to test for non-statistical effects. Results are presented for the isomerization of cyclobutene and 1-cyclopropylcyclobutene to the corresponding buta-1,3-dienes and the isomerization of 2-methylcyclopentadiene to 1-methylcyclopentadiene. No evidence for significant deviations from statistical behaviour has been observed.